Deer eat twigs, branches, leaves, fruits, grass, nuts, alfalfa and fungi, depending on the availability and the season. During the spring, deer prefer sweet clover, wild beans and poison ivy, while in the winter they subsist on coralberry, wheat and rye.
During the summer, droughts and rainfall can bring out extremes in deer eating patterns. Deer are preferential grazers but have also evolved to eat only the parts of plants that are nutritious and palatable, such as when they eat the seed heads and grain of sorghum.
The yearly diet of deer is heavily dependent on the availability of certain plants. During the fall, deer prepare for the winter months ahead by foraging for high-fat foods such as acorns and other nuts, as failing to build an adequate fat supply can result in a deer struggling to survive the cold. To combat deer mortality rates in the winter, some humans embark on supplemental feeding programs that are often detrimental to the overall deer population. Deer are sensitive to changes in diet, taking one to two weeks to adjust without illness, and a limited quantity of food often causes them to compete with each other for resources, leading to the starvation of fawns and other weaker deer.