Seasonal Rhythms Determine Dramatically Different Eating Patterns

By John J. Ozoga

 Given good nutrition, female whitetails are extremely prolific, bucks approach their maximum potential body and antler growth and deer populations flourish. Conversely, poor nutrition leads to physically stunted deer that exhibit poor productivity and high natural mortality rates. Traditionally, forest management served as the primary means of improving deer nutrition, especially on public lands.
However, nowadays many private land owners utilize food plots and commercially available food and mineral supplements, in order to enhance deer nutrition. All too often, however, the whitetail’s natural biological rhythms are rarely considered. That is, 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, and 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.


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 micro-organisms (bacteria and protozoa) produces nutrients that can be readily absorbed and used for energy. The residual materials pass into the other chambers, the omasum and abomasum. Highly digestible food may pass through the rumen in a few hours, whereas more fibrous or lignified (woody) material may 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 in 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.


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 out and consuming the most nutritious and easily digested plants available. 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 is also evidence that 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 upon 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, professor at Texas A&M University, also observed that deer have a special problem with lignin. “Not only is it indigestible,” he noted, “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 selecting the proper mix of nutrients to meet their immediate dietary needs.


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. Since their diet changes so dramatically with the seasons, it’s also important to note that 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 two to three weeks for the rumen microbes to completely adjust to a new diet.


Commencing about mid-March, in response to increasing hours of daylight hours (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 huge amounts of nutritious forage in the form of succulent new herbaceous growth high in protein, energy and essential minerals and vitamins are essential to herd welfare. Bucks also require minerals and vitamins for antler growth, but researchers still debate the exact amount. 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 a northern range, the deer’s spring diet can change from being nutritionally poor to excellent within a few days. There are many complex nutritional relationships that make diet diversity important for whitetails. Even good deer foods vary in their specific nutrient value. Early forbs, legumes 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 tends to aid in the digestion of others. Researchers have learned although some plants may 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 may 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 the whitetail’s nutritional demands. Whatever the reasons may be, poor nutrition during the spring period will impact the health and well-being of all deer. An inadequate spring diet will cause poor growth among young deer, retard buck antler growth and contribute to poor fetal development, ultimately leading to high newborn fawn mortality.


 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 relatively small fawns because of it. If fawns are to achieve their maximum skeletal size and body weight prior to winter, they require nourishing forage that has from 14 to 22 percent protein content. When researchers compared performance on diets containing 8, 13 or 20 percent protein, female fawns were found to make maximum gains on 13 percent protein while male fawns performed maximally on 20 percent. The fawn also needs minerals in their diet for proper growth. By comparison, yearlings, which also are still growing, require 11 percent protein, whereas mature animals may require 6 to 10 percent protein in their diets for body maintenance. Some researchers suggest that if crude protein levels in deer forage fall below 6 to 7 percent, rumen function is seriously impaired. Individual plant species and plant parts change in their nutritive value with maturity. Certain forbs, grasses and even sedges may be succulent and highly digestible when they first appear, but become hardened and fibrous at maturity. Therefore, in a chosen feeding area, an expanding deer herd can systematically and drastically reduce, or even eliminate, certain preferred plants. At the same time, other plants may increase either because they’re less palatable, resistant to grazing, or both. Although severely overgrazed range may 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.


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 season when their nutritional needs change and when patterns of deer range use change. Energy-rich foods high in carbohydrates such as acorns, beechnuts other starchy mast crops, as well as apples, cherries, grapes and a host of wild-growing and cultivated crops are choice foods because they promote fattening. When available, a deer will eat about 1.5 pounds of acorns daily 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 may lose 20 percent or more of their body weight during their four or five weeks of strenuous rutting activity and enter winter relatively lean. Because fawns must simultaneously grow and fatten, they seldom achieve their maximum size and fatness until December. Given favorable nutrition, however, they may double their body weight between weaning and the start of winter. As a result, fawns are particularly sensitive to the adverse effects of deer overpopulation, drought or early snow cover that may bring about food shortages for them in autumn. 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 kcal 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 percent or 6.6 percent). As a result, we concluded that level of protein in the autumn diet of fawns had minimal impact upon their wellbeing, 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 may be fairly fat, but stunted. Autumn nutrition also affects older female whitetails. The pattern of coat molt, rate of fattening and the conception rate (and date) among adult does may be quite variable but will hinge heavily upon their nutritional status prior to the rut. Yearling does are especially sensitive to nutritional stress because they must put on appreciable body growth during the summer months — as much as a third of the yearlings might fail to breed if subjected to nutritional shortage prior to the rut. We often see nursing does in red summer coats longer than does that fail to raise fawns. One reason for this difference is chemistry, 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. The other reason is energy. Both processes drain the doe’s energy reserves, and she cannot accomplish both at once.” Molting, a process that averages about three weeks in duration, is metabolically expensive. The four to five pounds of hair produced by the average adult deer each season requires a diet especially high in protein. 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 under nourishment than the estimation of [body fat].” Come autumn, deer still wearing red — most likely nursing does and fawns — probably don’t have much body fat. However, the ones already molted into their brown-gray winter coat — more likely adult bucks — may 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.


The freezing temperatures and snow cover that accompany early winter in the North 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 percent of their body weight over winter but few can withstand a 30 percent 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 become very energy conservative. Their adaptive traits involve an array of timely behavioral and physiological adjustments, including shelterseeking 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 percent. Healthy deer reduce their mid-winter food intake by about 30 percent, 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 the black bear. 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 can not 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 upon 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 hay — passes through the digestive tract explains why deer can “starve” with a full stomach. In order 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 build-up of lactic acid in the rumen. Each deer has a certain starvation threshold beyond which it can no longer survive. 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 winter 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.


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 may enhance the whitetail’s diet, they should not be expected to completely replace natural forage.