White-Tailed Deer/Population Health and Evaluation

Relative herd health is the condition of a white-tailed deer population relative to the ability of the habitat to support it. Herd health is an important component in any deer management scenario. At any point in time there is a finite supply of nutritional resources available to support deer. If the nutritional needs of the population exceed the habitat's ability to provide for those needs (i.e., a state of overpopulation), then both herd health and habitat quality or land health are compromised. Decreased herd health can negatively impact population parameters, such as fawn production, body weight, and antler size. In other words, the ability of most deer to fulfill their genetic potential would be limited. Of potentially greater concern is the long-term impact of overpopulation on land health. During the process of selective foraging to meet their nutritional needs, excessive deer populations may damage habitat quality by reducing available forage and/or plant diversity. This effect often is long-term if reversal requires natural changes in plant communities.

Herd health is critically important to land managers interested in white-tailed deer management. Monitoring herd health enables a manager to assess the relative ability of an "average" animal to fulfill its genetic potential. This ability is directly related to the quality of forage available throughout the year, which is why the food habits data presented in this publication are of such value. Realize that not all management goals require fulfillment of genetic potential; however, the information generated from a yardstick which accurately measures this ability can be applied to any management context.

Although deer herd health is very important to a management program's success, it is difficult to accurately measure this phenomenon. One preliminary problem with measuring herd health is identification of the target population; a population is the basic deer unit upon which we apply our management tools. Proper application of any tool to modify a specific population parameter requires clear identification of the boundary of the group of animals to be impacted. Identifying this boundary is complicated when working with a highly mobile animal in diverse habitats, such as with deer in the Cross Timbers region.

Once a population is identified for sampling, natural variation within the population dictates that fairly large sample sizes be used for population-level inferences. The sample size needed to make valid inferences varies among locations and should be evaluated separately for each management program. The actual sampling process can further increase variation for those variables sensitive to precollection animal stress or sampling conditions; thus variables sensitive to these biases should be avoided. In general, the more samples available on which to base decisions, the better the decisions.

A wide variety of variables can potentially be measured to assess relative herd health. Which variables to measure depends on the intensity of management and the eventual need to fine tune deer herd characteristics. For example, if recreational harvest of young male and female deer is the objective, then the only herd health variables included in a monitoring program might be body weights and antler measurements of harvested yearling bucks and fawn crop estimates from fall spotlight or cruise surveys. However, if commercial harvest of a specific number and quality of animals is planned, then a more intensive monitoring program may be required to document the same information plus body weights of all deer, antler development of all bucks, age-specific mortality rates, and other variables.

The practicality of variable measurement also is important to consider. Many physiological variables are inherently too variable or are too sensitive to sampling procedures to be practical within a management context. Cost and facility requirements are two considerations that often preclude using an additional set of variables.

For a variable to have practical application it must pass the following considerations. In the first place, does the variable provide specific information about the population? In other words, does it provide something of specific value relative to a population parameter of interest? For example, an estimate of fawn crop can be directly related to incremental growth of the population, which in turn directly impacts harvest rate decisions. Secondly, do the type and accuracy of the data outweigh the cost and effort associated with sampling? For example, in most cases the cost and accuracy associated with contracting a helicopter to produce a single survey for fawn crop estimation could not be justified for a general recreational program. Additionally, many habitats in the Cross Timbers can not be sampled effectively using a helicopter.

The inherent seasonal changes in physiological processes and resultant nutritional needs of the deer should also be considered when selecting which herd health indices to monitor. Variables which allow you to "backdate" herd health to an earlier season are of great interest. Often the population parameters of interest, such as antler size, are determined during an earlier season and are somewhat independent of the conditions present during the hunting season. For example, antler development is related to availability of nutritious, palatable forbs and browse during spring and summer whereas the amount of body fat on a fall-harvested buck will depend on the supply of mast (e.g., acorns) and other high energy foods during late summer and fall. Without a grasp of this phenomenon, understanding the apparently contradictory information provided by a sample of small-antlered bucks with abundant fat reserves would be difficult. Because habitat quality can vary dramatically between seasons, health indices which can backdate the health of a population can be very enlightening. Seasonal variations in forage quality on the NFWU and the resultant effects on deer condition will be discussed elsewhere in this publication.

So which variables are worth considering when planning a herd health monitoring program within the Cross Timbers region? The following variables should be measured for most deer management programs: age, body weight, antler size, and reproductive attainment. Age is not a herd health variable per se, but it must be measured for each deer so that body weight and antler size information can be interpreted properly. Some programs would also benefit from measuring fat deposits and reproductive potential. Methods of sampling and some interpretation guidelines are provided for each variable.

Age of harvested animals is of value when interpreting health indices and when evaluating the age structure of the population. Health indices can be interpreted properly only if grouped within specific age classes because most variables normally change with age. Data can be grouped across ages once age no longer influences a variable. For example, body weight of does should not increase significantly beyond 2.5 years of age, so data can be grouped for all does greater than or equal to 2.5 years of age. The age structure of harvested deer can be used as a gross estimate of the population's age structure if the animals are harvested randomly (i.e., no selective harvest) and if the sample size is adequate.

The age of harvested deer can be estimated by the replacement pattern of the first set of teeth (milk teeth) and by the eruption pattern and relative wear of permanent teeth (Figure 4 - see PDF version). This technique is quite reliable for assigning deer to three age-groups: fawns, yearlings (1.5 years old), and older deer (2.5 years or older). Aging accuracy declines dramatically as animals get older. However, some herd health variables, such as antler measurements, should be classified by specific year classes (e.g., 2.5, 3.5, 4.5, 5.5 and older), so attempts at assigning specific ages to deer 2.5 years and older may be justified. A collection of jaws which exhibit the representative tooth characteristics should be available at the deer processing area for reference.