Factors affecting diet selection by sheep. 2. Height and density of pasture

Abstract

Boards of artificial pasture were prepared by threading the top two leaves of tillers from either ryegrass or kikuyu grass, in their vegetative stage of growth, through 5-mm-diameter holes in pressed hardwood sheets. The holes were spaced in rows from 10 to 50 mm apart, and, with one, two or three tillers per hole, tiller density ranged from 346 to 25 980/m2. Sward height was varied by pulling tillers different distances through the holes before they were fastened to the underside of the boards, and the mass of herbage dry matter ranged from 0.04 to 7.61 t/ha. In series I experiments, the effects of sward characteristics on ingestive behaviour of two sheep were examined, whereas, in series I1 experiments, pastures that were consumed at different rates were offered in pairs and the preference of one sheep observed. The rate of pasture intake was related to sward height only when tiller density was constant and to herbage mass per unit volume (bulk density) only at similar sward heights. Intake rate was better described by herbage mass per unit area but, at herbage availabilities of less than 1 t/ha, intake rate was several-fold greater when sheep grazed tall, sparse pastures than short, dense pastures. The best predictor of pasture intake rate was herbage mass per area effectively covered by one bite. Intake per prehending bite declined with a reduction in both sward height and density, and was also best described by herbage mass per area effectively covered by one bite. Prehending bite rate doubled as intake per bite declined from 200 to 10 mg dry matter. The rate of jaw movements during grazing was virtually unaffected by sward characteristics; prehending bites accounted for 20% of jaw movements when intake per bite was 200 mg dry matter and 80% of jaw movements with short, widely spaced swards. When two pastures with different sward characteristics were offered together, sheep generally preferred the one they could eat faster.