BEN H. KOERTH, Institute for White-tailed Deer Management and Research, Stephen F. Austin State University, Nacogdoches, TX 75962.
Abstract: Density of brush cover sometimes limits management of rangeland animals. Herbicides have been shown to be an effective tool to manipulate brushland habitats for both wildlife and livestock as long as appropriate herbicides, patterns and rates of application are observed.
That woody plants dominate the vegetation cover of most rangelands is axiomatic. It has been estimated that more than 88% of Texas rangelands support brush densities severe enough to cause problems in effective management (Scifres 1980). As Blakey (1947) so aptly stated, “Encroachment of brush jungle upon formerly open forest and prairie range is insidious in that it has both good and bad effects upon certain wildlife species, and in some areas has the constant potential for near total exclusion of all valuable forms.” While it is widely recognized woody cover plays a critical role in wildlife habitat, having some brush cover is becoming increasingly recognized as desirable in livestock production as well. However, it often is advantageous to manipulate the composition, height, shape, canopy cover and relative availability of different species for use by rangeland animals.
Quality of an animal’s habitat is determined largely by the structure and composition of the vegetation. To benefit an animal population, the habitat must meet the needs of that animal. Quality habitat can be defined, at least in part, as a function of the interaction of woody and herbaceous components. The “domain of presence” of an animal population can be defined as the set of woody and herbaceous combinations that will support that population. Peak animal abundance theoretically occurs at the optimum mix of woody and herbaceous cover. As woody cover goes above or below optimum, animal abundance would be expected to decline until conditions are reached that will no longer support the population (Koerth 1996).
In simple form, herbaceous production is inversely related to woody cover. As woody cover increases, herbaceous cover decreases. Different animal species are adapted to exist at different levels along this woody-herbaceous continuum. Therefore, an understanding of the basic requirements of the species to be benefited must be understood. As there are limits to the amount of woody cover required by different species, it often is desirable to manipulate the habitat to favor certain populations. After years of controversy, it is becoming accepted that herbicides can be used to manage wildlife habitat as long as appropriate rates and patterns of application are used.
Herbicides, in the context of this discussion, are compounds exhibiting phytotoxic properties. In other words, a chemical used to control, suppress, kill or severely interrupt the normal growth processes of plants. While toxic to certain plants, rangeland herbicides exhibit a low order of toxicity to birds and mammals. There remains no evidence, when applied properly, that herbicides currently labeled for rangeland use will bioconcentrate and pose direct harm to rangeland wildlife or man (Johnson 1971).
Every brush management technique, including herbicides, has a unique set of strong points and associated weaknesses. The following, adapted from Scifres (1986), are the advantages and disadvantages of using herbicides to manipulate rangeland vegetation.
1. A variety of application methods are available ranging from individual plant treatment to broadcast application.
2. Aerial application methods are fast compared to mechanical methods and are independent of rough terrain and plant growth forms.
3. Applications result in little or no soil disturbance.
4. Stems and trunks of defoliated species and foliage of herbicide-resistant species remain to provide screening cover and shade.
5. Herbicides are effective in suppressing selected woody species.
6. Soil-applied herbicides have minimal drift and can be applied during relatively broad periods of the year.
7. Broad-leaved herbaceous plants also may be suppressed, hereby further reducing com-petition from more desirable species.
1. Many plants suppressed by herbicides are favored food plants of some wildlife species.
2. Foliar sprays are usually restricted to certain phenological stages of plants.
3. Application of foliar sprays may be hampered by environmental conditions such as wind speed and temperature.
4. The spectrum of species controlled in complex systems may not be sufficient to allow maximum production of the desired species.
5. Suppression of susceptible species may allow herbicide-resistant species to increase and ultimately form stands more difficult to manage than the original cover.
Methods of application
Methods of herbicide application basically can be categorized as aerial or ground methods. Use of a fixed-wing or rotary-wing (helicopter) aircraft is probably the most common. Aerial applications have the advantage of being able to cover relatively large areas in a short time. Also, aircraft are not limited by rough terrain or size of the woody cover to be treated. Herbicides can be applied either as a foliar spray or in a dry, pelleted form.
However, aerial application methods are not limitless. A suitable landing strip is necessary for fixed-wing aircraft. In addition, aerial applications are relatively broad-brush treatments and do not allow for much discrimination in plant treatment other than with herbicide formulation. Non-target species receive the same treatment as the species we hope to suppress.
Because herbicides do not leave an immediate physical mark on the area treated, aerial treatments are applied by flying the aircraft from point to point marked by flaggers at each end of the treatment area. Thus, aerial applications tend to be in straight lines and may not result in a physically appealing treatment pattern.
On the other hand, ground applications allow more flexibility. While broadcast treatments are an option, individual plant treatments permit the ultimate in selectivity. Particular species or even specific plants within species can be targeted for control. The trade-off being the time necessary to physically move across the area. Also, ground applications are limited by rough terrain and size of plants that may prohibit vehicle access. Ground applications are better suited to small areas than aerial methods.
Effects on wildlife habitat
The principle short-term impact on wildlife habitat is defoliation and loss of cover, browse and herbaceous food plants. However, changes with herbicide treatments are more subtle than with mechanical methods. It may take as long as two weeks for leaves to turn brown and a month or more to defoliate. With some soil-applied herbicides such as tebuthiuron, complete defoliation of susceptible species may not be complete for up to two years. Stems, trunks and all but the smallest twigs remain to provide screening cover and shade.
Because different plant species are susceptible to different herbicides and rates, only generalizations can be made about specific impacts. Predictions of future outcomes must be made on a case-by-case basis. However, there inevitably will be some loss of browse and broad-leaved plants following herbicide applications. This reduction in the potential food supply, however, is often short-lived. Restoration of herbaceous plants is seemingly largely dependent upon precipitation patterns following treatment.
The major long-term impact of herbicide applications on habitat is suppression of susceptible species, thereby reducing the complexity of the vegetation. However, changing vegetation composition is not always negative. Altering the forage composition may actually increase the diversity of the food supply compared to the untreated state. Selective treatments also can create relatively stable shrub communities and somewhat less stable herbaceous openings when interspersed within larger, mixed woodlands.
Patterns of application
Extensive coverage with broadcast herbicide applications is seldom recommended where wildlife is important. Large blocks of defoliated brush often exceed the threshold of suitability for many wildlife species (Beasom and Scifres 1977).
However, herbicide applications in patterns that retain sufficient untreated areas to provide food and cover have been used successfully in wildlife management. Probably the most commonly used practice is strip spraying. A herbicide dose is applied within the strip to provide maximum control of woody plants. Treated strips typically are alternated with untreated strips of approximately equal width. This pattern is the easiest to layout and application particulars are uncomplicated. However, while effective, the resultant pattern may be aesthetically unappealing to some.
To avoid the straight edge approach of strip spraying, Scifres and Koerth (1986) implemented and evaluated an alternative, generically called variable rate patterning (VRP).
In simplest form, a VRP is installed by applying herbicide at one half the normal dosage in treated and untreated strips in two directions. The second set of strips is applied over the same area, but perpendicular to the first. The resultant effect is not the checkerboard visualized from the treatment design, but rather a very asymmetrical pattern from responses of differing vegetation communities with different herbicide rates. This pattern provides a selection of habitats at varying levels of succession interspersed with blocks of mature, untreated areas for cover.
Whatever the treatment pattern, herbicides can be effectively used for manipulating vegetation for both wildlife and livestock. However, it is imperative to know the species composition of the proposed treatment site to predict plant and animal response. The most effective treatments maintain a high degree of structural and botanical diversity to provide requisite habitat needs for the target animal species.
Beasom, S. L., and C. J. Scifres. 1977. Population reactions of selected game species to aerial herbicide applications. J. Range Manage. 30:138-142.
Blakey, H. L. 1947. The role of brush control in habitat improvement on the Aransas National Wildlife Refuge. Trans. North Am. Wildl. and Nat. Resour. Conf. 12:179-185.
Johnson, J. E. 1971. The public health implications of widespread use of the phenoxy herbicides and picloram. Bioscience 21:899-905.
Koerth, B. H. 1996. Chemical manipulation of plants. Pages 321-337 in P. R. Krausman, ed. Rangeland wildlife. The Society for Range Management, Denver, Colo.
Scifres, C. J. 1980. Brush management. Principles and practices for Texas and the southwest. Texas A&M Univ. Press, College Station. 360pp.
Scifres, C. J. 1986. Integrated management systems for improvement of rangeland. Pages 227-259 in M. A. Sprague and G. B. Triplett, eds. No-tillage and surface-tillage agriculture: The tillage revolution. John Wiley & Sons, Inc., New York, N.Y.
Scifres, C. J., and B. H. Koerth. 1986. Habitat alterations in mixed brush from variable rate herbicide patterns. Wildl. Soc. Bull. 14:345-356.Comments: Dale Rollins, Professor and Extension Wildlife Specialist
Updated: Mar. 18, 1997