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What Is The Most Important Factor in Whitetail Growth?

by Dr. James C. Kroll   |  October 31st, 2017 0

I’m amazed at the progress made in whitetail management over the years, especially in managing food sources on private lands. Yet despite the advancements, there’s still confusion about the single most limiting factor in deer nutrition: energy.


It takes more than protein to produce big antlers. Research shows digestible energy is the most limiting factor in whitetail growth. (Photo by Ralph Hensley)

In the late 1970s and early ’80s, our Institute for White-tailed Deer Management & Research began an ambitious project focused on habitat dynamics. Graduate student Don Adams was given the onerous task of assembling everything known about forests, forages and deer interactions.

This work involved creating models for topics such as forage composition and growth after a timber harvest (clearcut vs. thinning), prescribed burning and site preparation. Then we needed additional models for deer population growth and reproduction. The work was supported by the U.S. Forest Service’s Wildlife Habitat and Silviculture Laboratory on the campus of Stephen F. Austin State University.

While we learned a great deal from this early work, the models continue to be modified and improved as a result of new discoveries here at the Institute. One of the things we learned is what really matters in deer nutrition.

In the beginning, I subscribed to the “protein school” of deer management. Our early work on food plots focused mainly on the crude and digestible protein of various cultivated species and varieties, such as legumes, cereal grains and broadleaf plants. The first question we often were asked, and even are still asked today, was, “How much protein does that plant provide?” This led to work on whitetail protein needs by many scientists.


This “two-stage”plot will produce high energy levels for months on end. Cowpeas are a warm-season annual, while corn comes into play later. (Photo courtesy of Dr. James C. Kroll)

Common reasoning at the time was that the more protein a plant contained, the better it was for deer. But as it turns out, a whitetail really doesn’t need more than about 16-17 percent protein in its diet to perform well. Anything above this level ends up lost through body waste — and can even be problematic.

A number of factors limit deer herds, including nutrition, predation, disease and competition. But does each of these have an equal effect on the herd? No.
In the mid-1800s, agricultural scientist Carl Sprengel came up with a theory (later popularized by Justus von Liebig) asserting that population growth is controlled not by the total amount of resources available, but by the single resource that is scarcest. At first, what became known as “Liebig’s Law of the Minimum” was applied to crop science. This law held that you can supply a host of nutrients to a crop with no increase in yield — yet, if you supplemented the one nutrient that was in least supply, the result would be an increase in production. Later, Liebig’s Law began to be applied to population dynamics of animals, as well.

Our computer modeling work began to create doubts about the idea protein was the limiting factor to deer populations. In fact, every model we generated relegated protein to third place! So what was the limiting factor? It was digestible energy, followed by available phosphorus. Protein was a distant third.

Remember, the whitetail is a ruminant. It has a four-chambered “stomach” consisting of the rumen, reticulum, omasum and abomasum. Such animals displaced many non-ruminants because they were more efficient in converting highly indigestible plants into food by fermenting them in their specialized stomachs. Ruminants developed a sophisticated relationship with a host of microorganisms (bacteria, protozoa, fungi and yeasts) that thrived in a large fermentation “vat” (the rumen).

The main byproducts of fermentation in the rumen and reticulum are volatile fatty acids (VFAs) and microbial proteins, which are absorbed through the lining of these chambers. The primary raw materials for the microorganisms are the cellulose, hemi-cellulose (fibrous plant materials), pectin, starch and sugars in plants. Up to 50 percent of cellulose and hemi-cellulose are digested by the rumen microbes; 60 percent or more of the starch is digested; and, up to 100 percent of sugars are converted into usable byproducts.

Some of the protein ingested by deer is broken down in the rumen; the rest moves farther down the digestive tract. The products produced during this breakdown include amino acids (building blocks of proteins), organic acids, ammonia and other products. Using the energy provided by VFAs, the microbes assemble these materials into new proteins that also are absorbed in the digestive tract.

If I haven’t lost you in this discussion, you now understand how much energy — especially digestible energy — means to deer and other ruminants. But what does this mean to deer managers?

The Southeast Deer Study Group was organized in 1977 by several young deer biologists as a vehicle for transferring new information to practicing managers and researchers. At one of the group’s meetings, an invited deer manager from South Carolina revealed a level of deer productivity unheard of at that time. The fellow ascribed his success to feeding corn!

Well, most deer biologists (including me) at that time strongly opposed feeding large amounts of corn, due to its incredibly high digestible energy. “If you feed too much corn,” I once asserted, “you’ll create a lethal condition known as acidosis, which will kill the rumen organisms!”

Needless to say, the manager giving the talk was almost thrown out of the group. But as with many other statements considered heresy in the early days of deer management, subsequent research supported some of his claims. In fact, I must confess that in the years since, I’ve never seen large-scale mortality from feeding corn.

After more than four decades of deer research and management, I’m convinced providing natural and cultivated forages rich in digestible energy, along with a supplement (where legal) rich in both digestible energy and modest protein, is sound management practice. However, as the saying goes, “The devil is in the details.”

When during the year do deer require the most digestible energy? You probably just said to yourself, Winter. However, our studies and those of others support that both bucks and does need more energy during spring and summer, when bucks are rapidly growing antlers and does are producing large amounts of milk.

We conducted a long-term study in which we allowed deer at the Institute to pick their own diets. We supplied free-choice feeders with protein pellets, corn, salt and minerals in a cluster formation (each monitored by trail cameras) so deer could “decide” what they needed, and how much of it, over the full year. Corn was used as our high-energy source.

When we examined feeder use by bucks, we were surprised to see the highest use of corn during April, July and August. But when you think about it, the reason is obvious: Spring and summer are times of rapid antler growth, and cell division takes a tremendous amount of energy.

We found highest protein use by bucks was during May and June. For does it was in May and September. Also, bucks and does tended to visit the corn feeders more often than protein feeders. Since this work, we’ve integrated energy and protein management into our program, with fantastic results.

Fortunately, there are several ways in which you also can do this, even without supplemental feeding.

Because whitetails utilize plants primarily for energy production in the rumen, the more digestible a forage is, the more energy is available. The whitetail is referred to as a “concentrate selector,” meaning it has a sharp nose and long tongue uniquely adapted to moving from plant to plant and selecting what to eat. The best parts are those that are either in the early growth stages or are inherently digestible.

Although whitetails would rather eat weeds, their mainstay food is browse (leaves and twigs of woody plants). Any management practice that increases the volume of young plant growth will produce larger amounts of digestible energy. We conduct prescribed burns periodically to keep deer foods within reach and to stimulate growth.

After a timber harvest, shrubs and young trees explode in volume and palatability. However, within 7-10 years many of the plants will have grown out of reach of feeding deer and will have become less digestible. So if you’re going to create high-energy deer forage, you’ll must frequently “disturb” the woods. An old-growth wilderness is a nice place to visit, but it’s a poor place for deer.

Food plots can provide the highest digestible energy to your deer, but you need to plant the right things at the right times. As the critical energy needs seem to be in spring and summer, warm-season food plots are more important than cool-season plots. However, the hot, dry conditions often experienced during this time frame work against growing good forages.

So in evaluating a potential food-plot variety, it’s actually more important to ask about the amount of digestible energy than protein content. Also, note how “nutrient dense” or “nutrient diffuse” a plant is. That’s another way of assessing how much energy is available per mouthful. Plants high in water content might be filling to deer, but those with both high digestibility and higher energy density are the best warm-season crops. This metric hasn’t previously been considered in selecting food-plot varieties.

In the North, we often plant oats in early spring to provide higher digestible energy for bucks coming out of winter. Our research shows oats are superior to other cereal grains in supplying energy. (They’re higher in both water- and alcohol-soluble carbohydrates.) Cereal grain preference by whitetails is oats, wheat, rye and triticale (a wheat-rye hybrid), in that order.

If you live where supplemental feeding is legal, during times of greatest need consider providing feed with a high energy content. At the institute we use a modest amount of corn supplied using a standard spincast feeder, timed to provide a larger amount of corn at critical times. This we place adjacent to a free-choice feeder providing 16-17 percent protein pellets.

The backbone of any deer nutrition program is rapidly growing, diverse stands of a variety of native foods kept within the “deer zone” (within four feet of the ground). About 30 percent of your land should be dedicated to forage production (natural and cultivated).

Boosting the amount of digestible energy available through the year — but especially in spring and summer — can greatly increase age-specific antler growth. So develop a plan that ensures your deer have ready access to highly digestible foods at this time. Strategies such as prescribed burning, roller-chopping and timber thinning/harvest will significantly increase the herd’s available energy. When coupled with a sound warm-season plot program, these steps are well worth taking.

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  3. Dr. Deer: Measuring Digestible Energy of a Food Plot
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  5. Understanding What Whitetail Need, Pt. 2
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