Brush management methods that presently have the most applicability in South Texas include mechanical and chemical techniques and prescribes burning. Each method has its strengths and weaknesses and should be considered in relation to management objectives.
Mechanical Brush Management
Mechanical brush management methods can be classified into two categories, those designed to simply remove the aerial parts of the plants and those remove the entire plant. Shredding and roller chopping are the primary methods for simple top removal. The effects of these practices are relatively short-lived since most brush species possess tremendous regrowth potential. However, these temporary effects can improve the accessibility and nutrient content of deer forage.
With shredding and roller chopping of mixed brush, a 50 percent canopy recovery has been observed only one year after treatment . However, all brush plants in treated areas are accessible to deer at least during the first year after treatment, and usually longer. Brush plants on untreated areas may be largely unavailable due to height and dense growth patterns of the mottes. Top removal not only increases the availability of browse species by reducing the plant height, but also increases browse allowing more tender regrowth to sprout. Deer readily feed on so-called unpalatable plants when the thorny stems are replaced by new, leafy shoots.
Not only is regrowth more palatable to deer than mature woody plants, but the nutritional quality of the immature growth is usually higher. Immature growth stimulated by top removal tends to be higher in crude protein content and more digestible than mature leaves or stems. Research has shown an eight-fold increase in the value of brush for forage (browse utilization x frequency of the use x plant density) after shredding, and a six-fold increase after roller chopping. While roller chopping may be less effective than shredding for improving the forage value of browse, it has additional advantage of increasing forb production through soil disturbance.
Although shredding and chopping may temporarily improve forage values, continues top removal of brush may result in thickets of root-sprouters such as mesquite and twisted acacia on the treated areas. Mesquite mast is considered important to deer during the summer and twisted acacia may be used by deer, but these species usually are undesirable in dense stands. Because most brush species are prolific sprouters, the effectiveness of shredding and roller chopping treatments generally does not last more than five years. The greatest forage values are associated with the first year following top removal treatment.
Grubbing, root-plowing and chaining are the primary methods of physical plant removal in South Texas. Little attention has been focused on the effects of grubbing on deer forage since it is an extremely selective method. The most efficient control by grubbing usually occurs on sites where woody plants are widely spaced and large enough to be seen by the equipment operator. Elimination of a browse species decreases the diversity of available forage and limits diets selectivity. Cool-season grasses may become established in pits left by grubbing, but grasses are relatively unimportant for deer nutrition.
Most studies on root-plowing have reported that the practice is devastating to white-tailed deer habitat because it destroys cover and plant diversity, unless brush strips or blocks are left untreated. However, because brush species generally constitute a significant portion of the deer diet in South Texas, root-plowing also has a detrimental effect on the year round availability of forage. Although root-plowing may essentially eliminate browse and reduce cover, the soil disturbance generally stimulates forb production . Therefore, newly root-plowed areas offer a good source of seasonal (spring and fall) feed for deer. However, the root-plowed areas will be used by deer only if cover is available nearby and there are alternate food sources to sustain the deer herd when forbs are not present.
Under proper environmental conditions, chaining is an effective methods of knocking down, uprooting, and thinning moderate to dense stands of large woody species. Like root-plowing, chaining large expanses of brushland can be detrimental to deer numbers, as well as deer nutrition, by reducing or eliminating available cover and browse species. The effects of chaining are generally not as severe as root-plowing since the smaller, more limber brush plants are seldom up-rooted and the larger shrubs that are broken off at the base often re-sprout with nutritious, palatable shoots. In addition, the low moderate soil disturbance (depending on treatment) will increase forb production most years.
Chaining may also result in extremely dense stands of prickleypear. A stacker rake must be used (prior to plowing and after chaining) to avoid this problem. Prickleypear is an important food plant for deer, especially during the summer and winter. However, it is most desirable when interspersed with a diversity of other forage species rather than growing in dense stands.
Chaining treatments reduce the density of most species, but differential reaction among species causes post-treatment communities to differ considerably in composition. The relative importance (density, frequency, size) of mesquite changes little after chaining. Although each additional chaining treatment decreases the density, frequency and size of mesquite, it remains the an important part of the woody plant community. Lime prickyash tends to be more susceptible to chaining, and its relative importance decreases with additional treatments. Spiny hackberry (granjeno), a high quality browse, increases in importance with additional chaining treatments.
Fire in South Texas brush communities significantly reduces woody cover during the first year after the burn. However, generally less than 15 percent of the woody plants are actually killed. Although fire does not kill many brush species, prescribed burning can reduce brush cover, alter brush composition and structure, and increase herbaceous cover. A major constraint to effective prescribed burning in South Texas is the amount and distribution of fine fuel required to carry the fire. A brush control treatment before burning may be required to produce adequate amounts and distribution of fine fuel. Therefore, prescribed, burning often is used in combination with other brush management practices and as a maintenance measure. Fire has proved to be more effective on areas where large brush mottes were first knocked down by mechanical means. The reduction of brush cover by shredding two or three years before the fire allows grass and forbs to grow, which provide fuel for a fire throughout the mottes. In addition, the chopped portions of old brush tops provide additional fuel for the burn. A rest rotation system of grazing also is necessary to promote adequate amounts of fine fuel.
Because brush species resprout from buds located on the stem base and below the soil surface on roots or on rhizomes, the effect of fire on these plants is similar to that of any method of top removal. In other words, prescribed burning reduces brush cover, especially following mechanical or herbicide treatments, and increases the forage value (availability, palatability, nutrient content) of brush. Increased browse availability and quality can benefit white-tailed deer, provided that other habitat components are adequate to allow utilization of the browse. Huisache plant that are burned tend to have higher levels or crude protein and phosphorous than unburned plants during the first six months after burning. The greatest differences in nutrient levels between burned and unburned plants occur during the first month of growth. The greatest utilization by deer and other browsers occurs during the first two months following the burn. Burned huisache plants tend to produce five to six times the number of “browsable” twigs as unburned plants. Maintenance of huisache plant in a low-growing bushy state can be achieved by burning at two-to-three-year intervals. Live oak thickets respond in a similar fashion.
A mosaic of brush cover patterns usually will result from burning in South Texas because of fuel load discontinuities associated with arid conditions and moderate to heavy grazing. This variability associated with “brush country” burns is often desirable for creating high quality deer habitat since it results in a vegetation “mosaic”. Deer tend to benefit most from small, hot burns within brush dominated habitats. This pattern increases forbs and valuable browse regrowth while maintaining security cover. Prescribed burning may even restore broadleaved plants to a range where repeated herbicide use has greatly reduced forb population. White-tailed deer make heavy use of burned areas in South Texas, especially in early spring when succulent forb growth is available.
Broadcast herbicide applications can have negative effects on deer habitat, but if applied properly they can improve the quality and availability of food plants and improve overall habitat. Treating relatively large acreages with herbicides may temporarily reduce white-tailed deer numbers. Although the standing remains of defoliated brush offer screening cover for deer, herbicides can reduce the diversity of browse species and the abundance of shade cover. In addition, broadcast herbicide applications reduce the diversity and abundance of forbs. Deer numbers may return to normal by the third growing season after broad-scale brush spraying. Generally, deer use grasses only in small amounts in the spring and fall; however, they consume more grasses in areas sprayed with a herbicide due to a lack of browse and forbs. In such cases, deer may suffer nutritionally since they have a low digestive capacity for grasses.
Research in the northern Rio Grande Plain showed that a substantial portion of the deer population evacuated a pasture where 80 percent of the brush was strip-treated with herbicides. However, when the forbs recovered and browse regrowth developed, deer returned in greater than normal numbers. A study in the coastal brushland found that spraying 80 percent of mature brush in alternating strips did not change deer numbers. The treated and untreated strips were 200 yards and 30 yards wide, respectively. The unsprayed strips apparently furnished adequate forbs which were important deer food items in this area. Spraying 100 percent of an adjacent pasture resulted in a 40 percent reduction in deer numbers. After two years, deer numbers approached pre-treatment levels.
Brush management in drainage habitats should be carefully considered since these sites (such as mesquite drainages) are considered the most important type of habitat for deer in South Texas. The structural features of these sites are preferred by deer for midday loafing and breeding. In addition, these moist, fertile bottomland sites have great potential for producing nutritious deer forage. Indiscriminate broadcasting of herbicides on these sites would be detrimental to deer numbers and/or nutrition. Brush treatment to thin dense stands or create small clearings woul be more appropriate. Research in South Texas has shown that spraying 70 percent of a mesquite bottomland was not detrimental to white-tailed deer. Any reduction in cover screen may have been mitigated by a general increase in quality, quantity and availability of browse. The three-to-ten-fold increase in grass production may have improved conditions for deer by reducing cattle use of forbs and browse.
Broadcast application of soil-applied herbicides at a rate of two pounds per acre or more increased forage production and botanical composition within two years in the northern and central Rio Grande Plain and in the Coastal Prairie. Although aerial applications of these herbicides at two pounds per acre effectively controlled whitebrush, rate as low as one pound per acre were detrimental to forb production and diversity. Rates that were high enough to partially control mesquite (4 lbs/acre) nearly eliminated forb production for two years following application.
One of the most beneficial herbicide applications for deer is the variable rate pattern (VRP), in which different rates of herbicide are aerially applied in strips at right angles to each other. This pattern creates numerous small blocks of vegetation treated with different herbicide rates ranging from none to heavy and results in a diversity of vegetation responses (Fig.3). This type of pattern provides deer with a good selection of food plants at various successional stages, while leaving scattered blocks of mature brush for cover.