Selection by grazing sheep of pasture plants at low herbage availability and responses of the plants to grazing

Abstract

Merino sheep grazing annual pasture at the beginning of the growing season when the amount of herbage on offer was small, preferred to graze Wimmera ryegrass Lolium rigidum or subterranean clover Trifolium subterraneum rather than capeweed Arctotheca calendula, and Erodium botrys was avoided completely. Behaviour observations showed that capeweed plants were usually avoided. When the plants were grasped they were sometimes pulled up by the roots and then dropped so that the number of capeweed plants in the pasture declined. Supplementation with oats reduced grass intake. Harvesting behaviour changed with pasture conditions: as grass height declined in the pasture, the rates of biting, stepping and head swinging increased. Pasture measurements showed that, whilst capeweed plants continued to increase in height during grazing, as did ungrazed controls, ryegrass and clover plants decreased or remained short. Herbage dry matter increased in all species, owing especially to basal growth. The proportion of shoots and petioles which were erect increased in ungrazed plants, but the proportion which were prostrate was much greater in grazed plants. Individual plants adapted their growth form in a way which counteracted the depredations of grazers. The ecological implications of these findings are important. Firstly, the sheep were not foraging optimally in terms of maximising rate of intake, since two abundant species were largely ignored even though food availability was low. Secondly, because of their selectivity the sheep were giving the capeweed and Erodium a competitive advantage which, in these pastures, will persist through the growing season.