While white-tailed deer are ruminants like cows, but their diet selection is much different. Cattle are grass-roughage eaters, have a relatively large rumen compare to body size, and depend heavily on grasses for their diet. Grasses are relatively low in crude protein and digestibility when compared with legumes or forbs (broadleaf weeds).
Because of these nutritive parameters, grasses have a longer residence time in the cow rumen. Longer residence time increases rumen microflora (bacteria and protozoa) degradation and digestion of the forage. Thus, for grass-roughage eaters like cattle and sheep, residence time is quite long and rate of food passage is slow.
White-tailed deer, on the other hand, are what people diet business call “concentrate selectors.” Their rumen is small relative to body size. Thus, their diet must be higher in nutritive value and capable of being rapidly degraded in the rumen. Therefore, white-tailed deer rely primarily on forbs and browse (leaves and twigs of wood plants), which are usually higher in crude protein and digestibility than grasses. Keep this in mind: grasses comprise only a very small part of the overall diet of the white-tailed deer, usually less than 10%.
Warm season perennial grasses that supply cattle with msot of their nutrients will not meet the nutrient and consumption requirements of white-tailed deer. Thus, good habitat combined with supplemental feeding should always be considered for proper deer herd health.
Only grasses that are rapidly degraded in the rumen, such as the small grains and ryegrass, are used to any extent by deer. This is why winter food plots containing these grasses work. Other useful introduced forages include both warm and cool season legumes. Native plants used by white-tailed deer include browse, forbs, soft and hard mast (fruits, acorns), and mushrooms.
Forbs and mast, while providing good nutrition, may not be available each year or at times of the year. Browse is usually the most important source of deer nutrition because of year-round availability.