Importance of nutritional and anti-parasite strategies in the foraging decisions of horses: an experimental test

The primary goal of foraging herbivores is to maximise the net rate of intake of digestible energy (or of a limiting nutrient). However, foraging strategies of herbivores are also sensitive to other selective forces (e.g. predation, parasites), which may modify their choice of feeding patches. Horses feed in spatially complex swards, and allocate their time among patches which vary both in terms of their nutritional benefits, and the risk of parasitism. It has long been suggested that horses allocate time among patches principally in relation to the risk of parasitism, though the nutritional costs and benefits of different foraging choices must play some role too. In this study, we investigated the roles of nutritional and anti-parasite factors in foraging decisions by horses. Six naturally parasitized and six unparasitized two-year-old geldings were allowed to graze from pairs of trays (112x72 cm) with swards at two heights (tall: 52 cm, medium: 15 cm) and two levels of contamination (280 g m(-2) of faeces, no faeces) in a latin square, pair-wise design with six different choices. In nutritional terms the differences between the swards were slight: the tall sward provided larger bite sizes and higher intake rates (+36%). Feeding motivation was not high, and the faecal stimulus was strong. The horses selected uncontaminated swards when the nutritional benefits were identical, and tall swards in virtually all circumstances. Contamination of the preferred tall sward (i.e. a test of the tradeoff between anti-parasite and nutritional strategies) had little effect on its selection by the horses. Their parasite status also had little effect: both groups selected the tall sward in the tradeoff (though this was only a trend for the non-parasitised group in one analysis). We conclude that patch selection by horses in these particular conditions was driven principally by their nutritional strategy.