FASTING AFFECTS INTAKE BEHAVIOR AND DIET PREFERENCE OF GRAZING SHEEP

In separate experiments, the effects of fasting on the intake behaviour and diet preference of sheep, Ovis aries, were explored by comparing animals that had been fasted for 24 h with control animals that had not been fasted. Intake and its components (prehension bite rate and bite mass) were compared in animals grazing from monocultures of perennial ryegrass, Lolium perenne, and white clover, Trifolium repens. The effects of fasting on diet preference were studied by releasing flocks of three sheep onto 0·25 ha plots of land, which were either 20% clover (and 80% grass) or 80% clover (and 20% grass), by area. Within each of the 12 flocks, the diet preference of one focal animal was recorded on the first and third days following fasting. Fasting led to a significant increase in the intake rate (42% higher for clover, 72% higher for grass), achieved largely by an increase in bite mass. Both fasted and non-fasted animals had a significantly greater intake rate when eating clover than when eating grass, but fasted animals spent a significantly lower proportion of their time grazing clover (82% versus 92% for non-fasted animals). These results show that fasting not only increased the animals' drive to eat (grazing time and intake rate) but also altered the diet composition. This study lends support to the use of state-dependent modelling for grazing behaviour and suggests that researchers should be wary of studies of diet selection or preference in which experimental animals were fasted. © 1994 The Association for the Study of Animal Behaviour.