Trade-offs between nutrient intake and faecal avoidance in herbivore foraging decisions: the effect of animal parasitic status, level of feeding motivation and sward nitrogen content

1. Three experiments were carried out to determine whether a trade-off betw een the benefit of increased nitrogen content of swards and risks of parasi tism exists in a grazing situation for sheep (Ovis aries L,). The effect of level of feeding motivation and parasitic status on the grazing behaviour of sheep faced with this trade-off was also investigated. Animals were pres ented with pairs of experimental swards (36 x 21cm), which varied in nitrog en content (high = N +, low = N -) and level of contamination with faeces f rom sheep infected with Ostertagisa circumcincta (Ostertag) (20 g Faeces pe r sward = F +; no faeces = F -), and allowed to graze for short periods. Fo r this study, a 'high parasite risk' grazing strategy is defined as taking more bites from an N+F+ sward compared to an N-F- sward, when presented tog ether as the trade-off choice. 2. Experiment 1 presented one choice (N+F- vs. N-F-) three times to 20 shee p divided into four animal treatment groups resulting from two levels of fe eding motivation (high and moderate) and two parasitic states (parasitized by O. circumcincta and non-parasitized), Experiment 2 presented four choice s (N+F+ vs. N-F-; N+F+ vs. N-F+;N+F- vs. N -F-; N+F- vs. N-F+)repeated thre e times to the same animals and treatments. 3. Experiment 3 presented the above four choices three times each to 24 she ep in four animal treatment groups created from two parasitic states (paras itized by O. circumcincta and non-parasitized) and two immune states (immun e to O. circumcincta and naive). 4. All animals selected the high nitrogen sward over the low nitrogen sward in experiment 1. When presented the trade-off choice, animals with a high level of feeding motivation took greater parasitic risks, than moderately m otivated animals in experiment 2. Both immune treatments took higher parasi tic risks than both the naive treatments in experiment 3. Subclinical paras itism resulted in increased rejection of the N+F+ award, reduced bite rates and grazing depths compared to nonparasitized animals, thereby reducing fu rther risk of parasitism in experiments 2 and 3. Non-parasitized, moderatel y feeding motivated (control) animals in experiment ? significantly rejecte d the N+F+ sward, adopting a low parasite risk strategy; nonparasitized, na ive (control) animals in experiment 3 adopted a similar grazing strategy. I n each of the remaining three choices in both experiments one sward held 3 clear nitrogen or faecal benefit (absence of faeces) which was selected for by all treatments.