Deer are widespread herbivores classified as “concentrate selectors” or “browsers.” This means they are highly selective eaters, consistently seeking the most nutritious and easily digestible parts of plants rather than grazing broadly like cattle. Their feeding habits and digestive anatomy allow them to thrive across diverse landscapes. Understanding their diet is a study in adaptation, driven by seasonal availability and biological needs.
The Core Diet of a Ruminant
The foundational diet of a deer is composed of three primary categories: browse, forbs, and mast. Browse refers to the tender leaves, buds, and terminal twigs of woody shrubs and trees, constituting a large portion of their year-round consumption. This category is always available, though nutritional quality drops significantly in winter.
Forbs are broad-leafed, non-woody plants, commonly called weeds, which are a highly sought-after food source. Examples include clover, ragweed, and various herbaceous plants. Forbs provide a high concentration of protein and easily digestible energy, making them preferred due to their superior nutrient profile, especially during spring and summer.
Mast includes the fruits and nuts produced by woody plants, serving as a critical energy reserve when available. Hard mast, such as acorns and hickory nuts, is rich in fats and carbohydrates, making it crucial for fat storage before winter. Soft mast, like berries and wild fruits, offers a quick source of sugars and vitamins during warmer months.
As selective feeders, deer do not simply graze on whatever is present. Browse, forbs, and mast make up over 85% of their overall diet, with grass consumption being relatively low. This strategy allows them to maximize nutrient intake with a digestive system built for high-quality, low-fiber forage.
Seasonal Shifts in Foraging
A deer’s diet shifts dramatically throughout the year in response to seasonal plant growth and physiological demands. During spring and summer, the focus is on growth, antler development, and reproduction, necessitating a high-protein diet (16% to 18% crude protein). Foraging concentrates on protein-rich forbs and the most succulent new growth of browse.
As autumn approaches, the focus shifts toward energy storage, preparing for the breeding season and cold months. Deer actively seek out hard mast like acorns to build up fat reserves, which are a concentrated source of carbohydrates and fats.
Winter represents the greatest nutritional challenge, as high-quality food sources become dormant. Deer must rely heavily on low-quality, high-fiber woody browse, such as the twigs of cedar or hemlock. Their metabolism slows down, and they depend on accumulated fat reserves to survive until spring growth returns.
Understanding Deer Digestion
Deer are classified as ruminants, possessing a multi-chambered stomach system adapted for breaking down plant matter through fermentation. The first and largest chamber is the rumen, which acts as a massive fermentation vat. This chamber houses billions of specialized microbes, including bacteria and protozoa, responsible for all primary digestion.
The deer initially swallows food quickly with minimal chewing, storing it in the rumen where microbes begin breaking down cellulose. Later, the deer regurgitates this partially digested material, known as cud, to chew it more thoroughly (rumination). This action reduces particle size and mixes the food with saliva, aiding fermentation.
Once re-swallowed, the food passes through the other three chambers: the reticulum, omasum, and abomasum. The reticulum filters out large particles, sending them back to the rumen, while the omasum absorbs water and minerals.
Finally, the contents move into the abomasum, the “true stomach” analogous to that of non-ruminants. Here, hydrochloric acid and digestive enzymes break down the microbes themselves, providing the deer with a large portion of its protein and nutrients. This specialized system allows the deer to extract energy from otherwise indigestible plant material.
Foods to Avoid and Supplemental Feeding
Interfering with a deer’s natural diet by providing human foods can have severe consequences. The specialized microbial community in the rumen is highly sensitive to abrupt dietary changes. Introducing high-carbohydrate foods like corn, bread, or other grains too quickly can destroy this delicate balance.
When a deer consumes a large quantity of high-starch food, the sudden influx of carbohydrates causes a rapid proliferation of acid-producing bacteria. This quickly lowers the pH of the rumen, leading to a condition called acidosis, or “grain overload.”
The acidic environment kills the beneficial, fiber-digesting microbes, causing the rumen to shut down. Acidosis results in severe dehydration and ulceration of the rumen lining, preventing nutrient absorption. A deer can starve to death with a full stomach because its digestive system is no longer functional. Supplemental feeding, especially during winter, is strongly discouraged due to the high risk of causing debilitating or fatal digestive upset.