Sheep avoidance of faeces-contaminated patches leads to a trade-off between intake rate of forage and parasitism in subsequent foraging decisions

We tested the hypotheses that the avoidance of faeces by herbivores leads to a grazing trade-off between nutrition and parasitism, and that the relative costs and benefits of the trade-off in relation to animal parasitic state determines their subsequent grazing behaviour. We divided 30 female sheep, Ovis aries, into three treatment groups, each of 10 animals. A parasitized and an immune treatment were created by daily dosing two groups of 10 sheep with the abomasal parasite Ostertagia circumcincta, for 3 weeks and 14 weeks before the experiment, respectively. The remaining sheep were maintained as nonparasitized (controls). Each of the three treatment groups was divided into two subgroups, each of which was placed in an experimental field plot and rotated around on a daily basis for 2 weeks. Each experimental plot was divided into a chequerboard of 100 patches with alternate patches contaminated with sheep faeces, Initially, all animals significantly selected to graze the noncontaminated patches resulting in disproportionate sward depletion. The significant avoidance of the faeces-contaminated patches by parasitized and nonparasitized sheep remained throughout the experiment. However, immune animals reduced their avoidance of faeces-contaminated patches during the latter stages of the experiment, resulting in a nonselective grazing strategy by the end. Compared with nonparasitized control sheep, immune sheep had increased rates of herbage intake and activity, and vice versa for parasitized sheep. We discuss the differences in grazing behaviour in relation to the sheep's state of parasite infection and the relative costs and benefits of the grazing trade-off. The behaviours used by herbivores to avoid the ingestion of parasites affects sward structure, creating a grazing trade-off between nutrition and parasitism, their response to which will determine their subsequent intake of nutrients and parasites, and thus fitness and survival