Foraging in a landscape mosaic: selection for energy and minerals in free-ranging cattle

Several studies have indicated the potential importance of nutrients, other than energy, in determining foraging decisions. A model was developed to test this idea, on the assumption of an intake maximization for different nutrients (energy, sodium and phosphorus). The model predictions were tested using field data from cattle grazing in a landscape mosaic of Pleistocene cover-sand and riverine grassland. Observations on foraging behaviour, food intake and diet composition were collected in thirteen 4-day-periods over 2 years. Habitat selection was determined by comparing the proportion of grazing time in different vegetation units with the available area proportion of the units. Two levels of habitat selection were examined: a micro-level (fine-scale, where vegetation units were considered separately) and a macro-level (coarse-scale, where vegetation units were combined to give selection at the landscape level). At the micro-level of habitat selection, no selection was apparent between the vegetation units of the riverine landscape, but the Deschampsia flexuosa unit was significantly selected for in the cover-sand landscape. At the landscape (macro-) level, the animals preferred the riverine landscape. The model revealed poor predictions of habitat occupancy on a micro-level. A much better prediction was obtained when vegetation units were combined at a macro-level. The D. flexuosa unit provided a higher energy intake, whereas the intake of sodium was higher in riverine grassland. Phosphorus proved relatively significant in determining habitat occupancy. Based on energy maximization alone, the model was a very poor predictor of habitat occupancy. It is argued that selection occurred mainly at the macro-level. The incorporation of different nutrient constraints in foraging models can then prove fruitful when seeking explanations of habitat occupancy. At the micro-level, difficulties for the animal in assessing nutrient availability may result in a less selective foraging pattern. However, the costs of increased selectivity may be greater than the benefits.