We demonstrated the influence of mob grazing on 9 cooperating ranches throughout South Dakota. The study took place from 2012-2014. We sampled soils and vegetation at each site. Three graduate students evaluated the impact of mob grazing on the 1) function of litter, 2) impact of pasture weeds, and 3) the harvest efficiency and trampling of vegetation. We also collected animal behavior data using pedometers on mob grazing versus a low stock density weekly rotation. A fourth graduate student investigated the economics of mob grazing. The final report includes the soil data to determine impact on soil carbon and nitrogen of mob grazing.
1) Evaluate vegetation responses to mob grazing.
2) Evaluate animal behavior in mob grazing versus low stock density weekly rotation.
3) Evaluate soil carbon and nitrogen in mob grazed and rotational grazed sites.
4) Evaluate economic models for profitability analysis.
1) Vegetation data collection
We conducted this mob grazing study across the different ranches in South Dakota (Figure 1). Stocking density ranged from 20,000 to 1,000,000 lbs of beef per acre with cattle rotated once per day up to nine times per day.
Pre and post grazing standing vegetation and litter biomass was sampled during the reporting period at 4 sites in South Dakota. Sites were located in Quinn (western SD), Selby (north central SD), Chamberlain (central SD), and Hayti (eastern SD) to look at harvesting efficiency, trampling, and biomass differences across a climatic gradient (Figure 2). Paired 0.25 m2 quadrats were clipped before and after grazing along 8 transects per site with 10 replicates per transect. Standing vegetation and litter biomass samples were placed in separate bags, dried at 70 C for 72 hours, and weighed. New trampled litter biomass after grazing was measured by placing green litter into a separate bag. Between 80 and 90% of the standing vegetation biomass was utilized (consumption + trampled) during mob grazing at the sampled sites (Figure 2). Harvest efficiency (percent of the pre grazed forage that is consumed by livestock) was 33% at Quinn, 43% at Selby, 63% at Chamberlain, and 34% at Hayti. To put this into perspective, moderate grazing in season-long grazing systems have a 25% harvest efficiency. Trampling increased from west to east in conjunction with forage production (Figure 2). Cattle trampled 18% at Quinn, 27% at Selby, 19% at Chamberlain, and 41% at Hayti.
We monitored soil temperature and moisture under different vegetation cover types (intact grassland vegetation, bare ground, and mob grazed vegetation) at 4 different sites across South Dakota. Soil temperature was typically higher for bareground treatments than ungrazed or mob grazed treatments irrespective of the far western site (Figure 3) or the eastern site (Figure 4). Soil moisture at the 2 inch depth was variable across treatments and sites but ungrazed vegetation often had lower soil water potential (likely from water uptake by intact vegetation; Figures 5-6). Results suggest that changes in groundcover from mob grazing may impact soil conditions that are important for microbial activity.
We evaluated litter decomposition at each of the four sites on the ungrazed, bare ground, and mob grazed treatments. There were no differences of litter decomposition of surface litter for the three treatments at the four sites (Figure 7). There was a difference in decomposition rates of among litter types (new litter, old liter, and filter paper) at the four sites (Figure 8). New litter tended to have greater decomposition rates especially at the more mesic sites (Chamberlain and Volga) compared with the drier sites (Eureka and Quinn).
At three sites, weedy plants were measured for their canopy volume before and after grazing at three sites in 2013 (Figure 12). Mob grazing reduced buckbrush volume compared to no grazing at Chamberlain. At Selby, mob grazing had a slight reduction in buckbrush volume compared with no change in the rotational grazed pasture. At Hayti, wormwood sage did not reduce in volume in either rotational or mob grazing. When an area was treated with 2,4-D, the cattle consumed the wormwood sage and completely defoliated the plants. It was likely that cattle were attracted to the plants due to the salty flavor of the herbicide treated plants. This may warrant further study.
2) Animal behavior
Bred heifers, weighing approximately 1000 lbs each, were allocated to mob grazing (200,000 lb/acre live weight) moved 3x per day and a weekly rotation (4000 lb/acre live weight). The grazing period was during the month of July on smooth bromegrass/Kentucky bluegrass dominated pasture. Forage samples were collected before and after grazing to determine utilization, trampling, and harvest efficiency. Pedometers were fitted to 3 heifers on each mob grazing paddock and on 2 heifers of each weekly rotation paddock. Pedometers recorded number of steps, lying time, and number of times the animal laid down (lying bouts).
Heifers in the mob grazing system were stocked two times higher than the weekly rotation. This resulted in a higher grazing pressure. The utilization was 1.5 times greater than the weekly rotation. Trampling was similar between the two systems. Harvest efficiency was almost 2 times greater for the mob system than the weekly rotation. Harvest efficiency as a function of body weight was not limiting intake in either system as bred heifers would be expected to consume about 2.5% of body weight. The daily activity recorded by the pedometers showed that heifers in the mob system rested 167 minutes less per day, laid down 10 times less, and took 1922 more steps per day than the heifers in the weekly rotation (Table 1). This resulted in heifers in the mob grazing system to walk about ½ mile more per day. It is unclear if heifer activity is influenced by the system or the herd size effect because they were confounded.
Mob grazing provided higher harvest efficiency compared to a moderately stocked weekly rotation. These data provide some insight into the high harvest efficiencies possible with mob grazing. With high harvest efficiencies comes high utilization. High utilization (>60%) of forage biomass from a pasture under season-long grazing conditions without rest is detrimental to forage growth. Under a more intensive rotation, such as mob grazing where the pasture gets grazed one day out of the entire year, the long recovery time allows the forage to regrow and build an adequate root system. In addition, the grass species grazed where more mature, and the higher level of utilization does less harm to the species compared to a more vegetative growth stage.
3) Soil Carbon and Nitrogen
The greatest change in soil carbon was with depth, i.e. the surface soil depth (0-4 inches) had 1.0 to 2.4 times more carbon than the 4-8 inch depth. Surface soil carbon was 0.5% and 0.3% lower in 2013 than in 2012 and 2014, respectively, irrespective of grazing treatment. The lower carbon in 2013 is expected following the drought in 2012. For the surface soil depth (i.e. the depth most impacted by vegetation and grazing impacts), soil carbon was similar in mob and rotation pastures across sites and years. In addition, we did not observe differences in soil carbon in grazed or ungrazed pastures (i.e. those allowed to rest for a growing season) relative to initial baseline soils collected in 2012 (Figure 10). Our results indicate that South Dakota mob grazing conditions do not lead to large increases in soil carbon (e.g. 1 inch per year) reported elsewhere.
Soil percent nitrogen tests yielded similar results as carbon. Soil percent nitrogen changed minimally by season, year, and site (about 0.01% difference on average). The greatest change in nitrogen was by soil depth, i.e. the surface depth (0-4 inches) was observed to contain about 1.2 to 1.8 times more nitrogen than the lower depth on average. Soil nitrogen did not increase from baseline measurements in spring 2012 following exposure to mob or rotational grazing (Figure 11).
4) Economic models
In order to test the economics of mob grazing, budgets were set up to compare 4 different grazing systems tested from a University of Nebraska mob grazing trial from 2011-2014 using yearling steers. The four systems were a continuous system, a four pasture one pass through system (4-PR-1), a four pasture twice pass through system (4-PR-2), and mob. Once the profitability of each system was found, static risk analysis was used from the actual budgets. It was found that mob grazing was the least preferred grazing system. Next, the selling price, ADG, and AUM were simulated in order to make the system dynamic. The simulated data was analyzed in a stoplight function, cumulative distribution function, stochastic dominance with respect to a function, stochastic efficiency with respect to a function, and negative exponential utility weighted risk premium function. It was found that on a per acre basis mob is the least preferred system when risks are not considered. As risk aversion increases, mob becomes the third most preferred system.
Next, in accordance to producer testimonies, the systems were tested to see how management changes affected the systems. Mob, continuous, and 4-PR-1 were given different ADG relative to the baseline 4-PR-2 system. The systems were test at a 5%, 12.5%, and 25% decrease in ADG. The purpose of this sensitivity test was to see if management changes could make the systems more profitable. Even with Mob having the same ADG as other systems, it was still less profitable due to the high labor cost of the system. A producer grazing yearling cattle would be economically better off to either graze each pasture twice such as the 4-PR-2 or continuously graze.
This study has provided insight into the mechanisms governing the trampling effect and adding litter to cover bare ground in relation to stocking density, plant height, and location (influence of local plant-soil-climate factors). Mob grazing did increase the distribution of cattle manure. Producers are getting an increase in harvest efficiency, probably 40-50%, which is almost twice that over season-long continuous grazing at a moderate stocking rate. It remains to be seen how the soil health will change, ultimately affecting plant productivity. This intensive grazing practice is particularly well-suited for highly productive grasslands (e.g., subirrigated meadows, irrigated pasture, and dryland pasture of high-yielding introduced or improved native grasses) where production potential per acre and ecosystem resilience is high. The economic analysis using animal data from a yearling study showed mob grazing to not be as profitable as other simple rotations or continuous grazing. An economic simulation needs to be performed using cow-calf production data and the increased grazing efficiencies (supporting higher stocking rates) needs to be conducted.
Producers are already using mob grazing in South Dakota.
Educational & Outreach Activities
Helms, E. 2015 Evaluation of mob grazing and its impacts on soil moisture and temperature and litter decomposition in South Dakota. Submitted to the Natural Resources Management Department, South Dakota State University. July, 2015.
Myer, H. 2015. Mob grazing as a perennial weed management tool in South Dakota grazinglands. Submitted to the Plant Science Department, South Dakota State University. July 2015.
McMurtry, B. 2015. An Economic Analysis of High-Intensity, Short-duration Grazing Systems in South Dakota and Nebraska. Submitted to the Economics Department, South Dakota State University.
Mortellaro-Brown, M. 2014. Harvest Efficiency, Forage Trampling, and Litter Decomposition of Mob Grazing. Submitted to the Natural Resources Management Department, South Dakota State University. May, 2014.
A tour was held at our Hayti, SD mob grazing producer site on September 10, 2014 that included nearly 200 participants across the northern plains region. Allan Savory spoke on his experience with mob grazing and our demonstration locations at Hayti were featured.
Helms, E. R., A. Smart, S. Clay, M. Ohrtman, D. Clay. “Does Litter Matter: Impacts on Soil Temperature and Moisture in High Stock Density Grazing”. February 2015. International Society for Range Management Meeting. Sacramento, California.
Helms, E. M. Mortellaro-Brown, A. Smart, S. Clay, M. Ohrtman, and D. Clay. Mob grazing in South Dakota. South Dakota Society for Rangeland Management Annual Meeting. October, 2012. Chamberlain, SD.
Ohrtman, M., S. Clay, A. Smart, D. Clay, and W. Schacht. Mob Grazing Effects on Pasture Utilization and Nutrient Deposition in South Dakota. Joint Annual Meeting for ASA, CSSA, and SSSA. November, 2013. Tampa, FL.
Mortellaro, M. A. Smart, J. Chang, M. Ohrtman, S. Clay, and D. Clay. Impact of High Stocking Density Grazing on Litter Decomposition. August 20, 2012. SRM 2013 Annual Meeting, Oklahoma City, OK
Myer, H., Sharon A Clay, Alexander Smart, and Michelle Ohrtman. Mob Grazing to Control Pasture Weeds in South Dakota. Agronomy Society of America Annual Meeting. November 4, 2014. Long Beach, CA. Poster Presentation.
Smart, A. Mob Grazing: what is it all about? Northern Plains Sustainable Ag Conference in Aberdeen, SD. January 24, 2015. Available on SARE’s Youtube channel https://www.youtube.com/watch?v=4YyiojlzF54&index=6&list=PLQLK9r1ZBhhFIETmMLo1dZBEVYZWXBIM1
Larry Janssen, Bronc McMurtry, Matthew Stockton, Alexander Smart and Sharon Clay. An Economic Analysis of High-Intensity, Short-duration Grazing Systems in South Dakota and Nebraska. Selected paper prepared for presentation at the 2015 Agricultural & Applied Economics Association and Western Agricultural Economics Association Annual Meeting, San Francisco, CA, July 26 – 28.
Ehmke, T. 2015. Mob Grazing Shows Possible Production, Ecological Benefits. CSA News. August, 2015.