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The quantity and quality of food, as well as the whitetail’s behavior, physiology and nutritional requirements change with the seasons. Here’s how it all comes together.
The population dynamics and physical welfare of deer depend on their nutritional status. With good nutrition, does are extremely prolific, populations flourish and bucks achieve maximum body and antler growth. Conversely, poor nutrition leads to physically stunted deer that experience poor productivity and high natural mortality rates.
Forest management traditionally served as the primary means of improving deer nutrition, especially on public land. Today, outdoor magazines are filled with articles (and advertising) extolling the importance of food plots and commercially available food and mineral supplements in order to artificially enhance deer nutrition.
All too often, however, the whitetail’s inherent biological rhythms are rarely mentioned. Unlike domestic livestock, the whitetail’s behavior, digestive processes, physiology — and resultant nutritional needs — change seasonally. Individual requirements also vary according to the animal’s sex, age, reproductive state and environmental pressures.
One cannot manage whitetails successfully without a basic understanding of how deer adjust, behaviorally and physiologically, to meet their nutritional needs, which change with the seasons.
Rumination and Fermentation
Whitetails are ruminants — cud chewers adapted to eating vegetation — and possess a compound, four-chambered stomach.
The first chamber, also the largest, is the rumen, where food is stored before being brought back up in “cuds” to be chewed. This system allows for fast and selective food gathering, a large capacity for food storage, and leisurely cudding and chewing — a clever “eat now, chew later,” adaptive anti-predator strategy.
After being regurgitated and chewed, food goes back into the rumen and reticulum chambers, where fermentation by microorganisms (bacteria and protozoa) produces nutrients that can be readily absorbed and used for energy before residual materials pass into the other chambers, the omasum and abomasum.
Highly digestible food might pass through the rumen in a few hours, whereas more fibrous or lignified (woody) material might remain in the rumen for days. Generally speaking, the more nutritious the food, the faster it passes through the digestive system. Obviously, the more food a deer can consume, the more nutrients it can assimilate, and the faster it will grow and gain weight.
The fermentation process, or breakdown of food by the rumen microbes, is the main difference between ruminants and simple-stomached animals. One advantage of such a digestive system is that it allows an animal to digest cellulose and other complex carbohydrates found in browse and other fibrous foods typically consumed by deer during the winter. This means deer can meet their energy needs from nutrients consumed in food, plus those synthesized by the bacteria and protozoa that live in their rumen.
Selective Feeders
Whitetails are highly selective feeders. Like other ruminants, they require some fiber in their diet for normal rumen function. However, unlike moose and elk, or domestic livestock, deer have comparatively less rumen storage capacity and less ability to digest highly fibrous or lignified materials. They must also feed more frequently.
As a result, whitetails must be more selective in their feeding habits, searching for and consuming the most nutritious and easily digested plants. Normally, they will muzzle or hold plant parts in their mouth, swallowing those that are succulent and easily digestible but rejecting others that are dry and high in fiber.
There’s also evidence deer can detect and avoid eating compounds that inhibit the action of rumen microorganisms. At low concentrations, these so-called “secondary compounds” seem to have little or no impact on rumen function, making whitetails’ habit of eating small amounts of a variety of plants a natural safeguard against consuming too much of any toxic substance.
Robert Brown, a professor at Texas A&M University, also observed that deer have a special problem with lignin.
“Not only is it indigestible,” Brown notes, “it can make other nutrients in the food less digestible by binding to them. And secondary plant compounds such as tannins and other phenolics can make both protein and cellulose less digestible.”
The whitetail’s diverse nutritional requirements largely explain why deer forage the way they do. That’s why they walk along slowly, eating “some of this” combined with “a little of that,” thereby unknowingly selecting the proper mix of nutrients to meet their immediate dietary needs.
Response to Change
The whitetail’s feeding habits are extremely variable and opportunistic, in addition to being highly selective. Their diverse feeding habits change with the seasons, allowing them to choose a wide variety of foods, including grasses, sedges, fruits, nuts, forbs and mushrooms, in addition to portions of those shrubs and trees that best meet their nutritional requirements.
Because their diet changes so dramatically with the seasons, it’s also important to note their digestive tract can change with diet, but gradually so. The amount of saliva produced, the lining of the rumen, and the rumen’s size, for example, change seasonally to compensate for the shift from eating succulent summer forage to a more fibrous winter diet, and back again to more luscious foods with spring green-up. However, it takes from two to three weeks for the rumen microbes to completely adjust to a new diet.
Ruminants have one important advantage over simple-stomached animals: Their rumen microbes can actually produce protein. When a deer’s diet lacks high-quality proteins, the microbes can simply create them using whatever amino acids and other nitrogen are available. Whereas simple-stomached animals must consume a sufficient amount of food containing high-quality proteins, a whitetail only has to be concerned with the quantity of protein in its diet. The rumen microbes can compensate for deficiencies in protein quality.
Spring: Metabolism Rises
Commencing about mid-March, in response to increasing hours of daylight (photoperiod), deer change immensely in basic physiological processes and general behavior. Their metabolism rises and they become more active. Pregnant does carry rapidly growing fetuses, young animals resume body growth and adult bucks start growing antlers.
It’s a time when copious amounts of nutritious forage in the form of succulent new herbaceous growth high in protein, energy and essential minerals and vitamins is essential to herd welfare.
According to Professor Brown, bucks require a diet consisting of 13 to 16% protein for favorable antler growth. He notes, however, like any ruminant, “Should his protein intake be low, the buck can recycle urea — a nitrogen-rich by-product of protein metabolism normally excreted in urine — from the rumen. Then rumen microbes can use the nitrogen to build proteins.”
Bucks also require minerals and vitamins for antler growth, but researchers still debate the exact amount. One study indicated a diet containing as high as 0.64% calcium and 0.56% phosphorus was required by bucks while growing antlers, but others suggest lesser amounts. However, the deer’s body cannot absorb calcium without Vitamin D.
The whitetail’s spring diet is probably more diverse, in terms of quantity and quality, than it is during any other time of the year; it can change rather sharply within a few days, as governed by soil type, rate of snow melt, temperature, amount of moisture and other factors. On Northern range, the deer’s spring diet can change from being nutritionally poor to excellent, sometimes within a few days.
Many complex nutritional relationships make diet diversity important for whitetails. Even good deer foods vary in their specific nutrient value. Early forbs and grasses, for example, tend to be highly digestible and contain high levels of protein, phosphorus and potassium. On the other hand, leaves of woody species, although poorly digestible, provide significant amounts of fermentable cell solubles and high calcium content.
Also, eating certain plants aids in the digestion of others. Researchers have learned although some plants might be high in protein or digestible energy, they are too low in nitrogen, phosphorus, magnesium or sulfur for adequate rumen function. But such nutrient-deficient plants might be utilized if they are eaten in combination with other plants high in the deficient elements. Hence, forages cannot be ranked low in quality simply because they do not meet all of the whitetail’s nutritional demands.
Whatever the reasons might be, poor nutrition during spring will affect the health and well-being of all deer. An inadequate spring diet will contribute to poor growth among young deer, retard antler growth and provide poor fetal development, ultimately leading to high newborn fawn mortality.
Summer: The Critical Growth Period
Good nutrition during summer is critical for favorable fawn growth. If the doe receives insufficient protein to support normal milk production, her milk will be of uniformly high quality, but the total amount produced will decline. Hence, a doe living on poor range might produce a limited milk supply and ultimately raise small fawns because of it.
If fawns are to achieve maximum skeletal size and body weight before winter, they require nourishing forage that has from 14% to 22% protein. When researchers compared performance on diets containing 8, 13 or 20% protein, doe fawns were found to make maximum gains on 13% protein, while buck fawns performed maximally on 20%. Fawns also need minerals, about 0.45% calcium and 0.3% phosphorus, in their diet for proper growth.
By comparison, yearlings require 11% protein, whereas mature animals might require 6% to 10% protein in their diets for body maintenance. Some researchers suggest that if crude protein levels in deer forage fall below 6 to 7%, rumen function is seriously impaired.
Individual plant species and plant parts change in their nutritive value with maturity. Certain forbs, grasses and even sedges might be succulent and highly digestible when they first appear, but become hardened and fibrous at maturity. Even certain highly nutritious agricultural crops such as winter wheat and alfalfa become less digestible when mature.
Therefore, an expanding deer herd can systematically and drastically reduce, or even eliminate, certain preferred plants. At the same time, other plants might increase either because they’re less palatable, resistant to grazing, or both. Although severely overgrazed range might not exhibit the stark, overused appearance one would expect, the land’s nutritional base and capacity to naturally sustain healthy deer steadily declines with continued overuse.
Autumn: An Increase in Deer Activity
With the shortening days of autumn, whitetails become more active — almost unbelievably so. Autumn is not only the whitetail’s breeding time, it is also that critical period when deer prepare for the forthcoming, stressful winter when their nutritional needs change and when patterns of deer range use change.
Energy-rich foods high in carbohydrates, such as acorns, beechnuts and other starchy mast crops — as well as apples, cherries, grapes and other wild-growing and cultivated crops — are choice foods because they promote fattening. When available, a deer will eat about 1.5 pounds of acorns per day, per 100 pounds of body weight.
Because fat reserves can be metabolized more readily than protein for energy needs when nutritious forage is scarce, storing fat in autumn is a mechanism that enhances deer survival during the winter months. Like other seasonal events in the whitetail’s life, the accumulation of fat is cued to photoperiod and is hormonally controlled. It is an obligatory process, meaning that all deer are inclined to become fat in autumn.
Adult bucks usually commence fattening earlier than other deer. They are also the first to molt into their winter coat, usually in early September, about the time they shed antler velvet. Prime-age bucks will be “hog fat” by early October, but might lose 20% or more of their body weight during their four or five weeks of rutting activity.
Because fawns must simultaneously grow and fatten, they seldom achieve maximum size and fatness until December. Given favorable nutrition, however, they might double their body weight between weaning and the start of winter. As a result, fawns are particularly sensitive to the adverse effects of overpopulation, drought or early snow that might cause food shortages.
The importance of digestible energy versus protein content in the autumn diet of fawns was demonstrated in our studies at Cusino. During a 10-week period, fawns provided diets high in energy (3,000 kilocalories per kilogram of pelletized feed) exhibited better body growth and fatness, as compared to those fed low (2,700 kcal) energy diets, regardless of feed protein content (16.2% or 6.6%).
As a result, we concluded that level of protein in the autumn diet of fawns had minimal impact on their well-being, whereas even minor reductions in the amount of digestible energy slowed their growth rate and decreased their level of fatness.
Surprisingly, however, even fawns on restricted autumn rations accumulate some fat, at the expense of additional skeletal growth. In other words, healthy fawns tend to be skeletally large and fat, whereas malnourished fawns might be fairly fat, but stunted.
Autumn nutrition also affects older does. The pattern of coat molt, rate of fattening and the conception rate (and date) among adult does might be quite variable, but will hinge heavily upon their nutritional status before the rut. Yearling does are especially sensitive to nutritional stress because they must add appreciable body growth during summer; as much as one-third of yearlings might fail to breed if subjected to nutritional stress before the rut.
We often see nursing does in red summer coats longer than does that fail to raise fawns. Chemistry is one reason for this difference, claims Canadian researcher George Bubenik.
Prolactin, “the hormone from the pituitary gland that, when declining and acting with other hormones, signals the body to produce the winter coat, is also the hormone that regulates lactation,” said Bubenik. “The level of prolactin associated with milk production is at odds with the low level associated with hair growth. Energy is the other reason. Both processes drain the doe’s energy reserves, and she cannot accomplish both at once.”
Molting, a process that averages about three weeks, is metabolically expensive. The 4 to 5 pounds of hair produced by the average adult deer each season requires a protein-rich diet.
And, according to Bubenik, “The drain on energy and protein reserves deer experience during the molt explains why animals in good physical condition molt first — before weak bucks and late-born fawns as well as before lactating does. In fact, a late onset of the development of wooly fur is a better [and easier-to-read] indicator of undernourishment than the estimation of [body fat].”
Come autumn, deer still wearing red coats — most likely nursing does and fawns — probably don’t have much body fat. However, the ones already molted into their brown-gray winter coats — more likely adult bucks — might already possess heavy fat deposits.
Generally speaking, if deer are able to meet their dietary energy needs in autumn, they will probably satisfy their needs for other nutrients as well. Forages that are high in digestible energy are usually immature plants that are also high in protein, minerals and other essential nutrients, but relatively low in fiber. However, autumn nutritional shortages can set the stage for severe consequences during winter.
Winter: Negative Energy Balance
The freezing temperatures and snow cover that accompany early winter cause deer to shift from eating succulent, highly nutritious herbaceous forage to subsisting upon less-nourishing woody browse. The change in diet results in a negative energy balance, meaning more calories are burned to meet basic body needs than are consumed in food. Deer can easily lose 15 to 20% of their body weight over winter, but few can withstand a 30% weight loss and still survive.
There are many trade-offs — involving nutrition, shelter and predator risk — in the whitetail’s bid for winter survival. Above all, they must conserve energy. Their adaptive traits involve an array of timely behavioral and physiological adjustments, including shelter-seeking behavior, reduced movement activity, reduced metabolism, voluntary restriction in food intake and intense socialization.
As winter progresses, whitetails gradually acclimate to the season. They shift into low gear, metabolically speaking. They show sharply reduced thyroid function, their heart rate decreases and they cut their metabolic rate by about 50%. Healthy deer reduce their mid-winter food intake by about 30%, even when highly nutritious feed is available. Therefore, instead of accelerating body heat production to compensate for cold exposure, the whitetail’s metabolism actually declines.
By mid-winter, acclimated whitetails adopt a form of dormancy, or semi-hibernation, quite similar to that demonstrated by black bears. In the process, deer become quite resistant to nutritional shortage and climatic stress.
The fermentation process is especially important to deer when only low-quality food is available, as is commonly the case during winter. However, deer are by no means super-ruminants — they cannot utilize some woody browse species as well as cattle can — and have difficulty surviving on highly lignified foods.
The rate at which deer can digest food depends on its cellulose content; succulent food being more rapidly broken down than fibrous foods. The very slow rate at which low-quality browse, such as spruce, balsam and timothy grass, passes through the digestive tract explains why deer “starve” with full stomachs.
For deer to digest high-energy foods, they must be in relatively good physical condition and harbor healthy rumen microflora. Starving deer generally exhibit altered rumen function due to decreased concentrations of rumen microflora and volatile fatty acids. When these animals consume large quantities of energy-rich food, such as corn, they can die of toxic acidosis — a buildup of lactic acid in the rumen.
Each deer has a certain starvation threshold beyond which it can no longer survive even though food becomes available. Physically stressed animals, in particular, incur irreversible damage to their rumen lining, and their rumen microflora lose their ability to digest cellulose.
The whitetail’s adaptive system of seasonally changing physiology is not infallible and does not guarantee overwinter survival. Toward the end of winter, the whitetail’s physiology changes, deer become more active, their metabolic rate rises and they need more food to meet their basic needs. Hence, food shortage during the late winter/early spring period can prove devastating to local deer herds.
Conclusions
The quantity and quality of their food, as well as the whitetail’s behavior, physiology and nutritional requirements change markedly with the seasons. Fortunately, deer have evolved the ability to select a mix of forages that balance their nutritional demands and are physiologically adapted to withstand rather severe nutritional hardship during mid-winter. It’s the periods of high energy demand during spring and autumn when food shortage can be so detrimental to their physical well-being and productivity.
The goal of deer habitat management should be to increase plant diversity. While food plots and supplements might enhance the whitetail’s diet, they should not be expected to completely replace natural forage for wild deer. In fact, some investigators suggest that practices favoring a single plant species or one forage type (such as agricultural food plots) should be avoided.
— John Ozoga has been D&DH’s top research contributor for more than 25 years. He is a retired deer research biologist.
Literature Cited
Brown, R.D. 1994. “The Nutrients of Survival and Growth.” Pages 203-207 in D. Gerlach, S. Atwater, and J. Schnell, eds. Deer. Stackpole Books, Mechanicsburg, Pa.
Ozoga, J.J. “Winter Feeding Patterns of Penned White-tailed Deer.” Journal of Wildlife Management, 34:431-439.
Plotka, E.D., U.S. Seal, M.A. Letellier, L.J. Verme, and J.J. Ozoga. 1981. “The Effect of Pinealectomy on Seasonal Phenotypic Changes in White-tailed Deer.” Pages 45-56 in C.D. Mathews and R.F. Seamark, eds, Pineal Function. Elserier North-Holland Biomedical Press.
Verme, L.J., and J.J. Ozoga. 1980. “Influence of Protein-Energy Intake on Deer Fawns in Autumn.” Journal of Wildlife Management, 44:315-324.
Verme, L.J., and D.E. Ullrey. 1972. “Feeding and Nutrition of Deer.” Pages 275-291 in D.C. Church, ed. Practical Nutrition. Oregon State University Press.
Verme, L.J., and D.E. Ullrey. 1984. “Physiology and Nutrition.” Pages 91-118 in L.K. Halls, ed. White-tailed Deer: Ecology and Management. Wildlife Management Institute, Stackpole Books, Harrisburg, PA.
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