Abstract :
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. BWwas 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×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×year interacted (P < 0.05) for
post-grazed samples (year 1: 57.0, 54.4, and 53.5; year 2: 56.8, 49.0, andYear and SR interacted (P < 0.05) in the percentage of grass in pastures post-grazing 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 grasseswas higher and that for
total forbs lower for sheep than for goats (P < 0.05). The preference value for ragweed, measured in year 2, was 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 47 g/day), and totalBWgain 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.
© 2005 Elsevier B.V. All rights reserved. 48.3 for SR4, SR6, and SR8, respectively (S.E. = 2.16).
