Foraging in a heterogeneous environment—An experimental study of the trade-off between intake rate and diet quality

Among the factors determining the choice of feeding sites by herbivores, the rate of intake of net energy and nutrients is a major one. For grazers, patches of tall grasses are ingested faster, but are digested less fully because of their high fibre contents; conversely, short grass is highly digestible but allows only low intake rates. Herbivores therefore face a trade-off between the quantity and the quality of their food. Little empirical information is available to test whether optimisation models of patch selection are applicable to horses. An experiment was therefore designed where the trade-off between sward height and quality was explored to test explanatory models based on rates of nutrient acquisition (digestible dry matter, energy and/or protein intake rates). Three groups of two 2-year-old saddle horses were grazed on pasture managed to produce three swards differing in both height and quality (vegetative to reproductive stages). They were offered binary choices in a Latin-square design to assess preferences; daily intake was measured to determine the consequences of their choices. Instantaneous intake rates (IIRs) were determined from bite rates at pasture, and bite mass estimated using trays indoors. The taller sward matured during the experiment, so the quality differences between swards increased. The horses selected the taller sward in the first period, and the shorter alternatives in the following ones. The rates of digestible dry matter (DM) and net energy intake were always higher on the tall swards; digestible protein was the best predictor of the horses’ choices. Patch selection thus depended on the nutritional context: when digestible protein was not limiting, the horses selected patches where food was ingested faster, and when protein supplies declined, they maximised their protein intake rate. Preferences were however partial: as the shorter swards allowed higher rates of digestible protein intake but lower net energy than the taller swards, the horses may have been balancing their protein and energy intake by feeding on both swards; alternative explanations are discussed.