Diet preference for grass and legumes in free-ranging domestic sheep and cattle: Current theory and future application

This paper reviews the current theory and potential practical applications of research on the diet preference for grass and legumes in grazing domestic sheep and cattle. Although much of this work has focussed on grass and clover as a model system, it has wider theoretical implications and potential for practical exploitation. Research in this field is of particular relevance with the recent increased interest in maintaining and enhancing biodiversity, both in agricultural systems and semi-natural habitats. One of the most consistent findings of previous research is that sheep and cattle both eat mixed diets, showing a partial preference of approximately 70% for clover. There is a diurnal pattern to preference, with a stronger preference for clover in the morning, with the proportion of grass in the diet increasing towards the evening. Research has also shown that sheep and dairy cattle achieve higher intakes from grass and clover when these are offered as separate monocultures compared with animals grazing a traditional mixed sward. The initial findings were from studies where the animals had free choice, but similar results have been achieved in dairy cows being allocated to clover between morning and afternoon milking and grass for the remainder of the day. The intake benefits, which have been attributed to a lower selection cost, have the potential to be exploited on-farm to increase intake and production. Our increasing understanding of the factors influencing diet selection raises the possibility of developing grazing management practices to maintain and possibly enhance biodiversity. Various theories have been proposed to account for the fact that ruminants eat mixed diets. Although some, such as spatial memory and visual discrimination have been discounted, others, such as perceived predation risk and balancing nutrient intake, appear to be more valid. However, further research is still needed to explore and validate hypotheses related to these theories.