Effects of the use of absorbents on fermentation characteristics, aerobic stability and in vitro/ animal performance fed silage from high moisture plant by-products.
Use of microbial inoculants during silage making have drawn interest to silage producers including those who are feeding their livestock on silage produced from by-products (e.g. pulps). Many farmers in the developing countries rely on agro-industrial by-products to feed their livestock, which is limited by the high moisture content of the by-products. This review pertains to issues related to silage production from high moisture plant by-products (e.g. pulps or pomaces), challenges involved in the ensiling of these resources, the use of additives (e.g. microbial additives), and growth performance of the animals that are fed silage from these resources. This information will be helpful to better understand the key roles of silage production from these resources.
The increasing demand for sustainable animal production is driving animal nutritionists to explore strategies for using high moisture by-products in animal feeding. Several researchers [1, 2] have reviewed the use of agro-industrial by-products as animal feed resources. These by-products are available during food processing and or beverage production, and are often produced in abundance, making it difficult to use them in a short period. Using these by-products in animal feeds will assist the food producing factories to reduce disposal costs while minimising the environmental impacts that these by-products would otherwise create .
The high moisture (>25%) content of these by-products makes it difficult to transport and handling during processing and storing . While disposing these by-products might seem as a solution, such an act is associated with potential environmental pollution . Subsequently, the high moisture coupled with high sugar content of these by-products allows for easy contamination by foreign materials and unwanted microbes, which leads to spoilage . Despite the negative factors that are linked with these by-products, they contain valuable nutritional properties such as crude protein, organic matter, fibre and oil . These resources should be processed and stored for animal feeding.
The drying of high moisture by-products to produce meals for animal feeding is technically feasible, but is costly and laborious . Research has shown that ensiling can be an alternative for processing and storing of these resources, provided all basic principles of ensiling are followed [7, 8]. Ensiling entails the preservation of plant/crop resources through anaerobic fermentation, usually by epiphytic bacteria that converts soluble carbohydrates to mainly lactic acid, and minor amounts of volatile fatty acids. The production of organic acids during ensiling reduces the pH to 3.8 to 4.2 for a good quality silage, which inhibits growth of undesirable microbes and results in an ideal preservation on the ensiled material . While ensiling represent an appropriate preservation method for forages, crops and high moisture by-products, it can also result in the losses of nutrients due to undesirable fermentation process in cases where lactic acid is not adequate . To overcome the nutritive losses of the ensiled material, different additives are used.
Additives are constituents that contribute to the reduction of losses, stimulate fermentation, and enrich nutritional value of silage . Such additives include chemicals, enzymes, absorbents and microbial inoculants . Chemical additives such as propionic acid, formic acid, sulphuric acid have been applied to high moisture (> 70% moisture) forages during ensiling for some decades. However, their use in silage is limited due to their toxic nature if not properly applied . Enzymes such as xylanase, cellulase etc. are usually added to forage at ensiling to partially degrade fibre to fermentable water-soluble carbohydrates (WSC) that are consumed by lactic acid bacteria (LAB). The LAB can use fibre as source of energy to produce lactic acid . The use of microbial inoculants ensures rapid and efficient fermentation of WSC to lactic acid and further predicts the adequacy of silage fermentation . Lactic acid bacterial inoculants have been introduced some decades as one of microbial additives that improve forage fermentation, aerobic stability of silage and silage utilization by ruminants . The most commonly LAB inoculants are obligate homofermentative, obligate heterofermentative and facultative heterofermentative LAB .
However, it should be noted that the addition of LAB inoculants to forage of low WSC (< 30 g/kg) content, could result to poor fermentation of the forage . Haigh and Parker  concluded that WSC content as low as 30 g/kg may be sufficient for a stable fermentation where an effective additive is added during ensiling. In many instances, a source of readily fermentable substrate for LAB is included with commercial bacterial inoculants.
Given that LAB inoculants have been used as silage additives for a long time, their utilization is however more prominent on the ensiling of forages/crops. However, research on the use of LAB inoculants during the ensiling of high moisture by-products is limited. The present study therefore reviewed the use of microbial inoculants on high moisture by-products with special emphasis on silage fermentation and aerobic stability and livestock performance.
2. Addition of high-moisture by-products to improve the ensiling of forages
High moisture by-products such as those from the fruit juice processing contains soluble sugar that can benefit silage making from low sugar forages such as alfalfa. For example, sugar beet pulp  and apple pomace  contain WSC of 26% and 12% respectively, can be used to improve the fermentation characteristics of silage from low sugar forages. Ke et al.  ensiled wilted alfalfa with or without pomaces (i.e. grape and apple) and reported a reduction in silage pH, reduced proteolysis and increased lactic acid production compared to the untreated silage. In contrast, pomace addition reduced silage aerobic stability compared to the untreated silage. Fang et al.  added 0, 5, 10 and 20% apple pomace in a total mixed ration that was ensiled for 90 days, and reported an increased silage ethanol production with the 20% inclusion of apple pomace. This was attributed to the increased sugar content of the silage mixture.
3. Use of absorbents to improve the ensiling of high moisture plant by-products
One of the major setbacks in ensiling agro-industrial by-products is their high moisture contents (>25%) that requires the by-products to be dehydrated or mixed with absorbents to improve the dry matter contents, compaction and ensiling process . When silage DM content is less than 300 g/kg, conditions for clostridial activity are favourable, resulting in high losses and silage of low nutritional value . To enhance the fermentation process and sustain nutritional quality during ensiling, various additives such as feedstuffs, nutrients and absorbents [24, 25], and non-protein nitrogen agents, chemicals and enzymes have been used .
High moisture by-products such as pulps and pomaces are difficult to ensile and may lead to seepages, causing nutrient losses. These by-products are usually ensiled with absorbents (i.e. dry sources) to improve both the dry matter and fermentation. The effects of adding various absorbents to high moisture by-products at ensiling are shown in Table 1. Adding absorbents to high moisture plant by-products at ensiling improved the fermentation (66%), silage aerobic stability (50%) and in vitro or animal performance by 74% of the responses. This variation in responses depends on the nutritive values and WSC content of the absorbents used. Nkosi et al.  ensiled potato hash with either
|By-products||Absorbent||Mixture (g/kg DM)||Fermentation response||Aerobic stability||Animal performance/||Reference|
|Apple pomace||Lucerne hay (Alfalfa)||345||7.8||Improved||Reduced||ND||Ke et al. |
|Citrus pulp||Dehydrated beet pulp||212.8||ND||Not improved||ND||ND||Megias et al. |
|Orange pulp||Wheat straw||296.6||ND||Improved||ND||IVDMD improved||Paya et al. |
|Wheat straw||210.3||ND||Improved||ND||IVDMD reduced||Denek and Can |
|Sweet potato vines||Sweet potato roots||163||ND||Not improved||ND||IVDMD reduced||Hadgu et al. |
|Sweet potato vines||Napier grass||197||ND||Not improved||ND||ND||Kabirizi et al. |
|Potato pulp||Dry rice||269.7||ND||Not improved||ND||ND||Zhang et al. |
|Dry bean straw||276.9||ND||Not improved||ND||ND||Zhang et al. |
|Dry maize stover||260.0||ND||Not improved||ND||ND||Zhang et al. |
|Potato hash||Poultry litter||364.0||33.8||Not improved||Reduced||IVOMD improved||Nkosi et al. |
|250||22||Improved||Improved||IVOMD improved||Nkosi et al. |
|Pineapple residue||Poultry litter||234||ND||Improved||Improved||IVOMD reduced||Nhan et al. |
|Pineapple||Rice polishing||230||ND||Improved||Reduced||Feed intake and weight gain improved||Nhan et al. |
|Grape pomace||Lucerne hay (alfalfa)||331||7.6||Improved||Ke et al. |
|Wet sugar beet pulp||Dry pelleted beet pulp||907||ND||Not Improved||Reduced||IVDMD improved||Leupp et al. |
|Tomato pomace||Ground maize grains||363||ND||Improved||ND||Feed intake improved||Galló et al. |
|Pumpkin chopped||Dried sugar pulp||292||ND||Improved||ND||ND||Łozicki et al. |
|Dried sugar beet pulp||289.6||57.8||Not Improved||ND||ND||Halik et al. |
|Banana fruit chopped||Dried sugar beet pulp||267.5||ND||Improved||ND||ND||Álvarez et al. |
|Tomato fruit chopped||Dried sugar beet pulp||263||ND||Improved||Álvarez et al. |
|Orange pulp||Dried citrus pulp||712||ND||Improved||Improved||IVDMD improved||Arbabi et al. |
|Dried sugar beet pulp||707||ND||Improved||Improved||IVDMD improved||Arbabi et al. |
|Wheat straw||606||ND||Improved||Improved||IVDMD improved||Arbabi et al. |
|Apple pomace||Maize plant||213||ND||Improved||ND||IVOMD improved||Ülger et al. |
|Sugar beet pulp||151||ND||Improved||ND||IVOMD improved||Ülger et al. |
|Pumpkin pulp||115||ND||Not improved||ND||IVOMD improved||Ülger et al. |
|Sugar beet pulp||Molassed beet pulp||186||249||Improved||Unaffected||IVDMD improved||O’Keily |
|Unmolassed beet pulp||188||75||Improved||ND||ND||O’Keily |
|Citrus pulp||Wheat bran||124||ND||Improved||Reduced||OMD improved||Kordi and Naserian |
4. Use of microbial inoculants during the ensiling of high moisture by-products
4.1 Microbial inoculants
Microbial inoculants are products that are added or inoculated to forages to increase the number of microbes (e.g. LAB) in the forage at ensiling and influence the fermentation process of the forage in the silo . The forage at ensiling is generally dominated by aerobic micro-organisms or facultative aerobes, with less population of LAB . Forages are usually inoculated with homofermentative and facultative heterofermentative LAB to enhance LA fermentation of forages. Homofermentative LAB produces 2 moles of lactic acid from one mole of glucose, and these products contain strains species such as
4.2 Silage fermentation characteristics
Bouillant and Crolbois first adopted the principle of microbial inoculation in 1909 when they applied LAB inoculants to beet pulp to improve fermentation . Later in 1934, Rushmann and Meyer (1979, were cited by ) documented that the rate of acidification during silage fermentation is dependent on epiphytic bacteria found on forages. Currently, there are several silage inoculants available on the market with inoculation rate that ranges between 104 and 106 colony forming unit (CFU)/g . Most commercially available inoculants contain homofermentative LABs, which are fast and efficient producers of lactic acid, and thus improve the silage fermentation. However, these LAB inoculants are mostly designed/produced to be used in the ensiling of forages to ensure enough LAB inoculation at ensiling. Most studies (e.g. [61, 62]) showed an increased in LAB population when LAB inoculants were applied to forages at ensiling. The response to LAB and enzyme inoculation to various high moisture by-products at ensiling are presented in Table 2. Literature shows that the response to LAB inoculation to forage varies a lot. Some reported positive effects in the terms of fermentation dynamics while some reported lack of response. LAB inoculation to high moisture by-products at ensiling have underwent the same pattern as with the forages. For instance, Parigi-Bini et al.  found that the inoculation of lactobacilli (
|By-Product||Treatment||Fermentation||LAB type||Inoculation rate||Ensiling days||Reference|
|Sugar beet pulp||pH||3.91||3.95||Not improved||-||90||Zheng et al. |
|Tomato pomace||pH||4.97||4.57||Not Improved||Sill All||105 CFU g−1 diluted with 2L water||70||Galló et al. |
|Sweet potato vines||pH||4.06||3.84||Improved||1.1 × 1011 CFU/g||Yacout et al. |
|Orange pulp||pH||3.8||3.5||Improved||1 g/kg diluted with 50 ml water (700000 U/kg)||90||Paya et al. |
|Potato hash||pH||3.5||3.14||Improved||Viscozyme® (hemicellulose and pectinase from ||100 ml enzyme diluted with 1 l of water||90||Nkosi et al. |
Mutavhatsindi et al. 
|Potato pulp||pH||3.97||3.94||Improved||Rhizopus oryzae||1×106 CFU g−1||40||Okine et al. |
|pH||3.97||3.95||Improved||Amylomyces rouxii||1×106 CFU g−1|
|Peach pomace TMR||pH||4.29||4.24||Not Improved||-|
3 x 105 CFU g−1
|56||Hu et al. |
|Potato hash TMR||pH||4.3||4.1||Improved||Lalsil fresh||-||90||Nkosi et al. |
|Pumpkin chopped||pH||3.96||3.78||Improved||Bacterial-enzyme- ||2 × 109 CFU g−1||70||Łozicki et al. |
4.3 Aerobic stability of silage
The aerobic stability is a term that nutritionists have used to define the length of time that silage remains cool and does not spoil after it is exposed to air . The aerobic deterioration of silage may increase the risk of proliferation of potential pathogenic or undesirable microorganisms thus affecting the performance of animals fed the silage. In most cases, aerobic deterioration of silage happens with silages that contain high residual sugars . It is noteworthy that
5. Effects of microbial inoculation to ensiled totally mixed rations (TMRs) on fermentation and aerobic stability
Due to the high moisture content in fresh high moisture by-products, it is more advantageous to mix them with other dry feed materials before ensiling. This technique helps to omit the time of mixing before feeding, minimize the risk of effluent production and avoids self-selection of feeds by animals [81, 82]. In some studies, TMRs that contained high moisture by-products (e.g. )  were formulated and ensiled. Nkosi and Meeske  reported an improved silage fermentation, aerobic stability and animal growth performance when TMR that contained potato hash was treated with LAB inoculant. However, Nishino and Hattori  reported improved silage fermentation but LAB inoculation was not worth in the aerobic stability of TMR silage. This might be attributed to the addition of various feed ingredients that might have helped to stabilize the TMR silage.
6. Animal performance
The production of silage will not be worth if it is rejected by animals during the feeding out phase. Animal performance includes feed intake, feed palatability, nutrient digestion, daily gains, milk and meat production. The results on the performance of animals fed plant by-product silage treated with LAB varies like when animals are fed LAB treated silages from plants/forages. According to Table 3, LAB inoculation to high moisture plant by-products improved animal performance in almost all the literature consulted, except for the sugar beet pulp silage reported by O’Keily . It is well known that improved performance by animals fed LAB treated silages are difficult to explain . However, Weinberg et al.  suggested that the interaction of LAB and rumen microbes and the alteration of LAB and rumen fermentation might be attributed to the improvement of animal performance with LAB treated silages.
|By-products||LAB type||Inoculation rate||Animal species/||Response||Reference|
|Potato hash||Bonsilage forte (||2.5 × 105 CFU g−1||Mutton Merino rams||Improved gross energy, crude protein and fibre digestibility.|
Improved nitrogen intake and retention.
|Nkosi et al. |
|Spent mushroom substrate||Mixture of ||1 × 109 CFU g−1||Cross bred rams||Improved EE digestibility and nitrogen retention.||Seok et al. |
|Spent mushroom substrate||Each strain at 0.12% v/w||Hanwoo steers||Improved ADG, FCR and FI||Kim et al. |
|Tomato pomace||Sil All 4x4 (||1 x 105 CFU g−1||Game||Improved||Galló et al. |
|Sugar beet pulp||Sil add ||0.5 g/kg FM basis||Hereford and Friesian steers||Unaffected||O’Keily |
|5 mg/kg FM basis||Increased DM digestibility.||Cao et al. |
|Cabbage waste||Silobac® (||5 x 105 CFU g−1||Increased DM digestibility.||De Rezende et al. |
|Yacon (||Chikuso-1 (||5 mg/kg FM basis||Increased DM digestibility.||Wang et al. |
|Avocado pulp||Emsilage® and Sil-All®||3.5 x 105 CFU g−1||Improved silage degradation||Nkosi et al. |
Inoculation of potato hash silage with
Methane, a greenhouse gas produced from enteric fermentation in the rumen, is a major concern in ruminant production globally. It is well indicated that by improving forage quality and digestibility, this gas production can be reduced. In terms of reduction gas production with LAB inoculation, very few studies have tested this effect in silages from high moisture by-products. Cao et al.  reported a reduced gas production with
7. Properties required in high moisture plant by-products for efficient silage fermentation
As indicated earlier, the low DM content in high moisture plant by-products is a concern since it can promote a clostridial type of fermentation if not improved prior to ensiling. According to Muck , most silages are produced at DM content that ranges from 300 to 500 g/kg, hence the DM of these by-products should be increased. This can be achieved by mixing with absorbents/dry forages. Also, the success of ensiling is determined by various factors that include the anaerobic conditions in the silo, WSC content, the buffering capacity of the pre-ensiled forage, and the epiphytic bacteria. Bacterial inoculation will be worthless if the by-products contain insufficient sugars, which should be consumed by LAB to produce lactic acid, which will reduce silage pH and preserve the crop .
Good quality silage can be produced from high moisture plant by-products with or without LAB inoculation. Increasing the DM content of high moisture plant by-products to >30% is required for efficient fermentation. It should be noted that there are no specific LAB inoculants designed for inoculation to high moisture plant by-products. The efficacy of LAB inoculation to forages depends highly on the type of crop/by-product, the strain and different doses and the ensiling management.