Performance and forage selectivity of sheep and goats co-grazing grass/forb pastures at three stocking rates

Differences among ruminant species in forage selectivity offer potential for efficient utilization of pastures with diverse arrays of plant species. One common management strategy that may influence forage selectivity is stocking rate (SR). Therefore, this experiment was conducted to determine effects of SR on performance and forage selectivity of growing sheep and goat wethers, co-grazing grass/forb pastures. Grazing was for 16 weeks in 2002 and 2003. Pastures consisted of various grasses, primarily bermudagrass (Cynodon dactylon) and johnsongrass (Sorghum halepense), and forbs (e.g., ragweed; Ambrosia spp.). Sheep (Khatadin) and goats (75% Boer) averaged 21 +/- 0.7 and 21 +/- 0.5 kg initial BW, respectively, and were 4-5 months of age when grazing began. Stocking rates were four (SR4), six (SR6), and eight (SR8) animals per 0.4-ha pasture, with equal numbers of sheep and goats. The nine pastures (three/treatment) were divided into four paddocks for rotational grazing in 2-week periods. Forage mass (pre- and post-grazed) and composition of grass versus forbs were determined by quadrat samples and transect analysis, respectively. BW was measured every 4 weeks and preference values for grass, forbs, and ragweed (10: highest possible preference; 0: consumption in proportion to availability; -10: no consumption) were determined from fecal microhistology and transect measures. There was a year x SR interaction (P < 0.05) in herbage DM mass before grazing (year 1: 2937, 3298, and 3351 kg/ha; year 2: 3033, 2928, and 2752 kg/ha for SR4, SR6, and SR8, respectively (S.E. = 174.4)). Post-grazed forage mass decreased linearly (P < 0.05) as SR increased (2279, 1693, and 1288 kg/ha for SR4, SR6, and SR8, respectively (S.E. = 102.6)). In vitro true DM digestibility of pre-grazed forage samples was similar among SR, but SR x year interacted (P < 0.05) for post-grazed samples (year 1: 57.0, 54.4, and 53.5; year 2: 56.8, 49.0, and 48.3 for SR4, SR6, and SR8, respectively (S.E. = 2.16).Year and SR interacted (P < 0.05) in the percentage of grass in pasture, post-racing determined by transect (year 1: 64. 69, and 74% year 2: 50, 66, and 73% for SR4, SR6, and SR8, respectively (S.E. = 8.4)) The preference for grasses was higher and that for total forbs lower for sheep than for goats (P < 0.05). The preference value for ragweed measured in year 2. as lower (p < 0.05) for sheep than for goats (-1.6 versus 0.2) and increased linearly with increasing, SR. Average daily, gain tended (p < 0.10) to decrease linearly as SR increased (61.51. and 47g/day). and total BW gain per hectare increased linearly (p < 0.05: 610.759 and 933 g/day for SR4, SR6, and SR8, respectively). In conclusion. post-grazing herbage mass > 1000 Kg/ha at most measurement times suggests that decreasing forage availability with increasing SR may not have been primarily or solely responsible for the effect on ADG by limiting DM intake. Rather, the effect of SR on available forage mass could have limited the ability of both sheep and goats to compensate for the effect of SR on forage nutritive value. (c) 2005 Elsevier B.V. All rights reserved.