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Effects of stocking rate on performance of sheep and goats co-grazing mixed pastures

G. Animut1,2, A. L. Goetsch1, G. E. Aiken3, R. Puchala1, G. Detweiler1, C. R. Krehbiel2, R. C. Merkel1, T. Sahlu1, L. J. Dawson4, and Z. B. Johnson5

1E (Kika) de la Garza American Institute for Goat Research, Langston University, Langston, OK, 2Animal Science Department, Oklahoma State University, Stillwater, 3USDA ARS Dale Bumpers Small Farms Research Center, Booneville, AR, 4 College of Veterinary Medicine, Oklahoma State University, Stillwater, 5Department of Animal Science, University of Arkansas, Fayetteville

Differences among ruminant species in forage selectivity offer potential for efficient utilization of pastures with diverse arrays of plant species. Therefore, this experiment was conducted to determine effects of stocking rate (SR) on growth performance of growing sheep and goat wethers co-grazing mixed pastures. Grazing was for 16-wk periods in 2002 and 2003. Pastures consisted of various grasses, such as bermudagrass (Cynodon dactylon) and johnsongrass (Sorghum halepense), and forbs (e.g., ragweed; Ambrosia spp). Sheep (Khatadin) and goats ( ≥ 75% Boer) averaged 21 ± 4.8 and 21 ± 3.7 kg initial BW, respectively, and were 4 to 5 mo of age when grazing began. Stocking rates were four (4), six, (6), and eight (8) animals per 0.4-ha pasture, with equal numbers of sheep and goats. The nine pastures (three/treatment) were divided into four paddocks, which were sequentially grazed in 2-wk periods. There were year SR interactions in herbage DM mass before ((P < 0.05) yr 1: 2,937, 3,298, and 3,351 kg/ha; yr 2: 3,033, 2,928, and 2,752 kg/ha (SE = 172.7)) and after grazing paddocks ((P < 0.09) yr 1: 2,535, 1,879, and 1,609 kg/ha; yr 2: 2,023, 1,507, and 996 kg/ha for 4, 6, and 8, respectively (SE = 111.3)). Year and SR interacted (P < 0.05) in the percentage of grass determined by transect in paddocks post-grazing (yr 1: 66, 69, and 74%; yr 2: 50, 66, and 73% for 4, 6, and 8, respectively (SE = 8.4)) but not pre-grazing (60, 64, and 64% in year 1 and 48, 57, and 60% in year 2 (SE = 7.0). Average daily gain tended (P < 0.10) to decrease linearly as SR increased (53, 44, and 41 g/d), and total BW gain per pasture increased linearly (P < 0.05; 214, 266, and 327 g/d for 4, 6, and 8, respectively). There was a tendency (P < 0.08) for an interaction in ADG between year and species (yr1: 62 and 27 g/d; yr 2: 59 and 37 g/d for sheep and goats, respectively (SE = 4.1)). In conclusion, increasing the SR of sheep and goats co-grazing mixed grass/forb pastures tended to linearly decrease ADG but increase total BW gain per pasture. Post-grazing herbage mass ≥ 1,000 kg/ha suggests that shifts in selection of plants or plant parts differing in digestibility may have contributed to the effect of SR on ADG.


 

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