After analyzing over a thousand deer forage samples from 25 states, I learned there’s only one type of plant that consistently meets a deer’s nutritional needs. However, most hunting properties aren’t producing enough of them to make a noticeable difference in body and antler size. The results of my research can help you steer your habitat management efforts to fill this gap and answer the most important nutritional demands of deer.
The Three Limiting Nutrients
Deer require many nutrients to survive and grow, including protein, minerals and vitamins. They meet their nutritional demands by selectively foraging on plants providing these resources. Although all of these nutrients are important, three are commonly considered to be limiting for deer during the growing season: protein, phosphorus and calcium.
Nutritional requirements for deer vary based on age and life history events, such as gestation, lactation and antler growth. Lactation is generally considered to be the most nutritionally demanding period for deer, and fawns receiving adequate milk production early in life is important for their growth and survival. A doe with twin fawns requires at least 14% crude protein, 0.3% phosphorus, and 0.34% calcium in her diet. These are the benchmarks we want native or planted forages on our hunting land to meet or exceed.
I’ve collected deer forages across much of the eastern United States as part of several research projects and have been interested in evaluating which nutrients tend to be most limiting. In general, calcium rarely is a limiting nutrient, as the vast majority of selected deer forages have at least 0.34% calcium. However, this isn’t the case for crude protein or phosphorus.
Based on samples from 131 forage species I collected across 25 different states as part of a research project working with Dr. Craig Harper at the University of Tennessee, we found that 43.9% of selected deer forages provide adequate levels of crude protein. However, only 18.8% of selected forages provided enough phosphorus to support lactation, which suggests that phosphorus likely is the most limiting nutrient for deer in most areas.
Which Forages Provide Adequate Nutrition?
Across sites where I have collected deer forages, broadleaf forbs consistently provide the greatest nutrition to deer during the growing season. Specifically, younger leaves of forbs – the plant parts deer selectively eat – provided an average of 18.6% crude protein and 0.28% phosphorus (see the chart above). Young forbs provided adequate crude protein in 73% of samples and adequate phosphorus in 39.7% of samples. Comparatively, young leaves of woody plants, such as shrubs and trees, averaged 11.7% crude protein and 0.15% phosphorus. Only 11.8% and 3.9% of samples from woody plants met minimum crude protein or phosphorus requirements, respectively.
These data clearly demonstrate that forbs are the only plant type that consistently meets deer nutritional demands. If deer on your property are making a living off maple, oak, Chinese privet, or honeysuckle leaves during the growing season, it shouldn’t be a surprise they aren’t as big as you might like! Fortunately, managers can easily promote several common forbs to meet these nutritional requirements.
For example, adequate levels of both crude protein and phosphorus were found in 80% of horseweed (aka marestail), 66.7% of jewelweed, and 60% of pokeweed samples (the photo at the top of this page shows a buck eating jewelweed). Other species such as common ragweed, giant ragweed, common lambsquarters and oldfield aster also provide high levels of both crude protein and phosphorus. Horseweed averages over 17% crude protein and 0.37% phosphorus, and common ragweed averages over 18% crude protein and 0.32% phosphorus. Not bad for a couple of “weeds”!
Several of these forbs are present on most hunting properties, but they aren’t produced in enough abundance to improve deer nutrition. So, let’s talk about methods for increasing that abundance.
How to Promote Native Forbs
It’s clear that forb abundance on a property is important to deer. Several practices can be used to increase their availability. In open areas, killing non-native grasses with herbicide followed by prescribed burning or disking is among the easiest ways to promote forbs.
In forests, reducing the canopy to allow at least 30% sunlight by either commercially harvesting trees or conducting Forest Stand Improvement, then frequently burning the stand with prescribed fire, can increase forb coverage. Invasive plants also compete with native forbs, and these should be aggressively controlled where present.
Relatively frequent disturbances are necessary to maintain forb abundance. Many of the forbs selected by deer are annuals and quickly decline in abundance in just a few years. On many sites across the eastern United States, areas burned less frequently than every two years will quickly become dominated by woody plants that provide relatively poor nutrition. The general three-to-five-year fire return interval that’s often recommended for deer is simply too long to maintain forb production, especially in the long growing seasons of the Southeast. The fire return interval can be lengthened slightly in areas with shorter growing seasons or less precipitation, but fire needs to be applied relatively frequently compared to what many currently apply.
What About Food Plots?
Food plots can also play a role in improving deer nutrition, but they shouldn’t be the primary source of high-quality forage on a property. Instead, they should be used to elevate nutrition during periods of greater demand such as antler growth and lactation, as well as providing high-quality forage during stress periods such as the late summer.
Hunters planting food plots often fail to consider forage availability during the mid to late summer, as their focus is on attracting deer during the hunting season. There’s certainly nothing wrong with this approach, but there are a few strategies you can use to increase the benefit of your plots to deer.
First, consider including longer-lived annual or perennial clovers such as arrowleaf and red clover to your fall mixes to extend forage availability farther into the summer. Second, you may want to dedicate separate acreage to warm-season plantings such as soybeans, cowpeas, or jointvetch. These plantings have excellent forage production during the summer and early fall that is high in both crude protein and phosphorus.
It’s best to leave dedicated acreage of warm-season plots rather than attempting to double crop a field, as many choose to disk their warm-season plots during August or September right when native forage quality is waning.
What About Feeding?
Supplemental feeding often is used with a goal of increasing deer body and antler size. Feeding was popularized in Texas, where rainfall often limits the production of high-quality forages during drought years. However, most deer managers don’t face rainfall limitations, and there are much less expensive ways of producing deer forage.
Feeding is an inefficient strategy compared to promoting native deer forages.
For example, Dr. Marcus Lashley and Jordan Nanney wrote an article that showed how supplemental feeding was 27 times more expensive than applying prescribed fire to hardwoods to produce the same amount of forage! Feeding can also cause other problems, including increasing the spread of chronic wasting disease (CWD) where it occurs, exposing deer and turkeys to harmful aflatoxins, and attracting turkey nest predators.
Given the fact that many native plants provide greater nutrition than you can buy in a bag, feeding isn’t the answer for most habitat managers. It is an inefficient strategy compared to promoting native deer forages.
Should You Fertilize Native Forages?
Especially when we consider phosphorus being an important mineral, many habitat managers become interested in fertilizing native deer forages. It’s reasonable to think that adding phosphorus to the soil would increase phosphorus in forage, but this isn’t the case with native forage plants.
In a study with Dr. Harper at the University of Tennessee, we found that fertilizing native forages didn’t consistently increase their nutrition, but it did make the plants grow taller and produce more biomass. Similarly, a recent study led by Dr. Harper found that soil nutrients didn’t greatly change native plant nutrients in 40 native plants collected across 16 states.
In general, native forbs are adapted to grow in a variety of soil types, so forage quality doesn’t vary much across soil types, but plants growing in more mineral-rich soils do produce greater biomass. I don’t recommend fertilizing as an approach to increase nutrition. Most hunting properties simply need more areas of forb production.
Conclusion
After analyzing over a thousand deer forage samples for research, it’s clear deer managers need to focus on the forbs to improve deer forage. Many properties aren’t producing enough forbs to shift the needle towards increased body and antler size. Consider managing to increase the abundance of forbs on your property to help deer meet their phosphorus, protein, and other nutritional requirements.