Diet choice in grazing sheep: A new approach to investigate the relationships between preferences and intake-rate on a daily time scale

For the herbivore grazing heterogeneous vegetation, optimal foraging theory has been proposed to predict which food components would be included in the diet on the basis of their relative profitability. We investigated the relationships between preferences and intake-rate in four groups of six ewes grazing down during 10 days conterminal monocultures of Lolium perenne L. (L)–Festuca arundinacea L. (F) 0.50:0.50 by area, at a stocking rate of 92 ewes ha−1. Both herbage species were initially offered at the same height (228 mm). The experimental design was repeated over 2 years, with two groups of ewes in each year.Diet composition and time spent grazing each species were measured daily for all animals, using the n-alkane technique and automated photographs. The organic matter intake-rate ratio between L and F (OMIRL/OMIRF) was calculated from the proportion of L in the diet (xL) and the proportion of grazing time spent on L (yL) using the following equation: OMIRL/OMIRF = (yL(1 − xL))/(xL(1 − yL)). The digestible organic matter intake-rate ratio between L and F (DOMIRL/DOMIRF) was calculated from OMIRL/OMIRF and OM digestibility of the grazed horizon in each species.There was an initial marked preference for L, but the preference for L declined linearly until day 6 of the trial, after which there was no preference for either species. The decrease in preference for L was mediated via an increase in the time spent grazing F rather than a decrease in the time spent grazing L. Ratio of digestible OM intake-rate between L and F (DOMIRL/DOMIF) was higher than unity before day 5, and close to unity thereafter. The number of transitions between the two herbage species during feeding bouts increased linearly from the beginning of the depletion until day 5, at which time it was doubled, then it levelled off. This pattern probably reflected the level of difficulty for the animal in choosing between both species, the highest difficulty occurring when alternatives offered a similar DOMIR. These results are consistent with the optimal foraging paradigm, but further research is needed to support the direction of causality between DOMIR and preferences. We observed a marked interindividual variability in preferences, highlighting the importance of the number of animals under study.