Virginia Tech Researchers Test the Sustainable Ag Benefits of Mob Stocking

BLACKSBURG, Virginia– Researchers at Virginia Polytechnic Institute and State University are looking at the environmental impacts of a relatively new grazing system that is gaining popularity across the country for its perceived production and land management benefits.

Emily Williams, a graduate student in the Department of Biological Systems Engineering, received a two-year Southern Sustainable Agriculture Research and Education (SSARE) Graduate Student Grant to study water quality of runoff and nutrient losses in mob stocking, compared to continuous and rotational grazing systems.

Mob stocking is a technique whereby high-density livestock populations are stocked in small areas for a short period of time, then removed to allow forages to recover. System advocates say mob stocking provides a number of sustainable ag benefits, including increased soil organic matter, increased plant species diversity, improved forage production, and better animal health.

“Rotational stocking systems have demonstrated increased production and improved water quality of runoff as compared to unmanaged, continuous grazing,” said Williams. “Ultra-high density stocking has recently emerged as a grazing system with potential for improving pastureland health and, in return, increasing production. However, very little research surrounding the environmental impacts of mob stocking exists. Those impacts, including runoff water quality and soil health, need to be evaluated to determine the validity of the claims that the practice is sustainable.”

Williams is comparing water quality of runoff and nutrient losses, specifically nitrogen and phosphorus, among the three grazing systems using rainfall simulations and then conducting statistical analyses of repeated measurements at two replicate farm sites.

“The stocking rate was held constant among the treatments, but the density differed.  In the continuous treatment we stocked 7 cows on 7 acres and they were left to graze unmanaged for the season.  In the rotational treatment, the 5-acre pasture was divided into 4 paddocks.  We stocked 5 cows in this pasture, but they were only in one paddock at a time.  They were moved every three days to the next paddock.  The cows were stocked on the rotational pasture for the entire season, too,” said Williams.  “For the mob treatment, stocking was deferred until forage was mature.  The 3-acre pasture was divided into 3 paddocks.  We stocked 40 cows on this pasture, but they were only allowed to graze in one paddock at a time.  They were moved to the next paddock after 1 day.  They were then removed from the site and the forage was allowed to mature again.  After forage had matured again in the fall, we mob stocked again.”

Williams said that the goal is to show the differences in water quality and nutrient losses among the three systems and how mob stocking stacks up against rotational grazing.

"Timing of weather events is critical for nutrient management," said Williams. "We gathered initial data (pre-stocking) and then ran monthly rainfall simulations after that. We timed the mob stocking and data collection so that a rainfall simulation would occur directly following removal of the mob cattle."

Despite the benefits touted with mob stocking, such as increased concentrations of manure or urine that act as natural fertilizer, Williams speculates that the higher density of livestock in a smaller area causes soil compaction, and results in increased runoff and more nutrient losses compared to rotational grazing.

“Farmers interested in conservation practices need to be able to make informed decisions on pasturing options. Maybe mob stocking is better in forage production than rotational and continuous grazing, but not so good when it comes to the environment, or it just may be the opposite,” said Williams. “We have to fully study mob stocking before we can promote it as a sustainable ag practice.”

Williams has just completed the first year of the two-year study, “Mob Grazing Effects on Nutrient Runoff in Cool Season Pastures (GS12-113).” Virginia Tech professor Cully Hession is the principal investigator (PI) on the grant.

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Published by the Southern Region of the Sustainable Agriculture Research and Education (SARE) program. Funded by the USDA National Institute of Food and Agriculture (NIFA), Southern SARE operates under cooperative agreements with the University of Georgia, Fort Valley State University, and the Kerr Center for Sustainable Agriculture to offer competitive grants to advance sustainable agriculture in America's Southern region.