Density-dependent diet selection and body condition of cattle and horses in heterogeneous landscapes

For some decades, grazing by cattle and horses is used as a management tool to achieve different nature management goals. For managers there are still questions to be answered about the effects of herbivore densities on their performance, vegetation development and biodiversity. This study examines the effect of density on diet composition, diet quality and body condition of cattle and horses. We expressed density as the ratio between consumption and net primary production of the preferred grasslands. Over a period of one year, we studied sward height and diet composition, diet quality and body condition of free ranging cattle and horses in two different study areas with different ratios between consumption and production. Our results showed that the amount of preferred high quality grasses in the diet of cattle and horses was lower when herbivore density was higher. As a result diet quality was lower and as a result of that body condition was affected. In October body condition of cows was lower and in March body of cows and mares was lower in the high density area. A striking difference between cattle and horses was that during the growing season and at high densities, the amount of preferred grasses in the diet of cattle decreased whereas that of horses increased. This was most likely caused by sward height which became probably too low for cattle. As cattle prefer grass heights of 9-16 cm, grass heights lower than those make it difficult for cattle to achieve a sufficient instantaneous intake rate. This means that in homogeneous areas and at high herbivore densities, horses can outcompete cattle. In this paper the effects of density dependent diet selection on vegetation development and conservation management are discussed.