Influence of the biomass, botanical composition and sward height of annual pastures on foraging behaviour by sheep.
Abstract
Merino sheep were given 24 h access to a plot that had 1 m2 patches of annual pasture that varied in biomass, height and botanical composition at Perth, Western Australia. The species involved were Vulpia myuros, Lolium rigidum, Trifolium subterraneum, Bromus diandrus, Erodium botrys and Arctotheca calendula. The study was carried out over 2 years and foraging behaviour was examined when the plants were in both vegetative and flowering stages of growth. Wt and height of each species were measured in each patch before and after grazing. Sheep were allowed to graze until approximately 50% of the total biomass available on the plot had been removed. There was a general trend to eat proportionately more from the patches having a high biomass, but this was more pronounced when overall biomass was low. In the first year the patches contained only a grass and clover, biomass was low and there was no discrimination between species. In the second year each patch contained 3 or more species and selectivity occurred even when biomass was low, with some species being preferred to others independent of the proportion of the species in the patch. Whilst proportion of a species in a patch was thus the major determinant of the proportion of that species in the total herbage removed from a patch, there were some secondary influences when the presence of one species either enhanced or depressed the consumption of another species. Results show that sheep grazing a pasture which varies spatially in biomass will tend to graze it to a uniform biomass by concentrating their grazing on areas with the highest yield. However, selection of species will occur particularly when there are several species present.