Plant species in the mob-grazed and rotational grazed sites at Hayti, SD in 2013 and 2014.
Short duration (≤24 h), high stocking density grazing systems (e.g., mob grazing) mimics historic prairie grazing patterns of American bison (Bison bison), and should minimize selective grazing. We compared mob [125 cow-calf pairs on either 0.65 ha for 12 h; or 1.3 ha for 24 h] vs. rotational [25 cow-calf pairs on 8.1 ha for 20 days starting in mid-May with or without 2,4-D application prior to grazing; and 15 days starting mid-April (no herbicide)] grazing systems based on forage utilization and impact to Artemisia absinthium (absinth wormwood) in a tall grass pasture of Eastern South Dakota. Grass height and density, and Artemisia absinthium patch volume were quantified pre- and post-grazing at sampling points along multiple transects. Mob grazing had >75% forage utilization, whereas rotational grazing averaged 50% (all consumption). Within a grazing season, three grazing systems suppressed Artemisia absinthium patches with rotation/spray (100% decrease) > mob (65 ± 10% decrease) > mid-May rotation (41 ± 16% decrease), whereas Artemisia absinthium patches in the mid-April rotation followed by summer rest dramatically increased in size. Artemisia absinthium patches <19,000 cm3 were browsed, whereas larger patches were trampled in mob-grazed areas, but avoided in rotational grazing. All Artemisia absinthium patches had regrowth the year following any grazing event.
- forage utilization
- mob grazing
- rotational grazing
- weed management
Grazing lands are managed to optimize forage and animal productivity, and minimize adverse impacts to soil and the surrounding environment. The annual economic impact of all weedy species in U.S. grazing lands is greater than all other pests combined , and has been estimated at 1 billion dollars for forage loss and 5 billion dollars for control costs . Weed infestations cause a variety of problems in grazing lands. Weeds can reduce forage vegetative quality and quantity; displace native plants and animals; reduce animal fertility, weight gains, or be toxic, resulting in fatalities; reduce meat and/or hide quality; increase management costs; and reduce land values [3, 4]. Tactics for weed management in pastures and grazing lands vary with the type of weed, livestock species, and applicability of other methods (e.g., mowing, biocontrol, herbicide treatment) [5, 6].
Livestock can help manage weeds by grazing or trampling and can improve pasture condition and competitiveness of desirable plants by increasing soil nutrients through manure and urine deposition . Weed species and stage of growth; livestock species; and stocking rate and duration influence grazing effectiveness on weeds [3, 7]. Unfortunately, cattle (
Rotational grazing often uses a ‘take half’- ‘leave half’ forage philosophy to maintain healthy, vigorous plant communities [12, 13]. Mob grazing has been promoted to mimic the world’s historic grassland ecosystems  with herds of large animals intensively grazing areas and moving often. The definition of mob grazing is subjective, but typically includes using extremely high stocking rates (100 head or more per ha) for short periods of time (moving every 12 or 24 h)  followed by recovery periods of 6–12 months. The goal of mob grazing is to have every plant within the enclosure eaten  or trampled , limiting selectivity or avoidance of specific species , and providing a more homogeneous grazing treatment. Barnes et al.  reported that grazing homogeneity correlated with paddock size, with pastures ranging from 1 to 8 ha in size grazed nearly uniformly, even if the same stocking rate per ha are used on larger areas.
Grazing impact for weed management is maximized when the target weed is most palatable, is the only forage option, or is made more palatable to livestock in some way (e.g., salt or sugar treatment) , and the desired vegetation is at its least vulnerable phenotypic stage . High animal densities maximize trampling, which incorporates plant litter, manure, and urine into soil, increasing organic carbon and soil nutrients . The combination of eating, trampling, and long rest periods is expected to increase productivity of more desirable forage [3, 18].
Mob grazing has been adopted by ranchers in Texas, SE Colorado, central Nebraska, Missouri, and other areas  where vegetative regrowth can occur quickly due to warm conditions, and high rainfall or irrigation capabilities. Under dryland conditions of the NGP, timing mob grazing to fit within the vegetative and environmental constraints of the area is difficult as growing seasons are short, and pastures often experience summer drought. McCartney and Bittman  reported on a mob grazing study that used 7–14 heifers ha−1 (dependent on seasonal timing) on about 0.3 ha paddocks at different intensities (light, grazed twice a year; to intense, grazed five times a year) in northeastern Saskatchewan. They observed positive [decline in smooth brome (
There have been few comparisons in the NGP among mob grazing and other, more conventional, grazing systems. Fundamental problems in grazing research often include small enclosure sizes and animal numbers, which provide data that are difficult to scale to commercial operations . Due to the expense, need for many animals, and labor and time involved to move cattle frequently, the study was managed by an Eastern South Dakota rancher who incorporates both rotational and mob grazing techniques into his cattle operation.
2. Grazing impacts to forage utilization and Artemisia absinthium
2.1. Experimental site
The effects of rotational and mob grazing stocking densities on Artemisia absinthium and surrounding forage utilization were compared in an eastern South Dakota rangeland location in the tall grass prairie habitat near Hayti (44.66°N, −99.22°W) in 2013 and 2014 . The dominant soil series of the rotationally grazed pasture were the: Poinsett-Waubay silty clay loams (Calcic Hapludolls/Pachic Hapludolls); Buse-Poinsett complex (Typic Calciudolls/Calcic Hapludolls); and Poinsett-Buse-Waubay complex (Calcic Hapludolls/Typic Calciudolls/Pachic Hapludolls) [https://soilseries.sc.egov.usda.gov/osdname.aspx]. Mob grazing pasture soils were similar to the rotational pasture with the addition Barnes-Buse loam complex (Calcic Hapludolls/Typic Calciudolls). The plant communities in these pastures were a mix of cool season native and invasive grasses, warm season grasses, and broadleaf species (Table 1).
|Mob-grazed sites||Rotational sites|
|Common name||Scientific name||Common name||Scientific name|
|Big bluestem||Western wheatgrass|
|Sweet clover||Absinth wormwood|
|Red clover||Kentucky bluegrass|
Growing degree days (GDD) were calculated to provide a reference for plant development between sampling dates and years. The GDD calculation [GDD = ∑ (maximum daily temperature + minimum daily temperature)/2 − base temperature] used the base temperature of 0°C, due to majority of cool season species with GDD accumulations starting on January 1 of each year. Precipitation (from January 1) was also determined. The rotational pre-graze samples in 2013 were taken on June 13, with 641 GDD and 243 mm precipitation (www.noaa.gov) with values similar to the 30-year (1980–2010) average. Post-grazing samples were taken July 22, with GDD of 1540 and precipitation totaled 343 mm. In 2014, samples were taken May 13 with GDD at the spring assessment (which was taken after the early spring grazing) 262 and 65.5 mm of precipitation. The fall assessment was taken September 16 with GDD of 2603, and total rainfall of 370 mm fall. Rotational grazing was done much earlier in 2014 because the rancher was concerned about low amounts of precipitation (nearly 60% below average) during the 2013 fall and winter.
GDD accumulations for mob-grazed areas in 2013 were 1801 (August 6) and 1855 (August 9) for pre- and post-graze samples, respectively. Precipitation totaled 376 mm before and after mob grazing. In 2014, GDDs were 1693 pre-graze (July 29) and 1817 post-graze (August 4) and precipitation for pre-graze and post-graze totaled 230 and 270 mm, respectively.
2.3. Grazing treatments
Stocking treatments (rotation vs. mob) were repeated, although cattle densities and time of grazing differed between the 2 years due to feeding needs and differences in forage growth due to low rainfall in 2014 (Table 2). Rotational grazing was conducted in 8 ha pastures with 25 cow-calf pairs (1560 kg ha−1). In 2013, in one paddock, the cow-calf pairs were allowed to graze for 14 days starting June 13 (referred to as ‘rotation’). In a separate paddock, generic 2,4-D ester at 1.1 kg ha−1  was applied 1 day before the start of grazing on June 13 with a grazing duration of 14 days (referred to as ‘spray/rotation’). In 2014, a different pasture was grazed by 25 cow/calf pairs for 15 days, starting April 27 and ending May 11 (referred to as ‘early spring grazed/summer rest’).
|Year||Grazing system||Stocking density||Grazing duration||Sampling date||Forage biomass1||Sampling date||Standing||Trampled2||Forage use efficiency3||Forage utilization4|
|kg ha−1||kg ha−1||kg ha−1||%||%|
Mob grazing was conducted for 12 h in a 0.65-ha paddock on August 8, using 125 cow-calf pairs (stocking rate of 223,250 kg ha−1 day−1) (Figure 1). In 2014, a different 1.3-ha area was mob grazed on July 30 for 24 h with 125 cow-calf pairs (stocking rate of 53,580 kg ha−1 day−1).
2.4. Forage amounts and utilization
Eight 50-m long transects were established in each paddock for vegetative production evaluation. Sampling points were placed every 5 m along each transect, with GPS coordinates (Garmin etrex 20, Garmin, LTD, Schaffhausen, Switzerland) recorded so that resampling occurred at the same points pregrazing and post-grazing. At the sampling points, pre-graze measurements (in 2013, rotational graze and spray/rotational graze—13 June; mob graze—6 August; 2014, mob graze—29 July) included vegetation height using a grazing stick , and ocular estimates of basal cover of living vegetation, litter cover, and bare ground (0–100%) in a 1 m2 area around the point. In 2013, vegetation in a 0.25 m2 area was clipped to within 1 cm of the soil surface, and bagged (n = 30). Litter under the vegetation also was collected. Samples were weighed, dried at 38 C to constant weight, and dry weight of vegetative biomass and litter per unit area were calculated. The biomass values and grazing stick estimates were compared at each sampled point.
A few days after grazing (in 2013, rotational graze and spray rotational/graze—22 July; mob graze—9 August; in 2014, mob graze—4 August), the same transects and sampling points were reestablished for post-grazing measurements. Vegetation height was measured using the grazing stick, and percent trampled vegetation (e.g., new litter; defined as living vegetation oriented less than 45° from the soil surface) was estimated in the same areas as pre-graze sampling.
In 2014 due to the producer’s needs, cattle grazed the designated rotational pasture in April and then this pasture was untouched for the remainder of the season (summer rest). Unfortunately, due to the early timing of the grazing in the second year, no pre-grazing measurements were taken for this pasture. Measurements occurred on 13 May, after the early season grazing was completed, and then resampled on 16 September (designated as regrowth after early spring grazed/summer rest). In addition, the transects which were sampled in 2013 were reestablished and vegetative height was quantified in May 2014 to examine recovery after grazing.
Another three 50-m transects were established in each pasture with vegetative height measured pre- and post-graze every 2.5 m along the transects.
2.6. Statistical analysis
Data analyses were performed using JMP®, Version 5.0.1, (SAS Institute Inc.). Forage amounts pre-graze were based on clipped biomass measurements and compared with the grazing stick method. The grazing stick equation, based on plant height, was:
This estimated biomass for a cool season mixed grass pasture [26, 27]. The 7.6 cm value accounts for basal stems and leaves that would not be eaten by grazing animals. Two-tail, two-sample homoscedastic t-tests were used to compare forage biomass with the grazing stick estimates. Grazing stick estimates were found to be statistically similar to the clipping method.
Forage biomass and
Binomial analysis of
was used to examine the influence of each treatment on
3. Measured impacts of grazing systems
3.1. Forage utilization
3.1.1. Mob grazing
Pre-graze forage coverage averaged 85% (grass and forb) in 2013 and neared 100% in 2014. In 2013, pre-graze forage biomass was estimated to be 2910 (±816) kg ha−1 with the clipped method and 2720 kg ha−1 with the grazing stick. These measurements were statistically similar. Pre-grazing biomass in 2014 averaged 4640 kg ha−1, the grazing stick method estimated 3980 kg ha−1, with estimates statistically similar. The discrepancy between direct biomass sampling and grazing stick can be partially explained by sampling method, as forage was cut to within 1 cm of soil level, but the grazing stick calculation subtracts 7.6 cm from forage height to account for unconsumed stubble. Whereas the clipping method provided excellent data, the process was labor intensive, slow, and required preweighing, drying, and postweighing. In addition, it was found that after mob grazing there was no biomass to clip. The grazing stick method provided a reasonable estimate of available forage.
In 2013, mob grazing forage utilization was about 80% (Table 2; Figure 2) with a harvest efficiency (amount consumed) of 62% (~1800 kg ha−1). The remaining 20% of the vegetation was trampled. In 2014, the same stocking rate (125 cow-calf pairs) was used, but the area was two times larger, had about 1.5 times greater pre-graze biomass, and grazing time was doubled from 12 to 24 h. Forage utilization in 2014 was 75%, similar to 2013. The amount consumed was 1600 kg ha−1, similar to the amount consumed in 2013, but due to the greater starting biomass, the harvest efficiency (percent consumed) was 34%, and the trampled amount was 40%.
3.1.2. Rotational grazing
In 2013, pre-graze forage amount averaged 2600 kg ha−1 and post-graze was 1190 kg ha−1 (Table 2). Both harvest efficiency (amount consumed) and utilization (amount consumed + trampled) were 45%, as new trampled litter was not observed. In the rotational/spray treatment, pre- and post-graze forage was 4530 and 2610 kg ha−1, respectively, which indicated that forage consumption neared 57%. As in the rotational area, there was little newly trampled litter.
The 2014 rotational pasture was grazed in April, which allowed recovery during the summer/fall of 2014. Forage after grazing was 870 kg ha−1. The rancher follows the ‘take half, leave half’ utilization recommendation [12, 13], so a reasonable pre-graze forage estimate would have been about 1300 kg ha−1. Grass forage increased from 11 (May) to 23 (September) cm in height (
3.2. Grazing impact on
A pre-grazing assessment of
3.2.1. Mob grazing
Matched-pair analysis of 2013 and 2014 combined indicated that about 65% of the
|Year||Grazing system||Pre graze ave. vol.||#Decrease/total||Ave. vol. of remaining||% Controla|
|2014||Summer recovery||2850||1/28||3 (NS)|
3.2.2. Rotational grazing
In 2013, 41% of the
In 2014, with no grazing pressure during the summer season, only 1 (3%) of the
3.2.3. Influence of initial
Artemisia absinthiumpatch volume on grazing system impact
When data were combined over both years, 28 of 38
In 2014, all tagged patches in the 2013 pastures were reevaluated to determine if patches and plants in the patches were still present and the amount of regrowth. Plants in the treated patches of the rotation/spray treatment, which provided excellent control of
Rotational grazing for 20 days at 25 cow/calf pairs in 8 ha had comparable results in forage consumption to mob grazing with 125 cow/calf pairs for 12 or 24 h. in 0.65 or 1.3 ha, respectively. There were other differences between the systems, most notably the vegetative growth stage of forage, which was more mature during mob grazing. Trampled vegetation was observed in the mob grazing areas but not the rotational grazing treatments. However, claims about building soil at rates of cm per year, or significantly increasing N and C content (which was measured and reported in Myer ), as often discussed in popular press articles [15, 19], could not be substantiated in this study. However, trampled litter and manure patches (measured as manure patches along the transects and reported in Myer ) were greater post-mob grazing compared to both pre-mob and post-rotational grazing.
McCartney and Bittman  and others [29, 30, 31], suggest that timing and grazing capacity for optimal forage utilization and weed control, with minimal harm to desired species, requires thoughtful management to improve or maintain rangeland health. Our results show that mob grazing (225,000 or 50,000 kg of cattle ha−1 day−1) could reduce biomass of
Mob grazing with cattle has been proposed as a grazing system to increase forage use efficiency and help in landscape restoration  and is likened to grazing patterns of the native plains bison. Kohl et al.  reported that bison and cattle differ in grazing, standing, bedding, and moving behaviors, with bison moving from 50 to 99% faster and foraging up to double the land area than cattle during the same duration. This is the precedent for the frequent moves when mob grazing cattle. In addition, cattle, when not pressured, tend to select high plant biomass, whereas bison tend to select intermediate plant biomass . Regardless of the inherent differences between these two species, when managed correctly, mob grazing with cattle can diversify grazing time, with frequent moves, and long rest periods . However, if managed incorrectly, high intensity grazing systems could increase weed infestations . For example, in 3 years, under medium grazing intensity (grazed five times year−1 with 6 cm of vegetation remaining after each grazing event) weeds increased by about 4 plants m−2, whereas under high intensity (grazed seven times year−1 until surface exposure), weed densities increased by 51 plants m−2 . Hart et al.  reported that stocking rates that alter grazing frequency and defoliation intensity, rather than grazing system, have greater potential to impact species composition. Plant diversity and complex mixtures of forage species are integral to healthy ecosystems and consistent yields [38, 39]. However, mob grazing, if repeatedly used in the same area and at the same seasonal timing, could decrease plant species diversity and richness, change functional plant traits (e.g., tall vs. short), but improve productivity of the remaining plants .
The animal of choice for grazing also can influence grazing results. Goats (
Herbicide applications are reported to be the most effective methods for
Healthy rangelands grow more grass which aids in
Once present, our study showed that grazing provided temporary reductions to
We found that mob grazing with cattle for 12 or 24 h in pastures where
Thanks to Mr. R. Smith, Hayti, SD for providing access to land and cattle. Funding support for this study was provided by the South Dakota Agricultural Experiment Station and USDA, NRCS Conservation Innovation Grant (CIG) 3FH560 “Demonstrating Mob Grazing Impacts in the Northern Great Plains on Grazing Land Efficiency, Botanical Composition, Soil Quality, and Ranch Economics.”
|Cattle||Bos taurus L.|
|Absinth wormwood||Artemisia absinthium L.|