Foraging in patchy grasslands: Diet selection by sheep and cattle is affected by the abundance and spatial distribution of preferred species

Abstract

Understanding the distribution of grazing activity and its management is valuable to ensure the sustainability and productivity of heterogeneous grasslands. Controlled behavioural studies can provide insight into the cognitive abilities of herbivores and suggest new approaches to improve their grazing distribution. We compared the behaviour and diet selection of sheep and cattle in 1-ha fescue (Festuca arundinacea S.) plots, in which the number and size of preferred ryegrass (Lolium perenne L.) patches were varied. Five different patterns were used to investigate the effects of ryegrass abundance and spatial distribution on the searching success of the herbivores. Both heifers and ewes grazed ryegrass more when its abundance increased from 1.5 to 6%, and also when ryegrass was aggregated into a few 8 × 9-m patches, rather than being dispersed into a larger number of 3 × 3-m ones. Neither the interaction between ryegrass abundance and patch size, nor that between herbivore species and patch size was significant. At 6% abundance, aggregating ryegrass into a single 24 × 24-m patch tended to further increase its use by heifers, but not by ewes. Fescue less than 1 meter from the nearest ryegrass patch was usually selected more than the fescue located further away. Fescue between 1 and 5 m from the nearest patch was never selected more than the fescue located further away. Our results, together with those for the diet selected by sheep, cattle and deer, when offered patches of a preferred food in aggregated or dispersed patterns, indicate that over a wide range of abundance, it is advantageous for herbivores to forage on patchy resources when the preferred vegetation is aggregated. This corroborates the prediction that foraging costs associated with patch distribution are involved in determining diet selection in patchy grasslands, but our results suggest that it is also influenced by some social characteristics of the animals, such as their social attraction and social tolerance.