The diet selected by deer is a combination of their foraging behaviour, forage preference and feed availability.
Red deer are best described as a mixed feeder, or somewhere between a browser and a grazer. This determines the diet they can select, the amount of food they can eat, rumen size and function as well as the amount of roughage they can eat. A true grazing animal (sheep) is ideally suited to grazing grass and legume pasture while a true browser (moose, roe deer) selects foliage (leaves, young tips, seeds etc.). Red deer therefore exhibit some of both which gives them the benefit of some flexibility in their diet.
This also means that deer have a smaller mouth, and therefore a smaller bite size, relative to their body size. In practice this relates to a longer grazing time compared to sheep on a similar pasture as well as a preference for the higher quality parts of the pasture to reduce the potential time spent grazing. In addition taller swards (greater than 6cm) allow deer to meet there feed requirements more readily.
Deer have also been shown to increase grazing time, bite size and rate depending on their physiological state (pregnancy, lactation) and the feed supply. During late lactation, when their energy demand is greatest, a hind will spend about 30% more time grazing than in early gestation. The type of forage also impacts on the deer’s diet as bite size and rate will differ between native hill pasture, improved perennial ryegrass pasture and a shrub/forb pasture. Forbs are any herbaceous plant in grassland that isn’t a grass. On hill pasture the deer are likely to take fewer bites and compensate by grazing longer to meet their requirements. Similarly bite size may be lower on grass pasture compared to a shrub/forb pasture. Chicory, red clover, plantain and lucerne as well as fodder trees may be included in a shrub/forb mixture.
Therefore the preference deer have shown for red clover, chicory and lotus over grasses can be explained by their higher quality and how easily they can be selected and eaten.