The two habitat variables that affect deer diets and nutrition are plant availability (quantity and accessibility) and quality (nutrient content and digestibility). Plants will vary in abundance, stage of growth and nutritional characteristics on a seasonal basis. Deer will attempt to maintain a quality diet and meet nutritional needs by adjusting diet components as the forage plants change the quality. If one or both of the above habitat variables are limiting, it will have a detrimental effect on deer nutrition.
Four basic categories of plants are available to deer for consumption: woody plants (browse), forbs (weeds), cacti and grasses. The proportion of each plant category or any particular species in the deer diet will vary among years, seasons, regions and individual deer. Availability of preferred plants is a key factor contributing to this variation. When a plant is green and growing, it is likely that a deer will eat it (at least in small portions). However, if a particular plant category (e.g.forbs) is largely unavailable within the home range of a deer, than its diet will reflect higher than “normal” percentages of browse, cacti and possibly grasses. Similarly, is a particular plant species is not present in the deer habitat, this species obviously would not be listed as a preferred deer food. Availability is a key variable and should be considered when comparing the results of diet studies.
On most South Texas ranches, availability of browse is generally not a problem. The exception is where brush has been eliminated and pastures have been seeded to grass. Mixed brush communities provide deer with sufficient amounts of moderate to high quality browse.
Forbs, on the other hand, are often scarce. High quality perennial forbs are uncommon because of past misuse of the rangeland. Annual forbs are highly dependent on soil moisture and are usually present only for brief periods in the spring and fall. Mid winters with adequate moisture can result in flushes of cool-season forbs.
Although forbs are considered to be the most nutritious and preferred food category for deer, brush and cacti may be more important in South Texas since they are consistently available and consumed by deer throughout the year. Grasses are the least important food category to deer, although use will increase slightly during the spring and fall if grasses are available. Young, cool-season grasses such as wintergrass, ryegrass and rescuegrass can be important sources of protein during the winter.
Forage accessibility is another important habitat component for the deer manager to consider. Brush characteristics such as height, density and growth form can limit browse accessibility. Brush mottes (thickets) can be extremely dense and impenetrable for grazing and browsing animals. In such situations, most browsing is restricted to the perimeter of the motte. The few weeds and grasses that survive under the brush thickets offer cannot be utilized. Brush management practices such as rootplowing, roller chopping, discing, dozing and prescribed burning can increase forage accessibility by creating openings or thinning dense brush thickets. Top removal practices (shredding, roller chopping, fire) can increase browse availability and nutritional quality by reducing the structure of tall, single-stemmed plants and promoting new growth of many brush species.
Leaves of shrubs can also become inaccessible because overgrazing.. This is a common problem with live oak in the Edwards Plateau and can occur with granjeno of guajillo in South Texas. When a range is overstocked and herbaceous (non-woody) forage becomes scarce, deer and other ruminants will depend heavily on browse. If animal numbers are not reduced, excessive browsing will occur and a “browse line” will develop four to five feet of the ground. Any leaves that sprout below this browse line will quickly be consumed. These may still be a great amount of browse remaining under these conditions, but if it is higher than five feet, it is inaccessible to deer.
Another accessibility problem can occur on large expanses of open or cleared range. There may be abundant forbs and small regrowth brush in a large cleared area, but few deer will venture more than 200 to 250 yards from the security of the perimeter cover. Although the forage is physically accessible, it may be behaviorally inaccessible to deer. The manager must be aware of what a deer perceives as available forage.
A third component of food availability is plant diversity. Diversity of food plants allows deer to select a quality diet from the available species as they fluctuate seasonally in nutritional quality. Diversity is particularly valuable if the species have different growing seasons. A variety a plants with different growing seasons will increase the probability for year round availability for quality forages. Deer can shift diet components in response to changing nutrient levels associated with the seasonal growth if each plant species. A diversity of plant species can also be important when mast crops such as mesquite beans, persimmons and acorns mature in a staggered manner, thereby increasing the availability over time of these important energy sources.
The quality of deer food plants can sometimes be improved through vegetation manipulation, (such as brush management). However, the manager has much less control over forage quality than quantity. Forage quality is associated with the growth stage of the plant, the plant species and environmental factors such as soil type and precipitation (soil moisture).
No single plant species maintains the year-round nutrient levels required by deer for successful growth and reproduction. However, some plant species are higher in nutrients than most other species in the same plant category. An example of a high quality browse species is spiny hackberry (granjeno) which maintains adequate levels of crude protein, energy and phosphorus year-round. Some browse species may maintain adequate year-round levels of a particular nutrient (such as crude protein), but may be seasonally deficient in energy or certain minerals required by deer. Pricklypear is high in dry matter digestibility (energy) but is relatively low in crude protein and phosphorus. This emphasizes the importance of maintaining a diversity of forage species for deer nutrition. The variation in nutrient content among several South Texas browse species, as well as seasonal fluctuations in nutrients, are shown in Table 1. Click on image to enlarge
Forbs are generally higher in nutrient content than browse species. On an annual basis, forbs tend to be 35 to 40 percent higher in energy content than browse species, similar in crude protein and 45 to 50 percent higher in phosphorus. Examples of higher quality forbs are bundle flower, ground cherry and lazy daisy.
Grasses are generally used by deer only when young tender shoots are green and growing. Grasses contain their greatest nutrient levels and are more digestible during this stage of growth. The preferred, native grass species average 10 to 15 percent crude protein, 0.17 to 0.27 percent phosphorus, and 45 to 55 percent dry matter digestibility (energy).