Summary and Implications
South central Iowa grasslands are dominated by cool season grass species with low productivity and little plant diversity which limits the forage yield and quality for grazing animals and habitat for native grassland wildlife. Strategic spring mob-grazing may reduce competition from cool-season grass species allowing early successional species, legumes, and native plants to establish while improving soil characteristics. Two blocks of three replicated pastures were divided into 5 equal-sized paddocks to determine the effects of early spring mob-grazing on pasture forage and soil characteristics. In each pasture, one paddock was not grazed (NG) and 4 were strip- (S; moved once per day with a back fence) or mob- (M; moved 4 times per day with a back fence) grazed beginning in May of 2011 (BL1) and 2012 (BL2) by 10 cows at a live forage DM allowance of 2% BW/d. Subsequently, one mob (MR) and strip (SR) paddock in each pasture was rotationally stocked to remove 50% of the live forage with 35-d rest periods beginning 60 d after spring grazing in yr 1 of each block. Measurements included pasture botanical composition determined by the line transect method, soil penetration resistance determined with a penetrometer, water infiltration determined with double ring infiltrometers, soil bulk density, and ground nesting bird habitat measured as visual obstruction to a 3.3x 3.3 ft board by image analysis of digital photos. In BL1 and BL2 after mob- or strip- grazing, the proportion of bare ground was greater (P< 0.05) in rotationally grazed paddocks than NG paddocks in most months. The proportion of annual grasses was greater (P< 0.05) in grazed than NG paddocks in July 2011 in BL1. In 2012, the proportion of legumes was greater (P< 0.05) than NG paddocks in M, MR, and SR paddocks in May, M and SR paddocks in July, and all grazed paddocks in BL 1 in October. But in 2013, the proportion of legumes was greater in MR and S paddocks in May and MR paddocks in October than NG paddocks. The proportion of warm season grasses in BL2 was less (P< 0.05) in MR and SR paddocks than NG and M paddocks in August 2013. In BL1, penetration resistance was greater (P< 0.05) in rotationally stocked paddocks compared to NG paddocks in May and October of 2012 and 2013, however reductions (P< 0.05) in water infiltration rates occurred in rotationally stocked paddocks only in October 2012 and 2013. In BL1, visual obstruction was less (P< 0.05) in M paddocks than S and NG paddocks to 15.7 inches high in October 2011. However in BL2, no differences in visual obstruction occurred in October 2012. A single mob- or strip-grazing event of a grassland in the spring may improve the nutritional value of the forage for grazing livestock and habitat for wildlife. However, the extent and longevity of these responses are related to soil moisture at the time of grazing and subsequent management and climatic conditions.
Iowa State University
Bisinger, Justin J.; Russell, James R.; Bear, Douglas Allen; Sellers, Joe; and Offenburger, Helga
"Enhancing Botanical Composition and Wildlife Habitat of Pastures in South Central Iowa through Soil Disturbance by Mob-grazing of Beef Cattle,"
Animal Industry Report:
AS 660, ASL R2888.
Available at: http://lib.dr.iastate.edu/ans_air/vol660/iss1/53