Effect of forage-to-concentrate ratio in diet on rate of solid and fluid passage through the rumen.
Fluid passage rates through the rumen influence digestion of soluble food nutrients, amount of short-chain fatty acids absorbed in the rumen and that pass out of the rumen, the amount of by-pass protein of dietary origin and the amount of microbial protein available to the host as a protein source, making modelling of passage imperative. Current research on passage rate should seek to incorporate various factors that affect rumen fill, and solid and liquid passage rates to develop intake and passage rate prediction models. The aim of this paper was to discuss factors that affect rates of passage of digesta and rumen digesta load. Ambient temperature, animal physiological status and reproductive status, fermentation and diet quality are major factors affecting digesta passage rates. The animal physiology also influences digesta passage rate. Computation of animal production level to account for all the physiological processes that affect passage rate is vital. Discrepancies on how ambient temperature and particle density (buoyancy) affect the passage rate of digesta in the rumen may cause uncertainty in calibration of temperature and buoyancy in prediction models. Corrected for diet properties, goats have similar passage rates to other ruminants.
- diet selection
- feeding behaviour
- intermediate feeder
- prediction model
Goats have become one of the most important livestock for resource-limited farmers around the world because they can survive in harsh climatic conditions from cold temperate regions (in the Siberia) to hot arid deserts (in the Kalahari). Key to their ability to survive in diverse climatic regions is their ability to walk through tight, narrow places and their capability of efficiently utilising vast plant feed resources. Goats are important as a source of protein (milk and meat) and wealth. Given the increasing importance of goats among resource-limited farmers in tropical and subtropical regions around the world, research on the physiological aspects of goat nutrition is vital for improved goat production. Feed availability and quality are the major factors affecting yields for and quality of chevon and milk in rural goat production systems. Enhanced milk and chevon productivity is largely dependent on their selective feeding behaviour supported by improved nutritional status through supplementation of poor-quality roughages with feeds of high nutritional value (e.g., protein and energy concentrates). Concentrates are generally expensive for resource-limited farmers in rural communities around the world, especially in Africa and Asia. Hence, cost-effective usage of these feeds is vital. Fundamental to cost-effective concentrate supplementation for improved productivity of goat farming systems in the tropics relies on accurate and precise prediction of roughage intake. Accurate prediction of roughage intake would enable farmers to calculate precise quantities of concentrates to be fed to achieve a cost-effective level of production of good-quality meat and milk.
One of the major challenges in developing sustainable and cost-effective feeding strategies for goats in rural production systems in Africa, Asia, and other parts of the world is the inability to accurately predict roughage intake in goats. This is partly due to limited information on the critical factors that affect intake, passage rates of digesta and rumen fill for goats. Nsahlai and Apaloo  examined the appropriateness of model in  to predict the intake in ruminants grazing on poor quality roughages in tropical regions and showed that the model, though structurally adequate, underestimated roughage intake, partly due to poor estimates of gut fill and rate of passage. Similarly, the authors [3, 4, 5] showed that the model of Illius and Gordon  overestimated retention time in browsing ruminants for particle sizes less than 2 mm. Nsahlai and Apaloo  pointed out that the model of Illius and Gordon  erroneously estimates rumen fill levels and passage rates as a function of body weight alone.
Given the role of fluid passage rates through the rumen in affecting by-pass proteins and fatty acids that are assimilated in the hindgut, passage of fluid would have a significant influence on milk protein and butterfat composition in dairy goats. This makes the study and modelling of digesta passage rate relatively important. Modelling of passage rates would necessitate prediction of roughage intake, microbial protein yield and milk composition in goats. Before any passage rate modelling exercise can be effectively carried out, factors that affect passage rates need to be reviewed and used to predict digesta passage rates.
Given the abundant literature on passage rates for cattle and sheep, and other ruminant feeding types, and limited data on goats, the chapter (i) identifies the major factors that affect passage rates in ruminants, (ii) explains the fundamental mechanisms by which each identified factor possibly affected rates of digesta passage from the rumen, (iii) gives suggestions of the major factors that can be considered as critical input factors for developing passage rate prediction models distinguishing studies on goats and (iv) determines whether goats are different with respect to other ruminants in terms of passage rates.
2. Rumen fill, gut capacity and its estimation
Gut fill is referred to as rumen fill with respect to ruminants based on the facts that the rumen is the only site in the gastrointestinal tract where distension has an effect of restricting digesta flow to a great extent . Maximal rumen load for dry matter is determined by allometric procedures as a function of body weight [1, 2]. However, a ruminant’s fill capacity also depends on the volume of digesta that causes rumen distension and on rate of flow of digesta from and rates of degradation of digesta in the rumen [6, 7]. As a result, criticism on determination of rumen fill based on body weight alone has been raised giving better models for rumen load based on body weight, mature body weight and dietary crude protein . This fill capacity may also be determined practically by manually emptying the rumen at a time when full gut capacity is reached and weighing out digesta at that time  or slaughtering animals upon meal termination . Rumen fill varies greatly with body weight and feeding habit. Rumen fill is approximated to be about 9 and 13% of body weight for browsers and grazers, respectively . Interestingly, there is no apparent approximation of rumen fill based on body weight for intermediate feeders such as goats, although it may be assumed to fall within the range of 9–13% when grazers and browsers are regarded as extremes.
Carrying out rumen evacuations to determine maximal rumen digesta load is not an easy task. It is assumed that the only or best way to know when an animal has reached its maximal gut capacity is when it stops or terminates feeding . This theory is supported by Boudon et al. , where termination of short-term feed intake was attributed to signalling from the rumen wall as a result of rumen fill. Taweel et al.  and Williams et al.  reported scenarios whereby maximal rumen fill was greater after termination of late afternoon feeding bouts just around sunset than bouts from other parts of the day. According to these results, measuring maximal rumen fill after termination of morning feeding maybe misleading. However, Baumont et al.  reported rumen fill to reach its first maximum after the main morning meals, with a daily maximal rumen fill being reached after the evening meal. Similar results were obtained by Thomson et al. , where maximal rumen fill was observed after termination of first morning meal and late evening meal at 0900 h and 2000 h, respectively, in grazing sheep. Assuming that at meal termination, ruminants would have reached maximal rumen capacity is misleading as well. This assumption is supported by Taweel et al.  and Chilibroste et al.  who reported findings where maximal rumen capacity had not been reached when grazing dairy cows terminated feeding bouts. Greenhalgh and Reid  reported similar results where sheep fed on hay and straw-terminated feed intake way before maximal gut capacity was reached. These and other experimental results therefore suggest the existence of different sets of rumen fill levels which are time dependent, either before or after feeding bouts. These fill levels may be a function of the rate of emptying of rumen digesta after eating has stopped. At any given time, rumen fill levels are a function of the rate of feed intake, rates of digestion and particle breakdown, and rates of outflow . As such, rumen fill levels or values are dynamic and thus should not be regarded as constants and times in which rumen capacity is measured should be taken into account as well.
Based on rates of passage and digestion, estimation of rumen capacity using mathematical procedures gives variable but useable results. Estimated rumen pool size based on passage and degradation rates at the point of meal termination gave values which were even twice as large when compared to average observed values obtained from the literature (Nsahlai, unpublished data). Failure of mathematical procedures to achieve tenable outcomes suggests that something uncertain takes place during the period after meal termination before evacuation.
3. Factors that affect rumen fill levels and rates of passage through the rumen
3.1. Animal species and feeding types
Ruminant livestock has different feeding habits  with cattle, buffalo and sheep classified as grazers and goats as browsers or intermediate feeders . Differences in type of diets and processes associated with feeding behaviour between these classes of animals may have an effect on rates of passage of liquid and solid phases in the rumen  and their rumen fill.
Sheep had lower mean retention times of solid in the rumen than cattle (58 vs. 65 h) . Lechner-Doll et al.  added that selective retention of particles is more pronounced in cattle than in sheep suggesting that the passage rate of large particles is greater in sheep than in cattle. Outflow rates of fine solid material are normally estimated by allometric procedures as an inverse function of body weight, which infers erroneously that the rate of passage in smaller ruminants is always greater than that in larger ruminants . Parra  showed higher passage rates for smaller herbivores than larger herbivores with diet quality held constant. Differences between cattle and sheep with respect to solid retention times depend on chewing activities of these species. Average chewing rates are higher in sheep (80–100 chews per minute) than in cattle (40–60 chews per minute), indicating different efficiencies in these ruminants. Consequently, cattle have developed pronounced selective retention mechanisms for large particles in the floating fibre mat found in the dorsal rumen to improve particle size reduction and perhaps nutrient extraction; hence, retain particles for longer periods compared to sheep [23, 24].
Oshita et al.  reported differences in passage rates and rumen fill levels among cattle as a result of different grazing strategies. Rumen fluid dilution rates were higher for rotational grazed cattle (12.2%/h) compared to cattle fed pasture in confinement (9.9%/h). Similarly, rumen volume was lower for rotationally grazed cattle (79.9 l) compared to cattle fed in confinement (110 l). Williams et al.  showed no variations in rumen pool sizes with pasture allowance and time of day. Lack of differences in rumen fill levels with increasing pasture allowance is due to great variations in outflow rates.
Great variation in passage rates between animals of the same class of ruminants may occur as a result of differences in feeding habits. Although very little or no evidence for this phenomena has been documented, it is highly likely to occur. Dorper sheep are less selective of feed, consumed more shrubs and bushes than Merino sheep during grazing in the Noorsveld Karoo, South Africa . Dorpers would be expected to have slightly faster passage rates than Merinos because they consume more browse. Hence, it would be expected that Dorpers spend less-time re-chewing twigs than Merinos resulting in more-intense rumen contractions that forced digesta out of the rumen quickly. Goats have a much similar feeding habit to these Dorper sheep as they utilise both graze and browse and are more selective of high-quality browse. It can be concluded that passage rate in ruminants is affected by interactions between diet, ruminant species and their climatic environment.
Molina-Alcaide et al.  observed no differences in particle passage rates in goats (intermediate feeder) and sheep (grazer) fed on various diets with average rates of 0.030 and 0.025/h, respectively. However, Clauss et al.  suggested that smaller browsing species had much greater solid and fluid passage rates through the rumen than grazers of a much similar size. A much different trend exists in larger individuals of each feeding habit. Larger grazers tend to show higher solid and fluid passage rates through the rumen than browsers of similar size [28, 29]. Surprisingly, intermediate feeders (such as goats) were not included in this comparison. Processes that occur in the rumen when different diets are fed coupled with the anatomy of the fore stomach associated with each ruminant feeding type are implicated in these differences. Fluid and solid passage out of the rumen occur through an opening between the reticulorumen and the omasum called the reticulo-omasal orifice . Positioning and size of the reticulo-omasal orifice may shed insight on the flow of liquid and solid digesta from the rumen. Hofmann  showed that the size of the reticulo-omasal orifice was greater in browsers than in grazers. It may be hypothesised that due to the larger reticulo-omasal orifice in browsers, a much greater volume of solid and fluid passes through the rumen per unit time than in grazers. This may cause browsers to have higher fluid passage rates than grazers in smaller animals .
With respect to the larger groups of animals, grazers possess larger omasum than browsers . One of the functions of the omasum is to absorb water , thus it may be logical to assume that there is a much greater water pulling effect (cohesion and capillary movement) of the grazers larger omasum than that of browsers. This could result in higher passage rates of fluids out of the rumen of grazers. Due to a greater receptive space of the omasum, the pressure difference between the rumen and omasum  is larger in grazers than in browsers. Hence, greater rates of passage of fluid observed in grazers may be due to a larger pressure difference. This may not apply to small grazing and browsing animals. Hence, a gap in knowledge on the relative sizes of the omasum in smaller grazers, intermediate feeders and browsers exists.
Indirect evidence suggests that browsing ruminants have shorter mean retention times for liquid and solid digesta in the rumen compare to grazers. These include postruminal absence of glucose transport mechanisms (GLUT transporters) in grazers which are present in browsers [4, 32, 33], deposition of large quantities of polyunsaturated fatty acids in browser carcasses compared to grazers [4, 34], lower efficiency of fermentation [4, 35] and total tract digestibility  in browsers than grazers and presence of large amounts of particles that are greater than 1 mm in faecal samples from browsers compared to grazers [3, 18, 37, 38]. These differences are partly a result of faster fractional passage rates of fluid and solid through the rumen of browsers compared to grazers.
Differences in viscosity of rumen fluid and saliva between grazers and browsers exist . Browsers have more viscous rumen fluid  and saliva  than grazers. The thicker and stickier the fluid digesta may have an effect of reduced movement of the fluid through the rumen due to increased attachment of water molecules to feed particles. Hence, fluid is less likely to escape from the rumen thus resulting in reduced fractional passage rate of fluid in the rumen of browsing animals. However, Silanikove et al.  obtained conflicting results to , where polyphenolic compounds increased the rate of fluid passage through the rumen. Polyphenolic compounds cause fluid digesta to be thick and sticky as a result of more viscous saliva production, which is a case in browsers . Hence, viscosity of rumen fluid increases due to the presence of polyphenolic compounds. The expected outcome is decreased fluid outflow rate. Contrary to that, increased viscosity due to polyphenolic compounds may increase the rate of passage of fluid. Fluid from the interstitial spaces may be drawn into the rumen in an attempt to wash off these polyphenols  as a physiological response by the animal against them. This occurrence may then result to increased rates of fluid passage through the rumen.
Due to observed differences in passage rates among ruminant species, possible differences in rumen fill may be expected given that the passage rate is related to the amount of digesta in the rumen at any given time. Molina-Alcaide et al.  showed that rumen fill and amount of rumen contents are larger for goats compared to sheep. It was concluded that goats possessed a unique characteristic of being able to maintain larger rumen fill levels without noticeable rumen distension than sheep when fed medium-quality diets. These results were not expected taking into account observations by Clauss et al.  showing that smaller browsing species had much greater fluid and solid passage rates through the rumen than grazers of a much similar size, suggesting that goats should have lower rumen fills than sheep. Cattle are expected to have a much larger gut capacity than sheep and goats when scaled to body weight. Parra  showed that metabolic rate increased as a fractional power of mass (Wx) suggesting that small ruminants have smaller rumen capacity per unit metabolic need. Hence, as a result, cattle would be expected to have a greater rumen capacity than sheep and goats. Due to the above-mentioned theories, small bodied ruminants with smaller gut capacity must compensate for this constraint by increasing passage rate to ensure they maintain adequate feed intakes to meet metabolic needs . This may help explain why sheep had higher passage rates compared to cattle . Body weight cannot be convincingly classified as a factor that affects mean retention time . At body masses less than 100 kg, Wenninger and Shipley  showed in cattle that there was no relationship between the body weight and mean retention time.
Differences in passage rates among ruminants exist as a result of differences in habitats in which they live and are adapted, which is dependent on the type of diet available. Silanikove et al.  showed that average fractional flow rates tended to be lower for desert goats (0.084/h) than non-desert goats (0.099/h). This translated to +39% higher fluid passage rate in non-desert goats per unit body weight. Again, mean retention time of solid particles was 10 h greater for desert goats with intake being predominantly limited by high levels of rumen fill . These findings indicated that desert (‘tropical’ or hot climate) goats may possess greater digestive capacity than other breeds of goats as a result of adaptation to feed and climatic conditions in the desert. Passage rate and rumen fill data for goats adapted to subtropical and tropical climates in sub-Saharan African are limited, thus necessitating data on how climatic adaptation influences passage rates and rumen fill.
Rumen capacity and fill levels at any given time vary according to breeds as well. Breeds better adapted to low-quality forages tend to possess increased rumen capacity for both digesta phases. Weyreter and Engelhardt  found that Heidschnucken sheep (well adapted to high fibre roughages) were better able to consume large amounts of fibrous diets compared to Merino sheep (less adapted to high fibre roughages). This suggested that Heidschnucken sheep have greater potentials in expanding their rumen capacity compared to Merino sheep. Black head sheep (cold climate or temperate breed) are unable to make such an adaptation relative to Heidschnucken sheep .
A new theory on passage rate is beginning to unfold based on anatomical features of the rumen in different ruminant feeding type. Clauss et al.  suggested that digesta passage patterns are correlated to and influenced by intraruminal papillation patterns. Differentiation between grazers and browsers using papillation patterns characterised grazers as having long, thick papillae and deep reticular crests and ridges. Browsers characteristically have short and much thinner papillae and shallower reticular crests compared to grazers. Presence of deep reticuloruminal papillae and crests caused entrapment of small particles in ridges of grazing ruminants than in browsers, causing longer retention times in grazers .
3.2. Level of nutrition and feed intake
Plane of nutrition may be referred to as the level of feeding and animal production level. Level of feeding is defined as the amount of feed the animal consumes relative to its level of feeding to meet maintenance requirements . Cases of hyperphagia increase demands for expanded rumen capacity so as to accommodate much greater digesta load . Quantities of feed ingested by ruminants depend on animal species, and the variability in intake levels occurs between breeds and/or individual within a breed .
Haaland and Tyrrell  observed that the rates of passage of fluid through the rumen increased by 13% when animals were fed at two times maintenance from feeding at maintenance level. The authors [50, 51, 52, 53] observed that an increase in dry matter intake was associated with linear increases in fluid passage rate. As an animal eats more dry matter, solid material entering the rumen accumulates, and there is a possibility of dry matter taking up space occupied by the fluid in the rumen thus exerting pressure on the rumen contents. With dry matter being more bulky  than liquid, there is a possibility of the bulk forcing liquid out of the rumen at a much faster rate as the pressure builds up in the rumen compared to low intake levels. In muskoxen, Barboza et al.  showed that elevation of feed intake by 74% increased gut fill by 31–34%. Hyperphagia increases gut fill, and gut fill is usually a result of reduced passage rate of solid material. On the other hand, this observation is inconsistent with studies where increased feed intake has been shown to increase passage rates. Although Lindberg  showed a strong relationship between liquid passage rate and feed intake in dairy goats, no correlation was reported between dry matter intake and mean retention time in addax . This suggests that high dry matter intakes may not necessarily influence passage rates through the rumen. Long mean retention times for particulate matter at high dry matter intakes in addax may have been due to a high reserve capacity of the reticulorumen. Accurate determination of the extent to which rumen capacity may expand to accommodate various types of forage diets in different ruminants would be important. This elicits determination of maximal rumen fill levels in ruminants. Body weight had high positive correlation to rumen capacity . Distension of abdominal cavities during the projected increases in rumen capacity have not yet been quantified and documented in any species . Estimates to which ruminant gastrointestinal tracts stretch to accommodate a given diet range roughly lies between 10 and 17% of the body mass in ruminants, with an upper limit of 20% for cattle. Goats and sheep reach this upper limit more frequently and easily than cattle . Body weight alone is not a good indicator of maximal rumen fill; Purser and Moir  reporting variation in gut capacity among animals of similar body weight. Tulloh and Hughes  reported larger rumen volumes in lactating cows than dry cows. Hence, rumen fill or volume is more a function of various physiological states.
3.3. Forage-to-concentrate ratio in the diet
Supplementation of predominantly roughage-based diets has become a major practice in ruminant nutrition. Protein concentrate supplementation of ruminants grazed on pasture increases the nutritional status of ruminants . Levels of concentrates added to predominantly roughage feed would affect the rate of passage of liquid and solid through the rumen. High roughage to concentrate ratio in the diet would lead to greater fluid and particulate passage rates from the rumen (Table 1). Passage rate is affected by roughage quality and the rate at which rumen digesta disappeared from the rumen is positively related to diet quality .
|Phase||Diet||F:C = 87.5:12.5||F:C = 75:25||F:C = 62.5:37.5||F:C = 50:50|
Bartocci et al.  reported an increase in passage rates of fluid and particulate matter from the rumen with an increase in the proportion of dietary fibre in diets fed to buffalo, cattle and sheep. All authors [61, 62, 63, 64, 65] reported that high proportions of concentrate in diets decreased the rates of fluid dilution and turnover in the rumen.
Although similar trends on the effects of forage-to-concentrate ratio on fluid dilution rate and fractional passage of solid were observed, a number of suggestions have been given toward explaining these observations. Forage-to-concentrate ratios can alter a number of processes in ruminants, and these processes have been implicated to changes in fluid and solid outflow rates from the rumen. These processes include the amount of saliva produced and the degree of stratification of rumen contents.
Froetschel  showed that cattle produced an average of 100–200 l of saliva in a single day when fed high-fibre diets. Saliva is mainly used as a buffering agent and lubricant as roughage digestion produces large amounts of short-chain fatty acids that may lower rumen pH. Increased amount of saliva forces ruminal wall contractions to escalate . An increase in these contractions may be stimulated by increased distension and tactile stimulation of the rumen wall due to filling by saliva. Distension of the rumen wall results to contractions, emptying the rumen fluid and solid digesta to prevent it filling. These contractions squeeze solid and fluid digesta out of the rumen at an increased rate increasing the efficiency of microbial protein synthesis. Bartocci et al.  observed decreased amounts of saliva production in animals fed high-concentrate diets that constituted 50% of the diet. Hence, reduced salivation may be responsible for lower passage rates in high-concentrate fed animals due to reduced rumen contractions. Another possibility is that occurrence of these increased contractions might be due to mineral ions present in saliva.
Due to the bulky nature of forage, high roughage diets may occupy a large space in the rumen. Bulky forage may force liquid out of the rumen at a much faster rate as competition for space increases. Tactile stimulation of the rumen wall by the roughage is a likely facilitator. Because of a much greater degree of tactile stimulation, rumen wall contractions may occur, thus forcing rumen fluid to pass through the rumen at a much faster rate. Okine and Mathison  showed that an increase in duration and amplitude of reticulorumen contractions resulted in an increase in passage rate of both solid and liquid matter out of the rumen. Due to less bulk, concentrates would occupy far much less space in the rumen than forages; thus, high-concentrate diets would induce low-amplitude rumen contractions due to reduced tactile stimulation of the rumen wall. Low power of contractions would force less fluid out of the rumen per unit time compared to roughages leading to lower fluid and solid passage rates. Rumen fill was reported to be greater in diets that had higher proportion of roughage because roughages contribute to rumen fill more than concentrates in view of longer retention times in the rumen and selective retention in the fibre mat. Lui et al.  observed reduced clearance rates of solid digesta in the rumen of animals fed bulky high fibre crop residues. Lui et al.  gave clear evidence of enhanced rumen fill levels as a result of high fibre/roughage content in ruminant diets. Concentrate particles are small and the chance of being trapped in the floating mat is minimal; thus, it passes out of the rumen at a much faster rate than roughage particles.
In the rumen, stratification occurs (Figure 1). Stratification involves separation of liquid and solid components into distinct layers according to density . Stratification is evident when a mat-like layer forms and floats on the liquid phase. Fibre promotes the formation of the floating mat  in roughage more than concentrate diets because concentrate particles are smaller forming more homogenous mixtures in the rumen. Formation and presence of a floating mat in the rumen stimulates ruminal wall contraction , possibly due to tactile stimulation of the rumen wall. These contractions may lead to a rapid outflow of liquid and fine-solid digesta through the rumen. Faichney  showed that entrapment of large solid particles in the filter bed of the rumen restricted their outflow. Entrapment increases retention time of large particles; hence, fibre-mat formation may be a factor labelled as affecting rate of passage of solid through the rumen.
However, the theory of stratification may be challenged. Moore et al.  showed that cottonseed hull diets, even though fibrous and elicit a faster rate of liquid flow through the rumen, do not promote stratification. Hulls are smaller and denser and form a more homogenous-like mixture in the rumen . Moore et al.  concluded that rates of fluid flow through the rumen increased because of increased intake of the hull diet. Contrary to that, Owens and Goetsch  reported that cottonseed hulls resulted in decreased passage rates of fluid in the rumen, thus supporting the theory of stratification. Further studies on the effect of cottonseed hulls on the rate of passage need to be done. The theory of stratification that supports increased flow rate of fluid through the rumen may be applied to higher passage rates in grazers than in browsers due to differences in diet. Grazers are mainly roughage eaters, and browsers are concentrate feeders . Hence, higher rates of passage of fluid are seen in grazers than browsers.
Stage of development of forage may also have an effect on the fluid dilution rate and solid passage rate . When a plant is young, it contains a higher proportion of water than old plants, with older plants tending to have a larger proportion of lignin. With older plants having large proportions of lignin than younger plants, it is expected that forages at a late stage of development may induce higher liquid passage rate. However, ruminants that graze on grass that is at an earlier stage of development have high fluid passage rates through the rumen than those grazing on mature pasture. Work by Adams et al. , Estell and Galyean  and McCollum and Galyean  showed that animals grazed on young pastures have higher dilution rates (18.3%/h) than animals grazed on mature pastures (9%/h). The presence of high mineral and water content in young forage may led to increased osmotic pressure in the rumen causing the relaxation of the reticulo-omasal orifice thus increasing fractional rates of passage of fluid through the rumen. Lignin and hemicellulose contents of forages may have a substantial effect on passage rates of both solid and liquid matter in the rumen. Mature forage contains a higher proportion of hemicellulose than young forages . Hemicellulose has hydrophilic properties  and capabilities of absorbing and holding water in the rumen are high. Due to hydrophilic properties of hemicellulose, fractional rate of passage of fluid through the rumen decreases because hemicellulose absorbs a greater proportion of fluid and reduces fluid outflow rate. This phenomenon is most likely to occur when high roughage diets are fed.
Van Weyenburg et al.  observed higher fluid passage rates in Lucerne hay than in grass hay. Analysis of the hemicellulose content in both feeds showed higher hemicellulose content in grass hay than in lucerne hay. The water holding capacity of the hemicellulose is approximately 260 g water/kg DM for grass hay and 59 g water/kg DM for lucerne hay . This suggests that the greater the hemicellulose content of forages, the greater the amount of water that forage can hold. The greater the quantity of water held by the forage then the lesser the proportion that leaves the rumen, resulting in a decrease in the fractional rate of fluid passing through the rumen. Seemingly, Froetschel and Amos  found no correlation between water holding capacity of digesta and fluid outflow rate, but a positive correlation between water holding capacity and ruminal fluid volume. More evidence of this subject is needed.
Dietary roughage quality affects rates of passage of solid material through the rumen . Rinne et al.  found out that clearance of digestible plant cell wall fractions of particulate matter was slower than indigestible fraction of matter. This is perhaps due to sorting of particles in the rumen by stratification  and entrapment of digestible material in the floating fibre mat. Within a feed particle, digestible portions of feed retain for longer periods in the rumen and degrade slowly to a high extent, whereas indigestible portions clear from the rumen through passage quickly because of their size and density. Plant particles undergoing fermentation produce gas, hence float and get entrapped in the floating fibre mat restricting their passage out of the rumen. As a result, fractional clearance rate of indigestible part of fibre such as lignin is more rapid than that of digestible fractions such as hemicellulose  and may reduce rumen fill . Contrary to this view, Baumont et al.  suggested that increases in lignin content of roughage would make it stay much longer in the rumen before being cleared through passage out of the rumen, increasing rumen fill as a result. Baumont et al.  was of the view that retention time in the reticulorumen depended on the rate of degradation of the degradable fraction and on the proportion of non-degradable fraction. This suggests that increased proportions of non-degradable fractions slowed down the rate of degradation of the degradable fractions, with overall effects of slowing down passage rate and increasing rumen fill. For microbes to get access to the digestible fractions of fibre, microbes must etch into and remove lignin so as to access these digestible fractions. Hence, as a result of high-lignin content, the rate of degradation is greatly reduced, thus increasing retention time in the reticulorumen. Grasses tend to contain high contents of neutral detergent fibre compared to browse leaves and legumes. Browse leaves are shown to contain much more lignin compared to grasses . Panjaitan et al.  reported mean retention times of lignin fraction that were three times greater than those of neutral detergent fibre fraction across four grass species.
Rumen fill is at times described based on fibre (neutral detergent fibre, NDF) fraction  on the basis that fermentation and passage rate of neutral detergent fibre through the rumen are slower than of any other dietary constituent. Fibre exerts a greater filling effect in the rumen . Indirect evidence on the effects of NDF content on rumen fill exists. Using sheep fed on alfalfa hay and orchard grass hay, Baumont et al.  observed higher dry matter intakes in sheep fed alfalfa hay relative to orchard grass hay, which was attributed to lower NDF content in alfalfa hay. Due to lower NDF content in alfalfa hay than in orchard grass hay, alfalfa had a lower filling effect on the rumen due to rapid rates of fermentation and passage through the rumen . In conclusion, low NDF content is associated with low rumen fill levels, suggesting a positive linear relationship between NDF content (x-axis) and rumen fill (y-axis), which reaches a plateau when rumen capacity cannot increase further with additional increase in NDF content.
Grazing herbivores have an ability to gradually modify rumen volume and increase passage rates in accordance with a reduction in roughage quality . Due to slower passage rates of the digestible fraction, ruminants fed on highly digestible feed may experience maximal rumen fill. Boudon et al.  stated that attainment of maximal rumen fill would limit feed intake in dairy cows grazed on highly digestible rye grass. Also, rumen fill in grazing animals varies greatly from the beginning to the end of a feeding session . On the contrary, Dove  suggested a relationship, whereby rumen fill played a major role in regulation of feed intake with decreasing digestibility of a feed. Digestibility is negatively related to lignin content and high lignin content caused slow passage rate in the rumen [78, 88]. This actually suggests that passage rate of solid material was slower for low digestible feeds. Slower passage rates increased rumen fill because feed stays for a much longer time in the rumen. Faverdin et al.  demonstrated a more or less similar phenomenon where the overall effect of indigestible feed components resulted in increased rumen fill of approximately 1 kg DM which resulted to a corresponding depression in feed intake of 0.6 kg DM/day. The most probable explanation for this would be a reduction in the rates of clearance of digesta from the rumen, mainly by passage.
3.4. Ambient temperature
As ambient temperature fluctuate during the course of the year due to seasonal changes, or as the day progresses from sunrise to sunset; animals respond to these changes in varied ways and to different extents. Temperatures that might lead to severely altered physiological processes would result in changes in rates of passage of fluid and solid through the rumen. These include temperature ranges above and below the thermo-neutral zone for ruminants .
Warm-blooded animals mainly respond to high environmental temperatures by panting, sweating or licking own body, which loses heat from body fluid via evaporation. Increasing plasma volume to increase heat dissipation  possibly via radiation may occur. Under extremely high temperatures, animals become reluctant to eat thus reducing dry matter intake  to cut down on heat production and heat increment due to feeding. In response to rather low environmental temperatures, warm-blooded animals shiver increasing movements of body parts to generate heat energy internally and generally tend to increase dry matter intake .
Low ambient temperatures generally lead to increased rates of fluid and solid passage through the rumen. Kennedy  reported a 21% decrease in mean retention time of solid digesta in the rumen as ambient temperature decreased from 21 to 0°C. This increase in the rate of passage may be attributed to occurrence of shivering and increased movements of various body parts . Contraction and expansion of muscles and organs in close association with the rumen may exert pressure on the rumen wall causing it to contract and decrease in size momentarily. Thus, exertion of some pressure on the rumen and its contents may force out rumen fluid and solid from the rumen at a much faster rate. Extents to which such an occurrence affect rates of passage of fluid through the rumen is virtually undocumented and may require further study. Increased rumen movement has been documented at low ambient temperatures , probably as a result of movement of organs in close proximity to the rumen. Such movements of the rumen are accompanied by increased power of ruminal wall contraction , which may squeeze rumen digesta resulting in it escaping from the rumen at a more rapid rate. Increased dry matter intake on exposure of animals to low ambient temperature  is also assumed to have an effect of increasing passage rate of fluid through the rumen due to a push effect.
In extremely high ambient temperatures, slower rates of fluid passage through the rumen are due to a decrease in the pushing effect on the ruminal fluid as a result of low intake. Contrary to that, Chaiyabutr et al.  observed that higher ambient temperatures resulted in an increase in the rate of fluid passage through the rumen even when a decrease in feed intake occurred.
Rates of passage of fluid from the rumen increased by almost double from an ambient temperature that is within the thermo-neutral zone of cattle to a temperature slightly above the thermo-neutral zone (Table 2). The observed increase in blood and plasma volumes indicated that animals responded to heat stress dissipating heat via evaporation and radiation through the skin thus cooling their bodies using blood water as a medium. Water has a high specific heat capacity with reference to biological systems and hence may be used to dissipate heat  in most animals.
|Rate of flow (l/h)||1.82||3.12|
|Rumen retention time (h)||18.7||13.5|
|k1 (per hour)||0.06||0.086|
|Blood volume (ml/kg)||63.95||68.08|
|Plasma volume (ml/kg)||47.45||50.83|
The rumen acts as a water reservoir . Water that contributed to an increase in plasma levels may have been from two sources, water intake and rumen, or both. Water may either enter blood through flowing across the ruminal wall ; however, proportions that go through this route are minute  or diffusion into the blood stream through the intestines . Assuming that water was rapidly mobilised from the intestines into the blood, the capability of the intestines to provide large amounts of water is unlikely. Since the rumen acts as a fluid reserve, it is likely that water would pass from the rumen into the intestines for absorption into the blood. As water from the intestines is lost into the blood, a high water concentration gradient between the rumen and the intestines is created. Suction power for water from the intestines would become high resulting in an increased flow of fluid through the rumen into the intestines . Most studies have reported contradictory results to those of Chaiyabutr et al.  on the effect of high temperature on fluid passage rates, thus making this appear as a special adaptation strategy of Swamp buffalo (
Warren et al.  observed increased levels of water intake with increasing ambient temperature. A study by Waybright and Varga  showed increased fluid passage rates of up to 64% in water-infused rumen. Tactile stimulation of the distended rumen wall triggers relaxation of the reticulo-omasal orifice and contraction of the rumen wall resulting in rapid flow and passage of fluid and particulate matter through the rumen . Studies by Warren et al.  stated that mean retention time was directly related to or affected by ambient temperature rather than feed intake as influenced by temperature. Desert species are expected to have faster rates of passage compared to species of similar rumen physiology from temperate regions . The study by Warren et al.  used Holstein cattle which are adapted to temperate climates; hence, it is expected that a temperate breed would respond to high ambient temperatures of above 32°C to a great extent. Ruminants that are well adapted to high ambient temperatures in tropical and sub-tropical climates may respond to temperatures of 32°C and above in a different way and probably to a lesser extent when compared to temperate breeds. A change in passage rate as a result of fluctuations in ambient temperature is very high, and the direction of change is unpredictable necessitating more research on the subject. Research needs to focus on the effects of differences in thermal resistance and/or thermal tolerance levels on passage rates in ruminant animals in the tropical regions. These suggest that studies need to consider season and place of study to index ambient temperature when modelling liquid passage rate to take into account the future effects of global warming on digesta passage kinetics. The research needs to consider accounting for the effect of ambient temperature on passage rate.
3.5. Stage of reproductive cycle and physiological state
The reproductive cycle can be subdivided into the lactational and non-lactational period, pregnancy stage, non-pregnancy stage and the number of days in gestation. During the productive cycle, animals undergo structural and functional changes during gestation and lactation . Behavioural changes like loss or gain of appetite and increased or decreased water intake may be observed during these stages . Rate of passage of liquid and solid material through the rumen may be altered by these changes. Gunter et al.  showed that rates of particulate and liquid passage through the rumen were higher for pregnant than non-pregnant animals, higher in lactating animals than their non-lactating counterparts, but lower during the late than the early stages in gestation (Tables 3 and 4). Helander et al.  suggested that different fractional solid and liquid passage rates should be used when formulating diets for pregnant and lactating ruminants.
|Liquid||Outflow rate (l/h)||0.7||0.5||0.4||0.5|
|Liquid||Rate of passage (%/h)||11.1||8.1||10.9||13.9|
|Liquid||Rumen volume (l/kg BW)||0.07||0.08||0.05||0.04|
|Solid||Rate of passage (%/h)||4.6||4.3||4.9||6.8|
|Solid||Gut fill (g/kg BW)||5.7||7.7||6.8||4.8|
|Solid||Mean retention time (h)||26.6||27.9||24.4||18.1|
|Parameter||Phase||d 102 gestation||d 118 gestation||d 132 gestation|
|Non-pregnant||Gut fill (g/kg BW)||Solid||5.6||3.8||5.0|
|Pregnant||Gut fill (g/kg BW)||Solid||6.1||6.6||7.7|
|Outflow rate (l/h)||Liquid||0.6||0.4||0.5|
During pregnancy, nutrient requirements for pregnant animals are higher than for non-pregnant animals . This is due to high demand for protein and energy used for foetal growth  and development. Rumen fluid contains dissolved protein , short-chain fatty acids  and microbial protein. Because of increased demand for the above mentioned nutrients, an increase in rates of passage of fluid through the rumen is observed as a physiological response to meet the increased demand for nutrients in pregnant animals .
During the lactation period, there is high demand for water [101, 106], minerals and soluble protein for the process of milk production [30, 106]. All nutrients for milk synthesis are absorbed across foregut walls and small intestines into the blood stream for transportation to the mammary gland. Rumen fluid serves as a water reservoir in ruminants and contains dissolved minerals and soluble proteins . High demand for water in the lower intestines may result in mobilisation of water stored in the rumen. Hence, ruminal fluid passes out of the rumen at a faster rate to meet animal’s requirements for water and minerals for milk production. When an animal is non-lactating, there is no demand in water for milk production; thus, the rate of passage of liquid through the rumen is much lower than in lactation. This is in accordance to Chaiyabutr et al.’s  theory that increased water demand in the lower gut might result in increased movement of water out of the rumen to meet demand in the lower tract (see Section 3.4). Consequently, the authors [102, 107] observed increases in dry matter intakes of about 20–30% from pregnancy to early lactation, which explains higher rates of liquid and solid passage through the rumen during lactation than during pregnancy [101, 108]. Work on sheep revealed increased rumen fluid volume of 15% during lactation compared to fluid volume at pregnancy  supporting the theory of increased water demand during lactation. Contrary to these findings, Hartnell and Satter  showed 10%/h higher fluid dilution rates for grazing non-lactating than lactating cows fed silage, suggesting the necessity of more data on the subject. Hence, investigations of effects of interaction between lactation and/or non-lactation period and diet type on dilution rate need to be done.
The rumen and pregnant uterus are in close proximity in the abdominal cavity . It is therefore common sense to assume that as a foetus increases in size there is likelihood that it exerts a pressure on the ruminal wall [76, 99]. This pressure may at least squeeze the rumen thus forcing out some liquid and solid particles with a much greater rate than prior to pregnancy. Increased occupation of abdominal cavity space by growing foetus in pregnant ruminants may have an overall effect depressing total rumen volume. Rumen fill would be expected to decrease exponentially in pregnant cows as pregnancy progresses. Dairy cows in early lactation have shown increased incapability of consuming enough feed to meet daily requirements for energy. To a certain extent, diminished rumen volume as a result of squeezing from growing foetus causes a reduction in available space for the rumen to expand in anticipation of increased feed intake. Hence, reduction of rumen fill is a result of pregnancy, due to a decrease in rumen volume. Forbes  reported an approximate decrease of 0.39 l/l in volume of ruminal contents as pregnancy progressed in sheep fed on hay. However, Kaske and Groth  observed increased rumen fill levels from mid pregnancy (60–80 days post conception) to lactation (35–55 days postpartum) with fill levels of 0.946 and 1.444 kg DM, respectively, in ewes. Percentage dry matter content of digesta increased modestly, mean retention times of liquid and small solid digesta reduced by 20–30% at late pregnancy compared to mid pregnancy, with fluid passage rates being approximately three times faster than small solids in sheep . Fluid outflow rate through the rumen increased by 20–36% between late pregnancy and lactation . Generally, rumen fill levels are expected to decrease with an increase in passage rates of solid and liquid digesta. Progressive increments in rumen fill levels in the course from mid pregnancy to lactation were suggested to be due to a gradual reduction in sensitivity of mechanoreceptors on the rumen wall . Such findings may suggest that reticulorumen volumes during various stages of the reproductive cycle may not depend on availability of space in the abdominal cavity alone. They may depend on numerous factors such as diet quality and nervous system response.
Time spent eating and the number of eating sessions were higher during pregnancy than lactation in ewes . Similarly, Kaske and Groth  showed a 19% increase in chewing frequency from mid-pregnancy to lactation in sheep. Duration of eating periods and perhaps increase in chewing times may have some effect on rates of liquid and solid passage through the rumen. Oshita et al.  showed higher fractional rates of liquid passage through the rumen in non-lactating cows grazed on rangeland (13.95%/h) than those fed fodder ad libitum in stalls (9.4%/h). Animals that graze on rangelands spend more time chewing and eating than those confined to pens [25, 53]. Cows have a greater frequency of rumen contractions during eating than during both rumination and rest . Processes of chewing and rumination stimulated rapid movement of material from the rumen into the reticulum  compared to resting. Typical values for frequency of rumen contractions are 1.4/min at rest, 2.3/min during ruminating and 2.8/min during grazing  for cattle. It is therefore assumed that frequency of rumen contractions in goats and sheep is not documented. The greater the number of ruminal contractions the greater the fractional rate of liquid and solid passage through the rumen . Thus, animals that spend more time grazing on rangelands have faster fluid and particulate passage rates through the rumen than stall-fed animals. Okine and Mathison  concluded that the major determinant of digesta flow through the rumen is a result of reticular contractions. Distension of reticulorumen wall would stimulate an increase in rumen contractions. Fractional passage rate of NDF out of the rumen increased by about 34% as a result of increased rumen contractions . One may tend to wonder the true effect of NDF on rumen fill. Earlier discussions pointed out that high NDF content is associated with increased rumen fill levels.
From an angle associated with reticulorumen contractions, fibre or NDF is a major contributor to increased tactile stimulation of the rumen wall. It may be argued that high levels of NDF in the rumen would increase the intensity and frequency of rumen contractions through tactile stimulation of the rumen wall. This would result to increased passage of digesta out of the rumen with an overall effect of reducing rumen fill.
So far, a general trend in results showed higher fluid and solid passage rates in lactating than non-lactating ruminants. However, contrary effects have been reported. Oshita et al.  observed 10%/h higher fractional passage rates for fluids in non-lactating than lactating cows when fed off silage. These results raise a question on effects of diet and lactation and diet and non-lactation interactions on rates of passage. Further research is needed to cover the gap in knowledge on these observations.
3.6. Particle size and functional specific gravity
Particulate matter is discriminated from moving out of the rumen at two major points in the gut, which are at the dorsal rumen and at the reticulo-omasal orifice  because of particle size and functional specific gravity. The likelihood of particles escaping from the rumen is strongly determined by particle size and density . These two factors are inversely related when fermentation has not occurred , but in the course of fermentation, Lirette and Milligan  observed a negative curvilinear relationship between functional specific gravity and particle size. Various work on effects of particle size and FSG on passage rate have reported similar findings. Allen and Mertens  suggested the passage of particulate matter depended on how much particles were present near the reticulo-omasal orifice during the second contraction of the rumen, suggesting that passage rate of solids depended on density. Functional specific gravity of a particle is defined as a physical measure of the weight of a given volume of a particle in the rumen relative to the same volume of fluid in the rumen . The functional specific gravity is determined mainly from the chemical makeup of the lignocellulosic matrix . Lechner-Doll et al.  showed a negative correlation between particle density and mean retention time in the rumen. Before fermentation occurs, a solid particle is intact and tends to be heavy (high functional specific gravity) enough to sink to the bottom of the rumen, close to the ventral part of the rumen where its chances of moving out of the rumen through the reticulorumen orifice is increased. So, at this point, movement is only prevented by particle size. Hence, particles tend to have differential passage rate, where it tends to be higher for unfermented particles.
In the course of fermentation after the lag phase (colonisation of feed particles by bacteria), gas is produced from and stays within feed particles . Gas production within particles increases buoyancy of large particles, and as a result, particles tend to float and become entrapped in the floating fibre mat. Probability that these trapped particles are cleared from the rumen through passage is reduced as they would remain trapped until fermentation is completed. Thus, the rate of passage is slow for particles undergoing fermentation. Overall, high fermentation rate may depress the functional specific gravity through increased buoyancy thus reducing the rates of passage. Smith et al.  showed that grasses containing higher levels of fermentable organic matter than legumes had much higher retention times in the rumen as a result of increased susceptibility of being trapped in the floating fibre mat. This supports a phenomenon whereby slower passage rates are associated with high fermentation rates and proportion of degradable matter. Thus, Rinne et al.  found that clearance of digestible plant cell wall fractions of particulate matter was slower than indigestible matter. Bayat et al.  also showed faster passage rates for indigestible neutral detergent fibre compared to that of potentially degradable neutral detergent fibre of a smaller particle size (Table 5).
|Particle size||Parameter||Grass||Red clover|
|Rumen digesta (kg)|
|Mean retention time (h)|
|Potentially degradable NDF|
Although increased reticulorumen contractions have been shown to increase passage rates of both solid and liquid through the rumen, this may directly apply to fermenting solid material because of variable functional specific gravity. The authors [98, 118] suggested that increasing the intensity of rumen contractions actually decreases the rate of passage of particles with low specific gravity from the rumen because contractions propel particles further away from the exit point, the reticulorumen orifice, before it even opens. Inferences on the eﬀect of reticulorumen contractions on passage rate should be specific on which fraction of solid matter and on the value of specific gravity.
The theory of the ability of particles to sink (sedimentation) and/or float (stratification) in the rumen resulting in passage out and/or entrapment in the rumen may be true for species of ruminants (grazers) where stratification occurs. There is overwhelming evidence that stratification does not occur in the rumen of browsing ruminants . Passage of particles out of the rumen in browsers is by mass flow, determined by abundance of digesta in the rumen and is normally a function of the occurrence of reticulorumen contractions . Reticulorumen contractions are one of the most important factors that lead to passage of digesta out of the rumen. More studies have to be done to clarify why browsers characterised by lower occurrences of rumen contractions may have faster passage rates of digesta compared to grazers. The authors [21, 121] suggested that lack of stratification was responsible and strongly linked to reduced particle retention times in the rumen of browsing ruminants. It is suggested that particulate matter in browsing ruminants flows out of the rumen at a rate that is proportional to fluid flow rate. It thus remains to be determined how and to what extent passage rates of fluid affect passage of small solids and vice-versa. Thus far, the selectivity factor (SF) is the only proposed measure of the relationship between mean retention times (passage rates) of solid and liquid particles in the rumen (SF = MRT particles ÷ MRT liquid). SF quotient values are used to describe ruminant ecological differences and find application in classification of ruminants into different feeding types . Given that rumen retention time is a function of roughage quality, SF may be used to describe physiological differences in the degree of adaptation of ruminants to different roughage qualities. Nsahlai et al.  proposed a relationship that took the form:
Reduction in size of large particles of feed is a prerequisite for particulate flow out of the rumen via the reticulo-omasal orifice and may be an important determinant of rumen fills . Particle size reduction occurs during rumination or rechewing of previously swallowed feed . The authors [123, 124] showed that resistance to particulate flow through the rumen increases with an increase in particle size. The rate of passage of particulate matter is inversely related to particle size . There is, therefore, a critical size that particle should reach for them to pass out of the rumen via the reticulo-omasal orifice . There are suggestions that critical particle size ranges from 1 to 4 mm [21, 123]. Small dense particles tend to fall into the ventral rumen just close to the reticulorumen orifice .
These small particles are capable of passing out of the rumen at the occurrence of the reticular contractions  because they would have reached a size that permits passage. Large particles that have a high density are prevented from passing out of the rumen  because of sedimentation of these particles at the bottom of the rumen . These particles would still be large and hence are unlikely to pass out of the rumen. The theory of critical particle size as a prerequisite for particulate passage out of the rumen may be questionable because larger particles than this are prevalent in faeces. The authors [129, 130] showed that reticulorumen contractions were accompanied by drastic increases in outflow of solid particles termed to be large particles (particles greater than 5 mm). McBride et al.  argued on how the so-called large particles are prevented from leaving the rumen yet the diameter of the reticulorumen orifice opening of 35 mm  is sevenfold greater than the critical particle size. Kaske et al.  revealed that when sedimentation was prevented in the rumen of sheep, outflow of 10-mm-sized particles was 40% of the outflow of 1-mm-sized particle, which shows that a great fraction of large particles do leave the rumen. An argument that can be raised is whether or not particle size is an important factor that leads to increased mean retention times in the rumen or it is the effectiveness of the floating mat in entrapment and sedimentation of large particles that determine passage rates to a greater extent than particle size.
Rates and extents to which solid particle size may be reduced depend on fragility of particles. Now, inclusion of particle fragibility as a factor that influences passage rate and ultimately rumen fill opens a new dimension to the current discussion. As noted earlier, high chewing frequencies have an overall effect of increasing passage rates through stimulation of reticular contractions. Chewing also reduces time for particle size reduction ensuring that particles reach a critical size that allows them to pass through the reticulo-omasal orifice swiftly. It can be hypothesised that highly fragile particles pass out of the rumen much faster than less brittle particles. This may be supported by the fact that brittle particles take a much shorter time to undergo particle size reduction, and thus would have a shorter retention time in the floating mat than less fragile particles. This gives more fragile particles a faster passage rate than less fragile particles. Egan and Doyle  explained a faster passage rate of indigestible fibre components such as lignin using this phenomenon. Taking a closer look at possible causes of particle fragibility, a contrary effect of fragibility on passage rate is developed. Increased fragility of plant fibre is caused by high lignin content. As a result, degradation rate of high lignin containing particles is reduced, hence more time is required by microbes to colonise and ferment digestible components of fibre. This would result in increased retention times of high lignin particles in the rumen for efficient fermentation. Hence, these particles are likely to be retained for a much longer time in the floating raft. This phenomenon may be aggravated when there are large-sized particles with high lignin content, whereby particles would be restricted by size from flowing out through reticulo-omasal orifice, resulting to reduced passage rates.
4. Are goats different in passage rates compared to other ruminants?
4.1. Background discussion
Generally, rates of passage of solid digesta are greatly dependent on the quality of diets ruminants consume. Botanical and nutritional compositional preference of plant feed sources in ruminants varies greatly. Although goats are classified as intermediate feeders , they are selective feeders. Goats demonstrate their botanical wisdom through a mastery of selecting high-quality leafy parts on shrubs, trees and grass stalks that are of higher protein and lower cellulose contents compared to sheep and cattle. This wisdom allows goats to specifically select diets that are able to provide enough net energy and protein to meet their requirements for maintenance for which sheep seem to fail to achieve . This implies that total tract digestibility and degradation rates of diets eaten by goats should be higher than diets eaten by sheep . Degradation rates of diets consumed by goats were higher than diets consumed by sheep , ensuring that goats maintain high intake levels to meet energy requirements (Table 6). Goats will spend more time eating per unit lucerne hay compared to sheep, due to their selective feeding behaviour . This mastery in the art of selection of high quality feeds is well documented in goats [36, 134, 135, 136] and is proposed to be one of the major reasons why goats have faster digesta passage rates than sheep and cattle. The art of botanical feed selection and preference differs between ruminants, with implications in differential passage rates in ruminants. Consequently, browsing ruminants have shorter mean retention times for liquid and solid digesta in the rumen compared to grazers largely because of increased diet quality.
|c (per h)||0.038||0.038||0.089||0.068||0.063||0.053|
|NDF consumed (g NDF/kg DM)||Digestibility of DM||Digestibility of NDF|
Feed residues obtained from troughs used for feeding goats had high crude protein and low NDF content compared to those obtained from sheep [36, 134, 137]. These results may be interpreted in two ways. Firstly, it may be that goats select for low crude protein content and high NDF in feeds compared to sheep. However, sheep select plant feed materials of high cell wall content when compared to goats on pasture . Goat selection for diets with low crude protein seems to be a phenomenon common to trough-fed goats. Secondly, the use of residues left behind after feeding and trough feeding of goats and sheep do not give clear results on diet and/or feed selection in these two ruminant species. Differential feeding behaviours occur in trough-fed goats and sheep. When fed from troughs, goats eat feed from top to bottom, whereas sheep eat from bottom to top. High crude protein and low NDF feed particles are finer than low crude protein and high NDF feed particles and are found at the bottom of feed troughs . This implies that goats fed using feed troughs are more likely to consume low crude protein and high NDF diets. Differences in diet selection between goats and sheep fed through feeding troughs warrant more research.
Botanical variation in diets consumed by goats and other ruminants are wide (Tables 6 and 7) and dependent on seasonal availability of different classes and types of feeds in each climatic region. Although predominantly grazers, cattle consumed diets that contained 84 and 48% woody plants in the late wet season and early dry season . Number of plants selected by goats and sheep (25 plants) grazing in a semiarid thornbush savannah were similar, but lower in cattle (10 plants). While total eating time was evenly shared between monocotyledonous and dicotyledonous plants in sheep across all seasons, cattle tend to select monocotyledonous plants (90% of total eating time) and goats consistently selected dicotyledonous plants (82% of total eating time) . Sheep diets contained lower lignin levels in the wet season compared to goats due to selection against browse by sheep .
|Crude protein consumed (g/kg DM)||Proportions (%)|
|DS||45 ± 5||100 ± 10||125 ± 15||Grass||5.3||14.2|
|INT||70 ± 10||115 ± 15||125 ± 15||Browse||45.3||40.2|
|GS||110 ± 10||175 ± 25||195 ± 15||Forbes||44.1||41.5|
The question on whether goats have faster passage rates than sheep, cattle and other ruminants by virtue that they select less of fibrous plant material is debatable. Generally, goats had faster passage rate than sheep (0.069 vs. 0.033/h when fed as a group; 0.054 vs. 0.029/h when fed individually) when fed formulated diets meeting requirements for maintenance and lactation . Other workers have reported faster passage rates of solid digesta [36, 133, 140, 141, 142], slower passage rates  and similar passage rates  in goats compared to sheep fed on the same diets. Schlecht et al.  observed faster passage rates in goats than cattle fed on the same diet (0.042 vs. 0.033/h when fed on bush hay; 0.053 vs. 0.042/h when fed on green feed).
Discussions on differences in passage rates between ruminant feeding types and species that do not consider effects of factors influencing digesta passage rates highlighted here lack descriptive and explanatory power. Given the large number of factors implicated in differential passage rates among goats and other ruminants, a digesta passage rate modelling exercise was used to test the null hypothesis that passage rates in goats are not different from other ruminants (grazers: cattle, sheep, buffalo, antelopes, mouflons, muskoxen, nilgai, blackbucks; browsers: moose, okapi, deer’s, dik-dik, duikers; intermediate feeders: goats, anoa, reindeer, gazelles and ibex).
Data were collected from studies that reported at least average values or ranges for body weights of animals used, measured fractional passage rates and/or mean retention times in the reticulo-rumen. A dataset was created bearing passage rates from wild and domesticated ruminants. Factors that affect passage rates were identified in each of these studies and included animal and feed factors. Quantification of factors that affected passage rates are described in . Process models developed as part of this study have been deposited into the Repository of Intelligent Models (REDIM) with accession number PRDA001762 and PRCN001814 for the estimation of solid and liquid passage rate, respectively, as indicated at http://www.redim.org.za/?search=PRDA001762 and http://www.redim.org.za/?search=PRCN001814.
After correcting for variation in 17 (liquid passage) and 23 (solid passage) factors that affect passage rates in the model, predicted solid and liquid passage rates for goats lay near the ideal prediction line and generally embedded with other ruminant feeding types (Figures 2 and 3).
This sparse distribution and entanglement of passage rates for goats within that of other ruminants strengthen the view that goats cannot be easily distinguished from other ruminants based on digesta passage rates; so, differences between goats and other ruminants are largely due to variation in diet quality.
Countless factors influence passage rates. Research has not considered effects of various combinations of factors on rates of passage of solid and fluid through the rumen. Mathematical models that seek to accurately predict passage rates, rumen fill levels and ultimately roughage intake should increase understanding of why part of the variation is not explained.
Animal and feed compositional attributes are the major factors to be included into passage rate prediction models. The role of animal physiology in influencing digesta passage rate is critical. Accounting for the influence of various physiological changes in ruminants, feeding level, stage of pregnancy and lactation, and growth in passage rate models can be done by computation of the feeding level based on total net energy requirements relative to net energy requirement for maintenance (animal production level, APL). It is evident that there are still discrepancies on how ambient temperature and particle density (buoyancy) affect the passage rate of digesta in the rumen. Indexing for buoyancy in solid passage rate, prediction models would likely involve determination of the extent of degradability of a particle taking into account the time available for digestion.
After correcting for variation in factors that affect solid and liquid passage rates, goats are not different from other ruminants with respect to passage rates suggesting differences between goats and other ruminants are largely due to dietary quality. More studies should be carried to ascertain the dynamics of digesta kinetics after meal termination in goats.
This work is part of a project funded by the National Research Foundation (NRF) of the Republic of South Africa (Project name: Intake of roughage by ruminant herbivores, GUN number: 87738).
Nsahlai IV, Apaloo J. On the suitability of the Illius and Gordon's model for simulating the intake and digestibility of roughage diets by ruminants. South African Journal of Animal Science. 2007; 37(4):275-289
Illius AW, Gordon IJ. Prediction of intake and digestion in ruminants by a model of rumen kinetics integrating animal size and plant characteristics. Journal of Agricultural Science. 1991; 116:145-157
Clauss M, Deutsch A, Lechner-Doll M, Flach EJ, Tach C. Passage rate of fluid and particle phase in captive giraffe ( Giraffa camelopardalis). Advances in Ethology. 1998;(33):98
Clauss M, Lechner-Doll M. Differences in selective reticulo-ruminal particle retention as a key factor in ruminant diversification. Oecologia. 2001; 129(3):321-327
Behrend A, Lechner-Doll M, Streich WJ, Clauss M. Seasonal faecal excretion, gut fill, liquid and particle marker retention in mouflon ( Ovis ammon musimon), and a comparison with roe deer ( Capreolus capreolus). Acta Theriologica. 2004; 49(4):503-515
Allen MS. Physical constraints on voluntary intake of forages by ruminants. Journal of Animal Science. 1996; 74(12):3063-3075
Forbes JM. Physical limitation of intake in ruminants and its interaction with other factors affecting feed intake. In: Engelhardt WV, Leonhard-Marek S, Breves G, editors. Ruminant Physiology: Digesta, Metabolism, Growth and Reproduction. Stuttgart, Germany: Ferdinand Enke Verlag; 1995. pp. 217-232
Fuller MF, Benavenga NJ, Lall SP, McCracken KJ, Omed HM, Axford RFE, Phillips CJC. The Encyclopaedia of Farm Animal Nutrition. Wallingford, United Kingdom: CAB International; 2004
Adebayo RA. Effect of Roughage Quality and Period of Meal Termination on Rumen Fill [thesis]. Pietermaritzburg, South Africa: University of KwaZulu-Natal; 2016
Balch CC, Campling RC. Regulation of voluntary intake in ruminants. Nutritional Abstracts and Reviews. 1962; 32:669-686
Boudon A, Peyraud JL, Faverdin P, Delagarde R, Delaby L, Chaves AV. Effect of rumen fill on intake of fresh perennial ryegrass in young and mature dairy cows grazing or zero-grazing fresh perennial ryegrass. Animal. 2009; 3(12):1706-1720
Taweel HZ, Tas BM, Dijkstra J, Tamminga S. Intake regulation and grazing behaviour of dairy cows under continuous stocking. Journal of Dairy Science. 2004; 87:3417-3427
Williams YJ, Doyle PT, Egan AR. Diurnal variation in rumen fill of dairy cows grazing Persian clover at different pasture allowances. Animal Production Science. 2014; 59(9):1388-1393
Baumont R, Brun JP, Dulphy JP. Influence of the nature of hay on its ingestibility and the kinetics of intake during large meals in sheep and cows. In: Jarrige, R, editors. XVI International Grassland Congress; Nice, France. France: French Grassland Society; 1989. pp. 787-788
Thomson BC, Cruickschank GJ, Poppi DP, Sykes AR. Diurnal patterns of rumen fill in grazing sheep. In: New Zealand Society of Animal Production; New Zealand. New Zealand: New Zealand Society of Animal Production; 1985. pp. 117-120
Chilibroste P, Tamminga S, Van Bruchem J, Van der Togt PL. Effect of allowed grazing time, inert rumen bulk and length of starvation before grazing on the weight, composition and fermentative end-products of the rumen contents of lactating dairy cows. Grass and Forage Science. 1998; 53:146-156
Greenhalgh JFD, Reid GW. Relative palatability to sheep of straw, hay and dry grass. British Journal of Nutrition. 1971; 26:107-116
Hofmann RR. Evolutionary steps of ecophysiological adaptation and diversification of ruminants: A comparative view of their digestive system. Oecologia. 1989; 78(4):443-457
Lechner I, Barboza P, Collins W, Gunther D, Hattendorf B, Hummel J, Clauss M. No 'bypass' in adult ruminants: Passage of fluid ingesta vs. fluid inserted into the rumen in fistulated muskoxen, reindeer and moose. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2009; 154(1):151-156
Bartocci S, Amici A, Verna M, Terramoccia S, Martillotti F. Solid and fluid passage rate in buffalo, cattle and sheep fed with diets with different forage to concentrate ratios. Livestock Production Science. 1997; 52(3):201-208
Lechner-Doll M, Kaske M, Engelhardt WV. Factors affecting the mean retention time of particles in the forestomach of ruminants and camelids. In: Tsuda T, Sasaki Y, Kwashima R, editors. Physiological Aspects of Digestion and Metabolism in Ruminants. San Diego, USA: Academic; 1991. pp. 455-488
Parra R. Comparison of foregut and hindgut fermentation in herbivores. In: Montgomery GG, editor. The Ecology of Arboreal Folivores. Washington DC, USA: Smithsonian Institute Press; 1978. pp. 209-229
Poppi DP, Minson DJ, Ternouth JH. Studies of cattle and sheep eating leaf and stem fractions of grass. II. Factors controlling retention of feed in the reticulo-rumen. Australian Journal of Agricultural Research. 1981; 32:109-121
Ulyatt MJ, Dellow DW, John A, Reid CSW, Waghorn GC. Contribution of chewing during eating and rumination to the clearance of digesta from the reticulo-rumen. In: Control of Digestion and Metabolism in the Ruminant. Englewood Cliffs, New Jersey, USA: Prentice-Hall; 1986. pp. 498-515
Oshita T, Sudo K, Nonaka K, Kume S, Ochiai K. The effect of feed regimen on chewing time, digesta passage rate and particle size distribution in Holstein non-lactating cows fed pasture ad libitum. Livestock Science. 2008; 113:243-250
Du Toit PCV. A comparison of the diets selected by merino and dorper sheep on three range types of the Karoo, South Africa. Archivos de Zootecnia. 1998; 47:21-32
Molina-Alcaide E, Martin-Garcia AI, Aguilera JF. A comparative study of nutrient digestibility, kinetics of degradation and passage and rumen fermentation pattern in goats and sheep offered good quality diets. Livestock Production Science. 2000; 64:215-223
Clauss M, Hummel J, Streich WJ. The dissociation of the fluid and particle phase in the forestomach as a physiological characteristic of large grazing ruminants: An evaluation of available, comparable ruminant passage data. European Journal of Wildlife Research. 2006; 52(2):88-98
Lechner I, Barboza P, Collins W, Fritz J, Gunther D, Hattendorf B, Clauss M. Differential passage of fluids and different-sized particles in fistulated oxen, muskoxen, reindeer and moose: Rumen particle size discrimination is independent from contents stratification. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2010; 155(2):211-222
Kennedy PM, Murphy MR. The nutritional implications of differential passage of particles through the ruminant alimentary tract. Nutrition Research Reviews. 1988; 1(1):189-208
Clauss M, Hofmann RR, Hummel J, Adamczewski J, Nygren K, Pitra C, et al. The microscopic anatomy of the omasum of free-ranging moose ( Alces alces) and muskoxen ( Ovibos moschatus) and a comparison of the omasal animal surface area in 34 ruminant species. Journal of Zoology. 2006; 270:346-358
Rowell A, Dyer J, Hofmann RR, Lechner-Doll M, Meyer HHD, Shirazi-Beechey SP, et al. The expression of intestinal sodium-glucose cotransporter in cervids. Zeitschrift für Säugetierkunde. 1996; 62(2):204-208
Rowell A, Dyer J, Hofmann RR, Lechner-Doll M, Meyer HHD, Shirazi-Beechey SP, et al. Abundance of intestinal sodium-glucose cotransporter (SGLT1) in roe deer ( Capreolus capreolus). Journal of Animal Physiology and Animal Nutrition. 1999; 82:25-32
Meyer HHD, Rowell A, Streich WJ, Stoffel B, Hofmann RR. Accumulation of polyunsaturated fatty acids by concentrate selecting ruminants. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 1998; 120(2):263-268
Van Wieren SE. Do large herbivores select a diet that maximises short-term energy intake rate? Forest Ecology and Management. 1996; 88(1-2):149-156
Hadjigeorgiou IE, Gordon IJ, Milne JA. Intake, digestion and selection of roughage with different staple lengths by sheep and goats. Small Ruminant Research. 2003; 47(2):117-132
Nygren KFA, Lechner-Doll M, Hofmann RR. Influence of papillae on postruminal regulation of ingesta passage in moose ( Alces alces). Journal of Zoology. 2001; 254(3):375-380
Clauss M, Hummel J, Vollm. The attribution of a feeding type to a ruminant species based on morphological parameters: The example of okapi ( Okapi johnstoni). In: Symposia of the Comparative Nutrition Society, editor. Comparative Nutrition Society; 21-25 August; Antwerp, Belgium. Silver Spring, USA: Comparative Nutrition Society; 2002. p. 123
Hofmann RR, Streich WJ, Fickel J, Hummel J, Clauss M. Convergent evolution in feeding types: Salivary gland mass differences in wild ruminant species. Journal of Morphology. 2008; 269(2):240-257
Silanikove N, Gilboa N, Nitsan Z. Effect of polyethylene glycol on rumen volume and retention time of liquid and particulate matter along the digestive tract in goats fed tannin-rich carob leaves. Small Ruminant Research. 2001; 40(1):95-99
Gross JE, Alkon PU, Demment MW. Nutritional ecology of dimorphic herbivores: Digestion and passage rates in Nubian Ibex. Oecologia. 1996; 107(2):170-178
Clauss M, Lang-Deuerling S, Muller DW, Kienzle E, Steuer P, Hummel J. Retention of fluid and particles in captive tapirs. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2010; 157(1):95-101
Wenninger PS, Shipley LA. Harvesting, rumination, digestion, and passage of fruit and leaf diets by a small ruminant, the blue duiker. Oecologia. 2000; 123:466-474
Silanikove N, Tagari H, Shkolnik A. Comparison of rate of passage, fermentation rate and efficiency of digestion of high fibre diet in desert Bedouin goats compared to Swiss Saanen goats. Small Ruminant Research. 1993; 12(1):45-60
Weyreter H, Engelhardt WV. Adaptation of Heidschnucken, Merino and Blackhead sheep to a fibrous roughage of poor quality. Canadian Journal of Animal Science. 1984; 64(5):152-153
Clauss M, Fritz J, Bayer D, Nygren K, Hammer S, Hatt JM, et al. Physical characteristics of rumen contents in four large ruminants of different feeding type, the addax, bison, red deer and moose. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2009; 152(3):398-406
Barboza PS, Peltier TC, Foster RJ. Ruminal fermentation and fill change with season in an arctic grazer: Responses to hyperphagia and hypophagia in muskoxen (Ovibos moschatus). Physiological and Biochemical Zoology. 2006; 79(3):497-513
Pearson RA, Archibald RF, Muirhead RH. A comparison of the effect of forage type and level of feeding on the digestibility and gastrointestinal mean retention time of dry forages given to cattle, sheep, ponies and donkeys. British Journal of Nutrition. 2006; 95(1):88-98
Haaland G, Tyrrell H. Effects of limestone and sodium bicarbonate buffers on rumen measurements and rate of passage in cattle. Journal of Animal Science. 1982; 55(4):935-942
Varga GA, Prigge E. Influence of forage species and level of intake on ruminal turnover rates. Journal of Animal Science. 1982; 55(6):1498-1504
Lindberg JE. Retention times of small feed particles and of water in the gut of dairy goats fed at different levels of intake. Journal of Animal Physiology and Animal Nutrition. 1988; 59(1-5):173-181
Kovacs P, Sudekum KH, Stangassinger M. Effects of intake of a mixed diet and time post-feeding on amount and fibre composition of ruminal and faecal particles and on digesta passage from the reticulorumen of steers. Animal Feed Science and Technology. 1998; 71(3):325-340
Seo S, Tedeschi LO, Schwab CG, Garthwaite BD, Fox DG. Evaluation of the passage rate equations in the dairy NRC (2001) model. Journal of Dairy Science. 2006; 89(6):2327-2342
Hummel J, Steuer P, Sudekum KH, Hammer S, Hammer C, Streich WJ, et al. Fluid and particle retention in the digestive tract of the addax antelope-adaptations of a grazing desert ruminant. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2008; 149:142-149
Adams D, Cochran R, Currie P. Forage maturity effects on rumen fermentation, fluid flow, and intake in grazing steers. Journal of Range Management. 1987;404-408
Clauss M, Schwarm A, Ortmann S, Streich WJ, Hummel J. A case of non-scaling in mammalian physiology? Body size, digestive capacity, food intake, and ingesta passage in mammalian herbivores. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2007; 148(2):249-265
Varga GA, Harpster HW. Gut size and rate of passage. In: Intake by Feedlot Cattle; Oklahoma, USA. Stillwater, USA: Oklahoma State University; 1995. pp. 85-96
Purser DB, Moir RJ. Rumen volume as a factor involved in individual sheep differences. Journal of Animal Science. 1966; 25(2):509-515
Tulloh NH, Hughes JW. Physical studies of the alimentary tract of grazing cattle. II. Techniques of estimating the capacity of the reticulo-rumen. New Zealand Journal of Agricultural Research. 1965; 8(4):1070-1078
Salem HB, Smith T. Feeding strategies to increase small ruminant production in dry environments. Small Ruminant Research. 2008; 77(2):174-194
Evans EW. An evaluation of the relationships between dietary parameters and rumen liquid turnover rate. Canadian Journal of Animal Science. 1981; 61:91-96
Okeke G, Buchanan-Smith J, Grovum W. Effect of buffers on ruminal rate of passage and degradation of soybean meal in steers. Journal of Animal Science. 1983; 56(6):1393-1399
Merchen N, Firkins J, Berger L. Effect of intake and forage level on ruminal turnover rates, bacterial protein synthesis and duodenal amino acid flows in sheep. Journal of Animal Science. 1986; 62(1):216-225
Owens FN, Goetsch AL. Digesta passage and microbial protein synthesis. In: Milligan LP, Grovum WL, Dobson A, editors. 6th International Symposium on Ruminant Physiology; 10-14 September 1984; Banff, Canada. New Jersey, USA: Prentice-Hall; 1986. pp. 196-223
Poore M, Moore J, Swingle R. Differential passage rates and digestion of neutral detergent fibre from grain and forages in 30, 60 and 90% concentrate diets fed to steers. Journal of Animal Science. 1990; 68(9):2965-2973
Froetschel MA. Effect of abomasal infusion of saliva on reticular motility and ruminal liquid contents of steers. Journal of dairy Science. 1995; 78(11):2396-2401
Okine EK, Mathison GW. Reticular contraction attributes and passage of digesta from the ruminoreticulum in cattle fed roughage diets. Journal of Animal Science. 1991; 69(5):2177-2186
Lui JX, Orskov ER, Chen XB. Optimisation of steam treatment as a method for upgrading rice straw as feeds. Animal Feed Science and Technology. 1999; 76(3):345-357
Tschuor A, Clauss M. Investigations on the stratification of forestomach contents in ruminants: An ultrasonographic approach. European Journal of Wildlife Research. 2008; 54(4):627-633
Moore J, Poore M, Swingle R. Influence of roughage source on kinetics of digestion and passage and on calculated extents of ruminal digestion in beef steers fed 65% concentrate diets. Journal of Animal Science. 1990; 68(10):3412-3420
Faichney GJ. The kinetics of particulate matter in the rumen. In: Milligan LP, Grovum WL, Dobson A, editors. 6th International Symposium on Ruminant Physiology; 10-14 September 1984; Banff, Canada. New Jersey, USA: Prentice-Hall; 1986. p. 173
Owens FN, Goetsch AL. Ruminal fermentation. In: Church DC, editor. The Ruminant Animal: Digestive Physiology and Nutrition. New Jersey, USA: Prentice-Hall; 1988. p. 145
Estell RE, Galyean M. Relationship of rumen fluid dilution rate to rumen fermentation and dietary characteristics of beef steers. Journal of Animal Science. 1985; 60(4):1061-1071
McCollum F, Galyean M. Influence of cottonseed meal supplementation on voluntary intake, rumen fermentation and rate of passage of prairie hay in beef steers. Journal of Agricultural Science. 1985; 60(2):570-577
Rencoret J, Gutiérrez A, Nieto L, Jimenez-Barbero J, Faulds CB, Kim H, et al. Lignin composition and structure in young versus adult Eucalyptus globulusplants. Plant Physiology. 2011; 155(2):667-682
Van Weyenburg S, Sales J, Janssens G. Passage rate of digesta through the equine gastrointestinal tract: A review. Livestock Science. 2006; 99(1):3-12
Froetschel ME, Amos HE. Effects of dietary fibre and feeding frequency on ruminal fermentation, digesta water-holding capacity, and fractional turnover of contents. Journal of Animal Science. 1991; 69(3):1312-1321
Rinne M, Huhtanen P, Jaakkola S. Digestive processes of dairy cows fed silage harvested at four stages of grass maturity. Journal of Animal Science. 2002; 80(7):1986-1998
Kennedy PM. Particle dynamics. In: Dijkstra J, Forbes JM, France J, editors. Quantitative Aspects of Ruminant Digestion and Metabolism. Wallingford, UK: CAB International; 2005. pp. 49-86
Egan JK, Doyle PT. Effect of intraruminal infusion of urea on the response in voluntary food intake by sheep. Australian Journal of Agricultural Research. 1985; 36(3):483-495
Baumont R, Prache S, Meuret M, Morand-Fehr P. How forage characteristics influence behaviour and intake in small ruminants: A review. Livestock Production Science. 2000; 64(1):15-28
Hummel J, Sudekum KH, Streich WJ, Clauss M. Forage fermentation patterns and their implications for herbivore ingesta retention times. Functional Ecology. 2006; 20(6):989-1002
Panjaitan T, Quigley SP, McLennan SR, Swain T, Poppi DP. Intake, retention time in the rumen and microbial protein production of Bos indicussteers consuming grasses of varying crude protein content. Animal Production Science. 2010; 50(6):444-448
Mertens D. Rate and extent of digestion. In: Dijkstra L, Forbes JM, France J, editors. Quantitative Aspects of Ruminant Digestion and Metabolism. Wallingford, UK: CAB International; 2005. pp. 13-47
Baumont R, Malbert CH, Ruckebush Y. Mechanical stimulation of rumen fill and alimentary behaviour in sheep. Animal Production. 1990; 50(1):123-128
Jung HG, Allen MS. Characteristics of plant cell walls affecting intake and digestibility of forages by ruminants. Journal of Animal Science. 1995; 73(9):2774-2790
Johnson TR, Combs DK. Effects of prepartum diet, inert rumen bulk, and dietary polyethylene glycol on dry matter intake of lactating dairy cows. Journal of Dairy Science. 1991; 74(3):933-944
Dove H. Constraints to modelling of diet selection and intake in the grazing ruminant. In: Hodgson J, Illius AW, editors. The Ecology and Management of Grazing Systems. Slough, UK: CAB International; 1996. pp. 257-275
Faverdin P, Baumont R, Ingvartsen KL. Control and prediction of intake in ruminants. In: Journet M, Grenet E, Farce MH, Thériez M, Demarquilly C, editors. Recent developments in the nutrition of herbivores. IVth International Symposium on the Nutrition of Herbivores; 11-15 September 1995; Paris, France. France: INRA Editions; 1995. pp. 95-120
Chaiyabutr N, Buranakarl C, Muangcharoen V, Loypetjra P, Pichaicharnarong A. Effects of acute heat stress on changes in the rate of liquid flow from the rumen and turnover of body water of swamp buffalo. Journal of Agricultural Science. 1987; 108(3):549-553
Kennedy PM. Influences of cold exposure on digestion of organic matter, rates of passage of digesta in the gastrointestinal tract, and feeding and rumination behaviour in sheep given four roughage diets. British Journal of Nutrition. 1985; 53(1):159-173
Bernard L, Montgomery MJ. Managing Intake of Lactating Dairy Cows. Tennessee, USA: The University of Tennessee Institute of Agriculture; 1997
Toole G, Toole S. New Understanding Biology for Advanced Level. 4th ed. UK: Nelson Thornes; 2006. p. 698
Parthasarathy D, Phillipson AT. The movement of potassium, sodium, chloride and water across the rumen epithelium of sheep. The Journal of Physiology. 1953; 121(3):452-469
Kamal TH, Shabaita MK. Climate effect on Friesians and buffaloes. 1. Blood volume using sodium dichromate. Journal of Dairy Science. 1968; 51(6):970
Warren W, Martz F, Asay K, Hilderbrand E, Payne C, Vogt J. Digestibility and rate of passage by steers fed tall fescue, alfalfa and orchardgrass hay in 18 and 32 degrees Celsius ambient temperatures. Journal of Animal Science. 1974; 39(1):93-96
Waybright TR, Varga GA. Effects of water filled bags in the rumen of wethers on ruminal digesta kinetics and total tract nutrient digestibility. Journal of Animal Science. 1991; 69(5):2157-2167
Reid A, Titchen D. Effects of vasoactive intestinal polypeptide on the reticulo-omasal orifice in lambs. Canadian Journal of Animal Science. 1984; 64(5):91-92
Coffey K, Paterson J, Saul C, Coffey L, Turner K, Bowman J. The influence of pregnancy and source of supplemental protein on intake, digestive kinetics and amino acid absorption by ewes. Journal of Animal Science. 1989; 67(7):1805-1814
Lunn D. Nutrient Requirements of the Beef Cow. Nutrifax Nutrition News and Information Update. Canada: Shur Grain, Nutreco; 2004
Gunter S, Judkins M, Krysl L, Broesder J, Barton R, Rueda B et al. Digesta kinetics, ruminal fermentation characteristics and serum metabolites of pregnant and lactating ewes fed chopped alfalfa hay. Journal of Animal Science. 1990; 68(11):3821-3831
Helander C, Nørgaard P, Jalali AR, Nadeau E. Effects of chopping grass silage and mixing silage with concentrate on feed intake, diet selection, chewing activity and faecal particle size of ewes in late pregnancy and early lactation. Livestock Science. 2014; 163:69-79
Hutjens MF. Dairy efficiency and dry matter intake. In: 7th Western Dairy Management Conference; 9-11 March 2005; Reno, Nevada, USA. USA: University of Illinois; 2005. pp. 71-76
Fox DG, Tedeschi LO, Tylutki TP, Russell JB, Van Amburgh ME, Chase LE, et al. The Cornell net carbohydrate and protein system model for evaluating herd nutrition and nutrient excretion. Animal Feed Science and Technology. 2004; 112(1):29-78
Lopez S, Hovell FDD, Dijkstra J, France J. Effects of volatile fatty acid supply on their absorption and on water kinetics in the rumen of sheep sustained by intragastric infusion. Journal of Animal Science. 2003; 81(10):2609-2616
Marston TT, Blasi DA, Brazle FK, Kuhl GL. Beef cow nutrition guide. USA: Kansas State University. Cooperative Extension Service; 1998
Faichney GJ, Brown GH. Effect of physical form of lucerne hay on rumination and the passage of particles from the rumen of sheep. Australian Journal of Agricultural Research. 2004; 55(12):1263-1270
Larsen M, Lund P, Weisbjerg MR, Hvelplund T. Digestion site of starch from cereals and legumes in lactating dairy cows. Animal Feed Science and Technology. 2009; 153(3):236-248
Kaske M, Groth A. Changes in factors affecting the rate of digesta passage during pregnancy and lactation in sheep fed on hay. Reproductive Nutrition Development. 1997; 37(5):573-588
Hartnell GF, Satter LD. Determination of rumen fill, retention time and ruminal turnover rates of digesta at different stages of lactation in dairy cows. Journal of Animal Science. 1979; 48(2):381-392
Forbes JM. Voluntary food intake in pregnant ewes. Journal of Animal Science. 1970; 31(3):1222-1227
Baile CA, Forbes JM. Control of feed intake and regulation of energy balance in ruminants. Physiological Reviews. 1974; 54(1):160-214
Frandson RD. Anatomy and Physiology of Farm Animals. 1st ed. Philadelphia, USA: Lea and Febiger; 1981
Dado RG, Allen MS. Enhanced intake and production of cows offered ensiled alfalfa with higher neutral detergent fibre digestibility. Journal of Dairy Science. 1996; 79(3):418-428
Evans EW, Pearce GR, Burnett J, Pillinger SL. Changes in some physical characteristics of the digesta in the reticulo-rumen of cows fed once daily. British Journal of Nutrition. 1973; 29(3):357-376
Lirette A, Milligan LP. A quantitative model of reticulo-rumen particle degradation and passage. British Journal of Nutrition. 1989; 62(2):465-479
Allen MS, Mertens DR. Evaluating constraints on fibre digestion by ruminants. Journal of Nutrition. 1988; 118:261-270
Sutherland TM. Particle separation in the forestomach of sheep. In: Dobson A, Dobson MJ, editors. Aspects of Digestive Physiology in Ruminants. Ithaca, New Jersey, USA: Cornell University Press; 1988. pp. 43-73
Smith LW, Goering HK, Gordon CH. Relationships of forage composition with rates of cell wall digestion and indigestibility of cell walls. Journal of Dairy Science. 1972; 55(8):1140-1147
Bayat AR, Rinne M, Kuoppala K, Ahvenjärvi S, Huhtanen P. Ruminal large and small particle kinetics in dairy cows fed red clover and grass silages harvested at two stages of growth. Animal Feed Science and Technology. 2010; 155(2):86-98
Jiang Z, Hudson RJ. Digestive responses of wapiti Cervus elaphus canadensis to seasonal forages. Acta Theriologica. 1996; 41(4):415-423
Nsahlai IV, Bryant MJ, Umunna NN. Utilisation of barley straw by steers: Effects of replacing urea with protein, source of protein and quantity of rumen degradable nitrogen on straw degradation, liquid and particulate passage rates and intake. Journal of Applied Animal Research. 1999; 16(2):129-146
Poppi DP, Norton BW, Minson DJ, Hendricksen RE. The validity of the critical size theory for particles leaving the rumen. The Journal of Agricultural Science. 1980; 94(2):275-280
Dixon RM, Milligan LP. Removal of digesta components from the rumen of steers determined by sieving techniques, and fluid, particulate and microbial markers. British Journal of Nutrition. 1985; 53(2):347-362
Wyburn RS. The mixing and propulsion of the stomach contents of ruminants. In: Ruckebusch Y, Thivend P, editors. Digestive Physiology and Metabolism in Ruminants. Westport, Connecticut, USA: AVI Publishing Company; 1980. pp. 35-51
Midasch A, Kaske M, Rehage J. Sonographic investigation of reticular contractions in sheep, cows and goats. In: Giesecke D, editor. Society of Nutrition Physiology; Frankfurt, Germany. Germany: DLG-Verlag; 1994
Poppi DP, Hendricksen RE, Minson DJ. The relative resistance to escape of leaf and stem particles from the rumen of cattle and sheep. The Journal of Agricultural Science. 1985; 105(1):9-14
Kaske M, Hatiboglu S, Engelhardt W. The influence of density and size of particles on rumination and passage from the reticulo-rumen of sheep. British Journal of Nutrition. 1992; 67(2):235-244
Welch JG. Physical parameters of fibre affecting passage from the rumen. Journal of Dairy Science. 1986; 69(10):2750-2754
Kaske M, Midasch A. Effects of experimentally impaired reticular contractions on digesta passage in sheep. British Journal of Nutrition. 1997; 78(1):97-110
McBride BW, Milligan LP, Turner BV. Short note: Endoscopic observation of reticulo-omasal orifice of cattle. Journal of Agricultural Science. 1983; 101(3):749-751
Bueno L. The Mechanical and Digestive Function of the Omasum [thesis]. France: University of Toulouse; 1975
Garcia MA, Aguilera JF, Alcaide EM. Voluntary intake and kinetics of degradation and of passage of unsupplemented and supplemented pastures from semi-arid lands in grazing goats and sheep. Livestock Production Science. 1995; 44(3):245-255
Domingue BF, Dellow DW, Barry TN. The chewing efficiency during eating and ruminating in goats and sheep. British Journal of Nutrition. 1991; 65(3):355-363
Pfister JA, Malechek JC. Dietary selection by goats and sheep in a deciduous woodland of North-Eastern Brazil. Journal of Range Management. 1986; 39:24-28
Rutagwenda T, Lechner-Doll M, Schwartz HJ, Schultka W, Von Engelhardt. Dietary preference and degradability of forage on a semi-arid thornbush savanna by indigenous ruminants, camels and donkeys. Animal Feed Science and Technology. 1990; 31(3-4):179-192
Morand-Fehr P, Owen G, Giger-Reverdin S. Feeding behaviour of goats at the trough. In: Morand-Fehr P, editors. Goat Nutrition. Wageningen, Netherlands: Centre for Agricultural Publishing and Documentation, Pudoc Press; 1991. pp. 3-12
Pfister JA, Malechek JC. The voluntary forage intake and nutrition of goats and sheep in the semi-arid tropics of North-Eastern Brazil. Journal of Animal Science. 1986; 63(4):1078-1086
Moleele NM. Encroacher woody plant browse as feed for cattle. Cattle diet composition for three seasons at Olifants Drift, South-Eastern Botswana. Journal of Arid Environments. 1998; 40(3):255-268
Tsiplakou E, Hadjigeorgiou I, Sotirakoglou K, Zervas G. Differences in mean retention time of sheep and goats under controlled feeding practices. Small Ruminant Research. 2011; 95(1):48-53
Katoh K, Sato F, Yamazaki A, Sasaki Y, Tsuda T. Passage of indigestible particles of various specific gravities in sheep and goats. British Journal of Nutrition. 1988; 60(3):683-687
Schlecht E, Richter H, Fernandez-Rivera S, Becker K. Gastrointestinal passage of Sahelian roughages in cattle, sheep and goats, and implications for livestock-mediated nutrient transfers. Animal Feed Science and Technology. 2007; 137(1):93-114
Domingue BF, Dellow DW, Barry TN. Voluntary intake and rumen digestion of a low quality roughage by goats and sheep. The Journal of Agricultural Science. 1991; 117(1):111-120
Moyo M, Gueguim Kana EB, Nsahlai IV. Modelling of digesta passage rates in grazing and browsing domestic and wild ruminant herbivores. South African Journal of Animal Science. 2017; 47(3):362-377