Habitat Requirements:
Food Requirements
Food is often the limiting factor in deer management in the Cross Timbers region. If biologists and land managers are to assess the quality and suitability of land as deer habitat and make informed management recommendations and decisions, some knowledge of regionally specific deer diets and nutritional ecology is imperative.

Diets of deer have been researched in various regions of the country, but unfortunately, the food component of deer habitat in the Cross Timbers had never been researched and published prior to this study. We therefore implemented a study in 1985 to fill the void in our knowledge of deer diets in this region. Previously, when land managers in the Cross Timbers were faced with a decision involving deer foods, they were forced to base that decision on information collected in other areas of the country, some bearing little or no resemblance to the habitat of the Cross Timbers. This was a difficult task because many plant species reported in other studies are absent or available in different quantities or qualities in this region. The extrapolations made from the other studies were crude at best.

The information presented in this publication about the food habits and diet selection of white-tailed deer is a result of the previously mentioned research project conducted on the NFWU. Due to the variability of vegetational communities within the region, this information is fairly site specific and probably does not represent the entire region with complete accuracy. However, all of the plant species identified as important deer foods in this study are adapted to at least a portion of the Cross Timbers and could occur in much of the region under suitable land management strategies. Most indicators of herd health and condition (these will be discussed in a later section) indicated that the deer herd on the NFWU was in good condition at the time of the study. The area was judged to be relatively good deer habitat at the time of the study based on deer herd parameters, deer condition, and vegetational quality, quantity, and diversity.

Methods Used to Gather Food Habits Data
Deer were sampled at times that should represent the most nutritionally demanding periods of a female deer’s life from a seasonal perspective; the rationale being that significant use of a given forage at these times should indicate its importance. It is not possible for the information presented here to include all of the forage species used by deer in the Cross Timbers. The vegetational community on the NFWU changed constantly throughout the study in composition, quality, and quantity. Obtaining a more comprehensive dietary representation of the NFWU deer herd would have required increasing the sampling frequency while maintaining an adequate sample size; a strategy which the NFWU deer herd could not support due to population size limitations. Potentially important forage plants, especially those used during only short periods of time, may not have been detected with the chosen sampling procedure. However, forage species consumed in significant quantities during the sampling periods should have particular importance, not only because deer consumed them in significant amounts, but also because they were being selected when the deer probably were trying to fulfill some of their most demanding nutritional requirements.

Adult female white-tailed deer were collected at approximately 3-month intervals beginning in late August 1985 and extending through May 1987. To adequately sample deer diets, five to ten female deer at least 1 year old (70 deer total) were collected per sample period. Most of these deer (55) were at least 2 years old. Sampling male deer was avoided to eliminate sex-linked variability for the physiological aspects of the project. However, diets of male and female deer usually are similar.

Seasonal collections were timed to coincide with nutritionally important, and sometimes stressful, periods of a female deer’s annual cycle. The spring collections, which took place in late May in both collection years, coincided with late gestation and/or early lactation. Both of these events result in high nutritional requirements and thus place additional demands on the habitat to fullfill these needs. Summer collections occurred during late August of both years. In addition to the demands resulting from lactation, this time of year in the Cross Timbers is typically very hot and droughty. Vegetation quantity and/or quality may be reduced in August resulting in less quality forage. Late summer may be a stressful period as it may be difficult for females to obtain adequate nourishment to fulfill the lactation needs of their fawns and maintain good body condition for themselves. Fall collections took place in early and mid November. These collections were representative of deer diets just prior to the peak of breeding season, as well as that period when deer are building fat reserves they will utilize throughout the coming winter. Collections for the winter sampling period were made in late January or early February. These collections were timed to coincide with the low food availability that is traditionally thought of as accompanying winter.

Collected deer were examined and samples of the rumen (the first compartment of a deer’s stomach) contents were collected and preserved for later analysis. Forage use was quantified using plant cell characteristics (microhistological analysis) to identify plant species or groups in the rumen samples. Efforts were made to identify all plants found in the rumen samples to the species level. However, when such differentiation between certain plants was not possible microhistologically, results were reported for the appropriate plant complexes. Plants comprising a complex were members of the same genus, with the exception that three complexes contained members of two genera.

The rationale behind using rumen contents for diet identification is that most of the forage consumed is held in the rumen and remains identifiable long enough to allow accurate sampling and identification. However, some highly digestible forages potentially were underestimated. Plants that made up at least 1% of the rumen contents of all deer collected during any one collection period were considered to be important food items for the appropriate season.

Vegetation on the NFWU was sampled to estimate species composition and abundance (in terms of biomass available to deer) within 10 days of the deer collections. This information was compared with the diet information derived from the microhistological analyses to establish relative preferences of forage species. Plants eaten by each deer were ranked according to percent composition in the deer’s diet and percent of total plant biomass (weight of live material per unit area) available to deer on the NFWU. In other words, they were ranked based on use and availability. The differences obtained by subtracting the rank of use from the rank of availability resulted in a preference ranking of dietary items for each deer. These rankings were averaged across all deer for each season to establish preference rankings for all deer collected in a single season. The rankings of the two corresponding seasonal collections were then averaged to come up with a combined seasonal preference ranking. Plant species or complexes found to be used during both corresponding seasonal collection periods and meeting the "important" criterion in both, were ranked higher than those used during just one of the periods. While these preference rankings may indicate deer selection processes a short time prior to deer collection, it is important to emphasize that all of the food items comprising over 1% of the diet are considered important.

Appendix A is a list of all terrestrial and emersed aquatic flowering plant species identified on the NFWU and their respective availabilities based on subjective analyses. This list is provided to help land managers in other parts of the Cross Timbers, as well as other regions, interpret the food habits information.