White-tailed deer eat a variety of foods, but browse makes up the majority of their diets. The whitetail dietary flexibility stops at grass, which they eat very little of. Whitetail did not develop into grazers like some of the other deer species. In addition, they did not develop the special teeth or stomachs that can efficiently grind up and digest the tough fibers in grasses (like the horses and bovines did for example). The types of deer that do graze (like the axis deer) prefer to follow behind the coarse grass grazers so they can eat the new-sprouting, more-tender shoots that spring up after the first “mowing.”
Whitetail, like all deer, have incisor teeth (the cutting teeth in front) on only the bottom jaw, and a cartilage pad on the front of the upper jaw (They have molars on both upper and lower jaws.) This tooth pattern causes them to pull out the grass rather than to cut it like the specialized grazers do. The tender base of the grass is low in fiber, more nutritious and more digestible. So while whitetail can digest some of the grasses’ most tender shoots, overall they would not thrive on grass alone.
Whitetail foods, for the most part, consist of a huge variety of low fiber foods, they are “concentrate selectors”. They eat tender shoots and leaves from all sorts of trees, vines, plants and bushes; fruits, vegetables, nuts (acorns are a real favorite), grains, mushrooms (a gourmet treat to deer) and mosses. Here in the South they eat Spanish moss (which is actually a bromeliad) and up in the cold climates in winter they eat frozen vegetation that has turned into a nutritious natural silage.
Many of the foods, even the good ones, that deer eat have poison in them, for example, tannin in acorns and oaks. But the foods are eaten in small amounts or are eaten when the plants are young so that they have a lower amount of toxins, and in some cases the toxins are balanced and neutralized by other vegetation so that they are not harmful. When tender shoots mature, they become tougher, have more fiber, have less nutrition and are more toxic, this is how the plants protect themselves, and how they discourage animals from eating them.
The deer’s diet can turn into a starvation diet if food runs out in winter, and deer will then eat the undesirable mature forms of the previously delectable foods, and in fact will eat almost anything. At times a starving deer will have a belly filled with food that it cannot digest and will die with a full stomach.