Hutchings, M.R., Kyriazakis, I., Gordon, I.J., and Jackson, F. (1999) 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. Journal of Animal Ecology, 68 (2). pp. 310-323.

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Abstract

1. Three experiments were carried out to determine whether a trade-off between the benefit of increased nitrogen content of swards and risks of parasitism 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 presented with pairs of experimental swards (36 x 21cm), which varied in nitrogen content (high = N +, low = N -) and level of contamination with faeces from sheep infected with Ostertagisa circumcincta (Ostertag) (20 g Faeces per sward = F +; no faeces = F -), and allowed to graze for short periods. For 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 together as the trade-off choice.

2. Experiment 1 presented one choice (N+F- vs. N-F-) three times to 20 sheep divided into four animal treatment groups resulting from two levels of feeding motivation (high and moderate) and two parasitic states (parasitized by O. circumcincta and non-parasitized), Experiment 2 presented four choices (N+F+ vs. N-F-; N+F+ vs. N-F+;N+F- vs. N -F-; N+F- vs. N-F+)repeated three times to the same animals and treatments.

3. Experiment 3 presented the above four choices three times each to 24 sheep in four animal treatment groups created from two parasitic states (parasitized by O. circumcincta and non-parasitized) and two immune states (immune 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 motivated animals in experiment 2. Both immune treatments took higher parasitic risks than both the naive treatments in experiment 3. Subclinical parasitism resulted in increased rejection of the N+F+ award, reduced bite rates and grazing depths compared to nonparasitized animals, thereby reducing further risk of parasitism in experiments 2 and 3. Non-parasitized, moderately feeding motivated (control) animals in experiment ? significantly rejected the N+F+ sward, adopting a low parasite risk strategy; nonparasitized, naive (control) animals in experiment 3 adopted a similar grazing strategy. In 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.

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