March 25, 2022
By Dr. James C. Kroll
When I began my career studying the white-tailed deer in 1973, there had been some early studies on their biology. However, most intricate details on what it takes to manage whitetails have come to light in the last 50 years! The amount of information related to deer management has grown at an exponential rate, almost as fast as technological advances in information discovery.
In my early days of research, I had to dig through scientific journals and attend annual conferences to keep abreast of new discoveries. In contrast, today’s wildlife scientist can learn about new discoveries by a few clicks on the computer. Prior to the turn of the 21st century, lag time between new discoveries about game management and actual implementation by practicing managers was 7-10 years. Today, the public often knows about new discoveries and theories before wildlife researchers. As a consequence, theories prematurely come to light long before they can be tested by tried-and-true scientific scrutiny.
Good science proceeds at a snail’s pace, while the Internet flies at light speed! As a consequence, landowners and deer managers have to wade through an ever-growing swamp of misinformation about how to manage whitetails. Nowhere is this more obvious than for deer nutrition. Ask the average deer hunter what the most important component of whitetail nutrition is, and the overwhelming answer will always be protein. This must be so, because every advertisement for a deer supplement or food plot seed variety guarantees the highest level of protein available!
Yet, it might surprise you that the amount of crude protein in native deer forage or even a supplement falls to a weak third in importance to deer nutrition. Research has shown the three most important aspects of good deer nutrition are digestible energy, phosphorus, and protein, in that order.
The Science Behind Whitetail Nutrition
Deer are ruminants, no different really than cattle, sheep and goats! Ruminants are animals that have a so-called four-chambered stomach, consisting of the rumen, reticulum, omasum and abomasum. This system allows animals to digest highly fibrous plant material, to release energy and raw materials for bodily functions.
The occurrence of the ruminant system gave these plant eaters a decided advantage over animals with a one-chambered stomach such as horses. The most critical part of the system is the rumen, a large chamber holding about 10 pounds of forage in whitetails, in which primary fermentation occurs.
The rumen is populated by many species of bacteria, protozoans, fungi and yeasts, that ferment and break down plant cell walls into their carbohydrate fractions to produce volatile fatty acids (VFAs). These VFAs are the primary source of energy for the deer. The organisms also produce amino acids, protein from non-protein nitrogen, vitamins B and K, and minerals.
From there, the decomposed plant material moves into the reticulum, a small chamber lined by structures shaped like the reticulating pattern on a giraffe. The reticulum consolidates the smaller material from the rumen and begins extraction of water for “recycling.”
The next chamber is the omasum, the walls of which have been likened to the pages of a book. Its function is to further absorb nutrients and water before the material reaches the true stomach, the abomasum. The pH of the first three chambers is between 6-7, approximately neutral; while the abomasum has a pH between 3-4, highly acidic. This breaks down the more stubborn, highly fibrous plant materials and digests the micro-organisms passing from the previous three chambers into the abomasum.
From there, the mostly undigested material passes into the small intestine, where more nutrients and water is absorbed, then into the large intestine and rectum, where the waste is rolled into pellets for defecation.
Let’s return to the rumen, where the important things happen! The deer absorbs the VFAs to use as an energy source for bodily processes, including body growth, heat generation and synthesis of critical chemicals (including proteins).
The foods deer eat include browse, weeds and grasses, plus fruits and nuts in season. The digestibility of these foods will determine the amount of energy obtained from them. Digestibility is mostly determined by the amount of non-soluble fiber they contain. As a plant grows from early spring to fall, the amount of non-digestible fiber declines rapidly. The most digestible plants, in order, are weeds, browse and grasses. The latter figure less in a deer’s diet than for cattle.
We generally describe the digestibility of a plant in terms of the amount of digestible organic matter (DOM). In the lab, we extract digestible material from plant samples using detergent water, followed by an acid bath. The remaining undigestible material is called the acid-detergent fiber; these include the highly indigestible part of forage, including lignin, cellulose, silica and insoluble forms of nitrogen.
A good DOM for a deer forage is greater than 60 percent. Some of the deer foods we test in late summer may have DOM values of 20 percent or lower!
In general, there is an approximate 90 percent correlation between the DOM of a deer food and the amount of digestible energy (DE) present. So, when DOM declines in natural deer forages, the amount of available energy also declines, significantly reducing resources for body and antler growth, milk production and surplus energy storage as white fat for the winter.
What it Means for Managers
It is safe to say there are few geographic areas or even seasons when deer have everything they need in the way of energy and nutrients to fully satisfy their needs. Yet, deer survive despite these deficiencies; however, there is a huge difference between surviving and reaching their optimum potential. A good management program includes manipulation of native vegetation to encourage healthy plant growth over most of the growing season. Strategies such as prescribed burning, disking, mowing, timber thinning and fertilization are extremely beneficial.
There also is a legitimate place for supplementation in the form of highly digestible crops such as cereal grains, legumes and chicory. In managing forage crops, it is not about their protein content, rather it’s the amount of digestible energy they provide when critically needed. Supplemental feeds, where legal, also can provide highly digestible material, and have been shown to greatly influence both fawn recruitment and antler growth.
Lastly, my research has shown deer orchards to be extremely beneficial in providing deer nutrition throughout an extended portion of the year. An orchard plot consisting of hard and soft mast that ripens and falls to the ground at distinct intervals throughout the later summer, fall and early winter is ideal. Once orchards mature and become productive, they can produce high levels of tonnage that’s highly digestible for deer.
Hopefully, this column has introduced you to a new way of thinking about and evaluating the nutrition of your deer. It certainly involves a lot more than mere crude protein!