Effect of long-term changes in relative resource availability on dietary preference of sheep for perennial ryegrass and white clover

Three replicate paddocks, each of 0.235 ha, containing adjacent monocultures of perennial ryegrass or white clover [50:50 by ground area, 6 cm sward surface height (SSH) at start of experiment] were continuously stocked with three yearling and four mature non-lactating, non-pregnant Scottish halfbred ewes for 12 weeks. Herbage intake, grazing behaviour and dietary selection were measured on seven occasions. Clover SSH declined rapidly over the first 5 weeks then stabilized at 1.2-1.6 cm, whereas perennial ryegrass SSH rose slightly initially, then declined gradually. Animals initially included proportionately c . 0.6 white clover in their diet but, by the end of the experiment, this had fallen to 0.3. Total daily herbage intake declined over the 12 weeks from 1.8 kg dry matter (DM) day(-1) at the start to 1.0 kg DM day(-1) . Total grazing time increased from 561 min day(-1) to 649 min day(-1) at the end of the experiment. The results suggest that, despite overall herbage depletion and a greater depletion of white clover than perennial ryegrass as a result of the initial partial preference for white clover, the animals traded-off a reduced total intake and an increased grazing time in an attempt to maintain their initial preferred dietary composition.