Effect of species composition and sward structure on the ingestive behaviour of cattle and sheep grazing South African sourveld

The effects of species composition and award structure on the ingestive behaviour of cattle and sheep grazing a mesic, low quality grassland in South Africa were investigated over the 1990-93 grazing seasons. Species composition had a significant (P < 0.05) effect on sheep bite size and on cattle and sheep bite rates but had no effect on dry matter intake rate (IR). Species composition could, however, affect IR over longer grazing periods than those used in the experiment. Sward structure had a major effect on ingestive behaviour. Cattle and sheep bite rates and cattle grazing time, were negatively correlated (P < 0.05) with plant height but positively correlated (P < 0.05) with sward greenness. Bite size and hence IR in cattle and sheep were strongly correlated (P < 0.001) with plant height. Cattle IR increased from 6 to 20 g/min over the range of heights encountered and appeared to reach an asymptote at a plant height of 20-25 cm. Sheep IR, expressed per unit of body mass, increased from 0.01 to an asymptote or maximum of 0.13 g/min/kg at plant heights of 10-15 cm. For sheep there was evidence of a non-asymptotic functional response at some sites with IR being maximized at certain sward heights but declining thereafter. This suggests the possible existence of a third, quality dimension to the functional response on these low-quality grasslands.