Most deer hunters learn early in their hunting career that some locations produce larger deer than others. Hunters in Iowa and Kansas, for example, have a much different impression of what constitutes a big buck compared to hunters in the Coastal Plain of Mississippi or Alabama. In this article, I’ll discuss our new research into the reasons for variation in deer body and antler size across the United States. I’ll also explain how you can change average deer size where you hunt.
3 Ingredients for Larger Deer
Deer body and antler size are influenced by three primary factors: age, genetics, and nutrition. Age effects are fairly straightforward, as deer grow larger until they reach maturity along a predictable growth curve.
Genetics influence the size of individual deer and likely also have some effect on the size of deer between different regions. However, we cannot manage for genetics in wild herds by either releasing deer with “improved” genetics or culling individuals with “poor” genetics.
The differences in deer sizes that hunters and managers have historically attributed to genetics often is more related to differences in nutrition between sites. Nutrition has strong effects on deer size, and several studies have highlighted the positive effects of improved nutrition on body and antler size.
What About Bergmann’s Rule?
There’s another factor that may influence the size of deer and other wildlife species: temperature. Bergmann’s Rule, as this relationship is commonly called, is named after an early German scientist who published about the relationship between animal size and latitude. Specifically, he noticed that individuals within a species were larger at sites further from the equator and hypothesized this was related to heat conservation.
Proponents of Bergmann’s Rule suggest animals located in cooler environments may be larger because they can retain more heat with a larger body-mass-to-surface-area ratio. Alternatively, animals in hotter environments may be able to shed heat faster with a smaller body-mass-to-surface-area ratio.
Regardless of the mechanism, there is a general trend in deer being heavier and having larger antlers in locations further to the north in the United States and Canada. Bergmann’s Rule likely doesn’t directly influence antler size, but larger bodied bucks are able to support the growth of larger antlers.
Evaluating the Effects of Various Factors
There are exceptions to this trend, such as where deer are large in regions of the South where forage is relatively abundant because of row-crop agriculture. There are also properties and cooperatives where managers have increased forage availability through habitat management and observed great increases in body and antler size. Disentangling the effects of temperature, landscape, and local forage availability was the goal of a recent research project that several colleagues and I conducted to help better inform hunters.
Dr. Craig Harper, Dr. Bronson Strickland, Dr. Marcus Lashley, Dr. Mark Wilber, and I worked with 35 properties across 21 eastern states to evaluate factors that influence doe body weight and buck antler size. Many of the properties were managed by NDA Deer Steward 2 graduates who kept excellent harvest data records. We were interested in comparing their harvest data with regional and property-level data that I collected over several years.
Regional Effects
We first considered the effects of landscape coverage of row-crop agriculture and temperature. Specifically, we used the percent coverage of agriculture within 6 miles of the property and the average annual temperature. Several studies have documented larger deer in locations with more agriculture, as crops such as soybeans and alfalfa provide high-quality forage during the growing season. Our data found a similar effect, with adult doe body weight increasing by 3.5 pounds for every 10% increase in row-crop coverage around the property. Antler size of mature bucks was also affected, with average antler size increasing 2.8 inches for every 10% increase in row-crop coverage around the property.
We also found support for Bergmann’s Rule, as doe body weight and buck antler size increased at sites that were cooler. Specifically, doe body weight increased 1.7 pounds and mature buck antler size increased 1 inch for every 1˚F decrease in average annual temperature. In general, these results match up with other research, as deer sizes appear to be determined by both landscape-level forage availability and climate.
Local Effects
We expected to find differences in deer size related to landscape factors, but the goal of our project was to evaluate whether site-specific forage availability would have similar effects. To measure this, I collected deer forage plants across all vegetation types on each property during the summer. I weighed the plant samples and sent them to a forage lab for nutritional analysis. In total, I collected about 5,000 forage samples and had nearly a thousand forage samples analyzed for nutritional content.
“For every additional 50 pounds of high-quality forage per acre across the property, average doe body weight increased 8 pounds and mature buck antler size increased 7 inches!”
I then calculated estimates of nutritional carrying capacity for each property, which is a measure of how many deer could be fed on an acre of land for a day by forages that meet a minimum nutritional requirement. For this project, we used a 0.3% phosphorus constraint as our nutrition cutoff, as phosphorus is likely the most limiting nutrient for deer in the eastern United States. These estimates of nutritional carrying capacity do not represent an exact number of deer an area should have, but they can be used to compare the availability of high-quality forages between sites.
We found that property-level nutritional carrying capacity increased doe body weight and buck antler size after we accounted for landscape effects. We grouped properties based on latitude and the percent row-crop coverage and then assessed differences in deer size within those groups. For every additional 50 pounds of high-quality forage per acre across the property, average doe body weight increased 8 pounds and mature buck antler size increased 7 inches!
Our results should be encouraging to deer hunters, as they clearly demonstrate that habitat management works! However, it is necessary to set realistic expectations for a property, as deer size is constrained by temperature and landscape-level forage. Average deer body and antler size on a property in Florida surrounded by pine forests is likely never going to match deer size in agricultural regions of Minnesota, for example. That shouldn’t discourage you from increasing forage where you hunt. You can still produce the largest deer possible in your landscape and location.
Deer hunters interested in increasing deer body and antler size on their property should consider habitat management practices such as forest stand improvement, early successional management, and planting food plots to provide additional forage. They should focus on increasing forb production by using frequent prescribed fire, especially during the late growing season. It also may be necessary to actively manage more of the property, as nearly all properties have unmanaged non-native grass fields and closed-canopy woods, for example, that could be converted to improve deer forage availability.
I’ll cover exactly how much these changes to habitat management can increase forage in an upcoming article, but achieving 50 pounds per acre of additional high-quality forage is easily attainable on most sites!
It’s About Habitat!
Where deer size is concerned, hunters put a lot of thought into factors that management cannot change, including genetics, soil fertility, and weather. Instead of focusing on these, I encourage you to consider the positive effect habitat management can have on deer using your property. Then, fire up the chainsaw and get the drip torch ready to have larger deer on your property in the future!