Grazing behavior and energy expenditure by sheep and goats co-grazing grass/forb pastures at three stocking rates

Abstract

A study was conducted to assess effects of stocking rate (SR) on grazing behavior and energy expenditure (EE) by growing sheep and goat wethers co-grazing grass/forb pastures. Grazing was for 16-week periods 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 that were rotationally grazed in 2-week periods. In weeks 3, 8, and 13 of both years, EE was determined on one goat and one sheep in each pasture via heart rate. In the same weeks, behavioral observations (position and activity) were made every 30 min of 13.5 h of daylight on two goats and two sheep in each pasture. Grazing behavior using IGER Grazing Behavior Monitoring System units was also measured over 24-h periods on animals used for EE measurement. Based on visual observations, grazing (52.7, 57.1, and 61.4%) and standing time (61.1, 66.3, and 69.8%) increased and idle time in daylight (24.2, 21.1, and 15.9% for SR4, SR6, and SR8, respectively) decreased linearly ( P < 0.05) as SR increased. Species interacted ( P < 0.05) with year in daylight time spent standing and ruminating. Grazing time during daylight was similar between species (56.1 and 58.0% for sheep and goats, respectively), although idle time was greater ( P < 0.05) for goats (23.6% versus 17.2%; S.E. = 1.41). Time spent ruminating in daylight was similar among SR but was greater for sheep in year 2 but not year 1 (year 1, 22.3% versus 19.0%; year 2, 27.8% versus 15.1% for sheep and goats, respectively; S.E. = 1.44). Based on the IGER units, the number of steps increased linearly ( P < 0.05) with increasing SR (2279, 2707, and 2788 for SR4, SR6, and SR8, respectively (S.E. = 96.4)), but was similar for the two species. As SR increased, time spent eating increased (7.4, 8.4, and 9.6 h) and time spent lying (11.0, 10.2, and 8.9 h), ruminating (7.9, 7.7, and 6.8 h), and idle (8.6, 8.0, and 7.6 h for SR4, SR6, and SR8, respectively) decreased ( P < 0.05). Goats spent less time eating (1.1 h) and more time idle (0.7 h) than did sheep ( P < 0.05). SR, species, and year interacted ( P < 0.05) in EE of wethers (year 1, sheep: 510, 569, and 572 kJ/kg BW 0.75; year 2, sheep: 572, 597, and 648 kJ/kg BW 0.75; year 1, goat: 524, 524, and 640 kJ/kg BW 0.75; year 2, goat: 499, 496, and 551 kJ/kg BW 0.75 for SR4, SR6, and SR8, respectively; S.E. = 17.0). In summary, influences of SR on grazing time and EE can vary with grazing season. With forage conditions of this study, SR had similar effects on grazing behavior of sheep and goats when co-grazing. Effects of SR on EE may contribute to impact on ADG by small ruminants.