Research Shows Feed Improves Dynamics Of Deer Population
SAN ANTONIO — The first phase of a 10-year study investigating the impact of white-tailed deer densities and supplemental feed has yielded results which were presented at the recent Deer Associates meeting here.
The study was conducted by a team of researchers at the Caesar Kleberg Wildlife Research Institute. It addresses the impact of supplemental feed on vegetation health, the nutritional health of the deer, and population dynamics such as impacts on antler size, growth rate and adult and fawn survival.
The study sites were on the Comanche and Faith ranches in Dimmitt County of western South Texas. Comanche Ranch owner Dan Friedkin and Faith Ranch owner Stuart W. Stedman provided funding for the project as well. Three different deer densities and supplemental feed treatments were implemented in six enclosures, each 200 acres in size, on each ranch. On each of the ranches two enclosures had a density of 10 deer per 200 acres, two enclosures had 25 deer per 200 acres and another two had 40 deer per 200 acres.
The density levels were based on historical levels recorded in South Texas. For example, the 40 deer per 200 acres is slightly higher than the highest published population estimate recorded in South Texas. Over the course of the study researchers maintained these prescribed densities through helicopter captures and harvest.
On each of the ranches deer in three of the six enclosures were provided supplemental feed. Two feeders located in the center of these enclosures provided a pelleted ration ad libitum 365 days of the year over the entire period of the study.
Annual deer captures were conducted. Various measurements such as sex, age, body weight and antler size were recorded, and the deer were tagged for identification purposes in the event they were captured again. From this data collection they were able to investigate population size, survival and fawn-to-doe ratios.
To determine the native vegetation composition of the deer’s diet, female fawns were raised from birth and habituated to people. Each doe was then equipped with a radio collar and translocated to one of the enclosures. After a period of time, graduate students simply followed the deer around to learn what native vegetation was being consumed.
The deer diet data were analyzed over the four different seasons during drouth and wet periods. Dr. Dave Hewitt, the Leroy G. Denman Jr. Endowed Director of Wildlife Research at the Caesar Kleberg Wildlife Research Institute, told listeners that what they learned, in part, is that deer change what they eat depending on the conditions. For example, they documented a “pretty dramatic” drop in the amount of shrubs consumed during wet compared to dry periods. The amount of shrubs in the deer diet declined during the spring as well. In the wet springs, about 80 percent of their diet was made up of annual forbs.
They learned that mast — prickly pear fruits, mesquite beans and various other pods and fruits — were particularly important during the summer. In fact, Hewitt said, mast accounted for almost 50 percent of their diet in both wet and dry years.
“In dry springs deer got pretty creative,” said Hewitt. “Cactus was a fair part of the diet, as were blackbrush flowers.”
Looking at the impact of deer density on deer diets, researchers did not identify a statistical difference in diets even with a four-fold difference in deer density. Hewitt said there was no statistical difference in proportion of forbs or mast in the diet. There was a difference in the amount of cactus in the diets, though it was not consistent.
“In the spring the low-density deer had more cactus in their diet, and in the winter the high-density deer had more cactus in their diets,” said Hewitt. “So we couldn’t say that the deer would have a greater use of cactus if the deer density was higher.”
He added that grass accounted for a larger part of the diet for deer in the high-density enclosures, but it was only about one percent, whereas in the lower-density enclosures, grass accounted for only half a percent.
In terms of diet quality, Hewitt said there was no change in digestible protein or energy between the different deer densities.
However, when supplemental feed was added to the equation, the vegetation consumption clearly changed. For example, Hewitt said that in all seasons except winter the deer with supplemental feed had more browse in their diet than the deer that did not have access to supplemental feed. Deer that had access to the pelleted feed also decreased their consumption of mast. There was no difference in the proportion of the diet composed of forbs.
Hewitt also noted that the deer that did not have access to supplemental feed were the ones that focused on the blackbrush flowers in the winter. Perhaps even more interesting, during the winter the deer that had access to supplemental feed weren’t eating the flowers but were instead eating dead leaves.
“It was almost like they were looking for something to balance out the high quality pelleted food,” Hewitt told listeners.
In terms of diet quality, researchers were not really able to detect a difference in digestible protein in the vegetation portion of the diet for the deer with supplemental feed versus those without. There was, however, a spring and summer of a wet year in which they did detect some differences in the amount of metabolizable energy in the diets. Specifically, the deer that had access to supplement had a lower amount of energy in the vegetation portion of their diet than those that did not.
Dr. Tim Fulbright, Meadows Professor in Semi-Arid Land Ecology, Research Scientist and Regents Professor, followed with the vegetation analyses comparing the different deer densities and then supplemental feed to those without supplemental feed. One observation, Fulbright told listeners, is that the abundance of preferred forbs on the landscape varied a lot more with variation of rainfall than it did with deer density.
He said five of the nine years of the study were drouth years, based on the Palmer Severity Index. Also, three years were “really wet,” and one year was “normal”.
During the drouth years, the canopy cover of the preferred forbs was low, Fulbright said. In fact, forbs were almost non-existent. They also found that the canopy cover of these preferred forbs did not change over time based on the different deer densities.
“If those deer were having an impact on the preferred forbs, we would expect to see a decline in their canopy cover over time, and there wasn’t,” Fulbright told listeners.
Not only was there no difference in the canopy cover of the preferred forbs, species diversity was not impacted, either. Additionally, researchers basically found that the same thing was true for the shrubs.
Fulbright opined again that highly variable rainfall — going from wet to dry and seldom anything in between — likely weakened the anticipated influence of deer density on vegetation. He also reiterated Hewitt’s comments about the deers’ ability to adapt and survive on whatever is available in the given season. That gave the class of vegetation that the deer were previously consuming time to recover. He opined that rest was another of the contributing factors as to why no difference was detected between high and low-density enclosures.
Fulbright also noted that the woody browse plants are “extremely well defended” from browsing herbivores by such anatomical attributes as thorns and compounds like tannins that don’t typically set well with a herbivore’s digestive system.
Turning to the supplemental feed part of the equation, Fulbright told listeners that there’s a perceived concern that providing supplemental feed to deer causes habitat degradation, specifically that the number of preferred deer foods decline. However, what the researchers found was that over time the preferred forbs actually increased in abundance. Likewise, the diversity of plants was greater overall in the enclosures with supplemental feed. Nor did they find a difference in the percentage of preferred forbs in the high deer density enclosures versus the lower density enclosures. Fulbright said this was because more than half of the deers’ diet was feed; subsequently, that reduced pressure on the vegetation.
“Needless to say, the vegetation dynamics in this western part of South Texas didn’t follow our traditional notions of how vegetation is supposed to change,” Fulbright told listeners. “We did not see a shift away from the climax vegetative state. What we learned was the environmental factors control changes in the vegetation more than deer.”
Dr. Charlie DeYoung, Research Scientist and Professor Emeritus, rounded out the presentation with the population dynamics and deer quality impacts of the study. Researchers investigated the impact of different density levels on antler growth, fawn-to-doe ratio, growth rate of fawns six months to a year old, growth rate of yearlings, buck and doe body weights, survival of adults and older fawns and population growth rate.
Starting with antler growth, DeYoung said they found no density effects on the proportion of spike buck yearlings under the no-feed treatment with increasing deer density. Increasing deer density also did not negatively impact Boone and Crockett scores of mature bucks.
As for the impacts of supplemental feed, forked antler yearlings increased by 48 percent, and on average mature buck antler B&C scores increased by 17 inches.
Enclosures without feed showed a density-dependent impact on the fawn-to-doe ratio. On the higher density enclosures there was “quite a decline” in the fawn-to-doe ratio.
On the fed sites the fawn crop averaged about 70 percent compared to 30 percent on the unfed sites.
Body weights for bucks and does, DeYoung said, declined for deer that did not have supplemental feed. Additionally, DeYoung said, there was a density-dependent effect on population growth rate in the enclosures without supplemental feed.
In those same enclosures, researchers found no density effects for antler size, adult survival, older fawn survival, growth rate of fawns or growth rate of yearlings. Likewise, there were no density effects for the supplemental feed treatments. However, DeYoung said, they observed an increase in aggressive interactions at the higher deer densities, particularly at feeder sites.
Supplemental feed, he reiterated, increased basically everything in terms of the deer themselves — body weights and antlers were bigger, fawn-to-doe ratios were higher, survival was higher and the population growth rate was larger.
Summing it all up, researchers found that on sites without feed, deer density levels had little effect on diets or vegetation. However, they did find some density effects on some population measures and deer quality measures. With supplemental feed, animal and population qualities improved significantly. Feeding, however, did not result in damage to preferred plants in the habitat.
In terms of management implications, DeYoung told listeners that without supplemental feed a manager should be careful about harvesting does. Also, he said, the research clearly shows that year-round free choice supplemental feed results in significant improvements to the deer themselves and the population overall, though he acknowledged “it’s pretty darn expensive.”
That said, he also pointed to a potential concern over disease transmission.
“If chronic wasting disease ever became established in South Texas, that would shut down feeding, because concentrating deer around feeders would further spread the disease,” said DeYoung. “We would lose this tool.”
Finally, the lifelong deer researcher stressed that the results are applicable to the thorn scrub region in the western portion of South Texas, assuming fairly good native vegetation.
“We just don’t know how applicable any of this would be to other areas of the state,” he concluded.