Energy is not really a nutrient. It is a property of other nutrients. Protein, lipids and carbohydrates have energy, whereas water, vitamins and minerals do not. Energy is usually expressed in terms of calories (c), or actually kilocalories (Kcal), short for 1000 calories. Some cattlemen may be familiar with the TDN system (Total Digestible Nutrients), where energy is expressed as a percent of the diet or pounds per day.
Energy is probably the most variable of the requirements, because it is so dramatically affected by the environment. Energy is required for basal metabolism, which is that needed to maintain body temperature in a normal environment, and to allow for respiration and a small amount of activity. Actual energy requirements are generally about twice maintenance. There are of course additional energy requirements for growth, reproduction, pregnancy, lactation and antler growth. Just as important, there are additional requirements for daily activity (walking, browsing, avoiding predators, or running from hunters). Deer need dramatically more energy to maintain their body temperature in cold weather, especially if they are forced to move during that time to seek food or avoid danger.
It has been estimated that the maintenance energy required by a 120-pound doe in winter is about 3,192 Kcal/day of digestible energy. Standing increases the energy costs over lying down by about 9 percent.
Energy – Feed Intake Relationships
An interesting thing about energy requirements is that they are directly related to body weight. That is, as the deer gets larger, of course it needs more energy, but it actually needs less per unit of body weight. This is also reflected in the whitetail’s food intake patterns. The larger the deer, the less it eats per unit of body weight. More importantly, energy requirements and food intake vary seasonally.
Both bucks and does consume the most food in late summer and early fall. This may be the most critical food period for deer. Bucks are growing their antlers and developing fat for the winter rut, does are lactating or weaning their fawns, and fawns are shifting from a milk diet to solid food.
Once winter begins and the breeding season starts, both bucks and does reduce food intake. They focus on the rut, and even though rutting and winter temperatures require more energy, they have prepared by storing fat earlier in the year. Deer can easily lose 15-30 percent of their body weight in winter, and recover with the spring green-up.