Deer are herbivores whose diet is intrinsically tied to the changing seasons and the specific plant life within their habitat. Understanding their natural foraging habits is crucial because their specialized digestive system is highly sensitive to abrupt changes or improper foods. The health and survival of wild deer populations depend on the availability of appropriate natural forage and the absence of harmful human-introduced substances. This article examines the natural diet of deer and highlights the dangers associated with feeding them foreign or toxic materials.
The Primary Natural Diet
The natural diet of deer is highly varied and shifts continuously throughout the year to maximize nutrient intake from available sources. Their feeding strategy is characterized by a preference for easily digestible, high-nutrient plants, classifying them as selective browsers. This means they prefer to nibble on the best parts of a wide variety of plants rather than grazing on large amounts of low-quality grasses like cattle.
The majority of a deer’s diet, often exceeding 85%, consists of three main categories: browse, forbs, and mast. Browse refers to the leaves, buds, and tender twigs of woody plants, such as shrubs and trees, which become a necessary staple during winter when herbaceous plants are scarce. Forbs are non-woody, broad-leaved herbaceous plants, commonly called weeds, providing high levels of protein and digestible energy during the spring and summer growing seasons. These leafy plants are important for antler growth in males and for pregnant or lactating females.
Mast, which includes both hard and soft varieties, is a favored seasonal food source. Hard mast, such as acorns and nuts, provides a dense pulse of fat and carbohydrates in the autumn to build up reserves for winter. Soft mast, including berries and wild fruits, is consumed in the summer and fall, offering sugars and vitamins. Deer are opportunistic feeders who consume hundreds of different plant species, but the bulk of their nutrition comes from these few high-quality, easily digestible sources.
Digestive System and Feeding Behavior
The deer’s digestive anatomy is that of a ruminant, featuring a four-chambered stomach that processes fibrous plant matter. The largest chamber, the rumen, functions as a fermentation vat filled with a specialized community of microbes, including bacteria, protozoa, and fungi. These microorganisms break down tough cellulose and plant fiber into volatile fatty acids and other compounds the deer absorbs for energy.
After swallowing partially chewed forage, deer retreat to a safe location to regurgitate and re-chew this material, known as “chewing the cud.” This mechanical process reduces particle size, making the food more accessible for the microbial population in the rumen. The composition of the gut flora is dynamic and shifts gradually to match the type of forage consumed.
Deer are considered “concentrate selectors” because their smaller digestive system requires them to seek high-quality, low-fiber forage for rapid and efficient digestion. This specialization explains why they cannot thrive on large amounts of low-quality, high-fiber grasses like bulk-grazing animals. This system is easily overwhelmed by an influx of inappropriate food.
Harmful and Toxic Foods to Avoid
A sudden shift to foods high in carbohydrates, such as corn, bread, pastries, and apples, poses a significant threat to a deer’s health. These high-starch items are rapidly fermented in the rumen, causing a swift overgrowth of acid-producing bacteria. The resulting high concentration of lactic acid lowers the rumen’s pH, leading to a condition called acidosis, or grain overload.
Acidosis destroys the necessary gut microbes and causes severe damage to the stomach lining, preventing nutrient absorption. The deer essentially starves with a full stomach and can die within 24 to 72 hours of consuming improper food. Processed human foods and grains should never be offered to deer.
Many common ornamental garden plants are highly toxic to deer, even in small amounts. Japanese or English yew (Taxus) shrubs contain potent cardiotoxins (taxines), which can cause sudden cardiac failure. Rhododendron and azalea species contain grayanotoxins, affecting the nervous and cardiovascular systems and leading to severe gastrointestinal distress. Oleander (Nerium oleander) is highly poisonous, containing cardiac glycosides that are lethal to most animals when ingested.
Health Consequences of Improper Diet
Consuming inappropriate food, or even a sudden abundance of high-quality food in winter, disrupts the delicate balance of the deer’s digestive system. When deer consume low-quality, non-nutritive material, such as too much low-fiber grass or woody material during periods of scarcity, they suffer from nutritional stress. This is often misidentified as starvation, because the stomach may be full, but the contents cannot be properly digested for energy.
The sudden introduction of processed grain, which causes acidosis, leads to severe dehydration, diarrhea, and a loss of coordination. Artificial feeding sites cause deer to congregate unnaturally, which increases the risk of spreading contagious diseases. Diseases such as Chronic Wasting Disease (CWD) are easily transmitted through contaminated saliva, urine, and feces left behind at crowded feeding locations. Supplemental feeding, even with the best intentions, compromises the animal’s natural ability to forage and weakens the health of the herd.