Pesticides are an imponant part of modern farming and ranching practices. Most of the pesticides used on ranches are herbicides, with insecticides used less frequently. Herbicides are used to control brush or weeds that limit forage production and hamper livestock handling. Sometimes, however, herbicides may reduce the value of rangelands as wildlife habitat. As stewards of the land and natural resources, ranchers should strive to use pesticides properly in order to minimize environmental risks on their land.
Pests such as cutworms, armyworms, grasshoppers and range caterpillars may infest pastures, but treating them with insecticides is not often necessary. Red imported fire ants can be a serious problem on rangeland in some areas of Texas, and insecticides may be necessary to control them. Because of the limited use of insecticides on rangeland, however, this publication will focus on the effects of herbicides on wildlife.
About 65 percent of the land in Texas is rangeland or pasture land used primarily for livestock grazing. Rangeland supports native plants such as grasses, forbs (herbaceous broadleaf plants), cacti and woody shrubs. These are food for both livestock and wildlife. Rangelands also are important as watersheds and recreational areas.
There are moderate to dense stands of brush on approximately 66 million acres of Texas rangeland. Excessive brush or undesirable brush species can make land of little use for grazing, so ranchers use a variety of methods, including herbicides, to control brush. During the 1980s, about 875,000 acres per year were treated with herbicides for brush control, and more than 2 million acres of pasture and rangeland per year were treated for herbaceous "weed" control.
Most landowners enjoy and appreciate wildlife on their property, and hunting and fishing leases are an additional source of income for many ranchers. If ranchers desire to manage their land for both livestock and wildlife, intelligent decisions must be made about the habitat. Wildlife and livestock usually are compatible. Wildlife and livestock goals can be achieved simultaneously, but they cannot be maximized simultaneously. Livestock production trade-offs will have to be made to accommodate wildlife.
The goals of most Texas ranchers fall into one of the following categories:
Herbicides affect wildlife directly when animals are exposed to chemicals, or indirectly when wildlife habitat is altered. The toxicity of a herbicide is usually expressed as its LD50 (lethal dose). The LD50 of a particular chemical is the dose that kills 50 percent of the animals exposed to it. For any species, the lower the LD50 the more toxic the chemical. Generally, herbicides have much lower toxicity to wildlife than insecticides. With the exception of paraquat (Gramoxone®), all herbicides registered for use on pastures and rangelands in Texas are of low toxicity to birds and mammals (Table 1). Only 2,4-D and paraquat are moderately toxic to fish. Common table salt and aspirin are more toxic than most rangeland herbicides.
The main risk to wildlife from herbicides is habitat alteration. Rangelands are home to many species of wildlife, as are the fencerows, turnrows, borrow ditches, field borders and shelterbelts near pastures. Wildlife rely on rangelands and pasturelands for food and shelter. Many "weeds" in the agricultural context (pigweed, sunflower, goatweed) are important food sources for a variety of birds and other animals.
Because herbicides kill plants and alter the mixture of plant species in the treatment area, broad scale, frequent use of herbicides can harm wildlife habitat. However, if applications are carefully planned and conducted, herbicides can make more forage available for both livestock and wildlife without destroying too much wildlife cover. In order for herbicide applications to be beneficial to both wildlife and livestock, the following must be considered:
When treating large areas (more than 200 acres), apply the herbicide in strips alternating with untreated strips or blocks. The minimum widths of untreated strips or blocks will vary with plant species composition, plant densities, herbicide used, topography, the particular requirements of the wildlife species inhabiting the area, and the landowner's goals. In general, untreated brush strips should be at least 50 yards wide, and preferably 100 yards. Brush strips may vary in size and configuration (straight lines, checkerboard patterns).
The variable rate pattern (VRP), in which different rates of herbicide are applied in strips at right angles to each other, can be one of the most beneficial application patterns for wildlife. This pattern creates numerous small blocks of vegetation treated with different herbicide rates ranging from none to heavy. As a result, some vegetation will be killed, some suppressed and some not affected at all (Figure 1). VRP application increases "edge" and leaves scattered blocks of undisturbed brush that wildlife can use for cover. The mosaic created by VRP application depends on the range site and the herbicide rates used, so no two VRPs are exactly alike. VRP application costs more than other types because of the additional flight swaths required.
When herbicide is broadcast on relatively large areas (more than 200 acres) of ranneland, wildlife numbers may be reduced temporarily. Animals may use only a portion of the treated area, usually near the edges. Some species such as quail may stay within 50 yards of the edge if there is no suitable brush cover (live or dead). Herbicide applications generally leave some brush which can serve as screening cover for deer, but defoliated brush gives little shade. Herbicides reduce the amount and diversity of food plants available, both browse and forbs. As a result, animals may avoid the area until plants recover (usually one or two growing seasons).
A substantial portion of the deer population will leave a pasture when most of it has been sprayed. However, when the forbs recover (generally in 3 to 18 months) and browse begins to regrow, deer may return in greater numbers than before the treatment. In one South Texas study, spraying up to 80 percent of mature brush in alternating strips did not change deer numbers, but broadcast spraying of all the pasture resulted in a large reduction in deer numbers, and the population stayed small for at least 2 years.
Brush management along drainages and in bottomlands should be done carefully, if at all, because these areas are especially important wildlife habitat. They usually contain many kinds of plants, especially woody plants, which provide loafing cover for quail and turkey, bedding sites for deer and nest sites for songbirds. These bottomlands also produce an abundance of food plants. Indiscriminate applications of herbicides should not be made on these sites. Selective or "spot" brush treatments to thin out dense stands or create small clearings would be more appropriate.
Research in South Texas suggests that treating less than 70 percent of a mesquite bottomland is not detrimental to white-tailed deer. The reduction in cover was offset by an increase in the quality, quantity and availability of browse. Grass also increased, and since cattle prefer grass to forbs and browse, there was less competition between cattle and deer.
Herbicides can be applied to plant leaves (foliar-applied), bark (basal-applied) or roots (soil-applied). Foliar applications can be made over a large area (broadcast) with aerial or ground equipment, or applied to individual plants (Fig. 2). Individual plants can be treated by spraying the foliage or the bark with a hand-held or power sprayer.
Soil-applied herbicides come in liquid (Velpar L®, Grazon PC®) or pellet form (Spike 20P®). In either case the herbicide is applied to the soil under the plant's "drip line," and the roots absorb the chemical. Pelleted herbicides such as tebuthiuron (Spike 20P®) can be applied with a "whirlybird" spreader or with a backpack blower (Fig. 3). The backpack blower is particularly useful for creating small openings or strips in brush.
The "best" method of application depends upon the mode of action of the herbicide used and the plant species to be controlled. Local conditions and hazards may also dictate that a certain application method be used. Pelleted herbicides and individual plant treatments are very useful when working near sensitive habitats.
Woody plants and forbs react differently to various herbicides (Table 2), especially to foliar broadcast treatments. Choosing the appropriate herbicide, herbicide rate and application method will ensure control of problem plants while lessening the effects on desirable plants and wildlife habitat. An advantage of herbicides over mechanical brush control methods (from the wildlife perspective) is that herbicides rarely kill more than 65 percent of the plants in a site containing mixed brush. Species such as lime pricklyash and lotebush are especially resistant to herbicides.
Some mixtures of foliar herbicides will control mesquite while having much less effect on important wildlife food plants (Fig. 4). However, some mixtures such as triclopyr plus picloram may be more damaging to species such as hackberry and chittam than triclopyr alone. Similarly, commercial mixtures such as Weedmaster® (Banvel® +2,4-D) or Grazon P+D® (picloram + 2,4-D) may be more damaging to desirable perennial forbs than 2,4-D alone. Pelleted herbicides (Spike 20P®) control various species of oak and whitebrush but have minor effects on other important wildlife plants. When possible, select the herbicide that will pose the least risk to plants important to wildlife. Consult Extension publication B-1466, "Chemical Weed and Brush Control: Suggestions for Rangeland," to select appropriate herbicides for particular plants.
A major concern when spraying herbicide is that the chemical may drift away from the target area to wildlife habitats nearby. Even some soil-applied herbicides may be absorbed by the roots of sensitive plants (such as oaks) near by. When applying any herbicide, the operator should minimize drift by:
Certain wildlife species are protected by state and/or federal laws under the Endangered Species Act. Texas currently has about 128 species (or subspecies) of animals on its Threatened or Endangered Species list. Also, 28 species (or subspecies) of threatened or endangered plants are found within the state. Herbicide labels list restrictions on their use in habitats occupied by endangered species. Species that may be mentioned on herbicide labels include the Attwater's prairie chicken, the Aplomado falcon and the ocelot, all residents of south Texas. The Attwater's prairie chicken may suffer when broadcast spraying kills the herbaceous plants it depends upon for food. On the other hand, the birds benefit when introduced brush species such as Chinese tallow and Macartney rose are controlled. These plants are invading and harming prairie chicken habitat. On such sites, herbicides can be used to treat individual plants or plant colonies, rather than broadcast.
There may be herbicide restrictions to protect endangered or threatened species in your area. Contact your county Extension office or the Texas Department of Agriculture to find out.
Integrated pest management (IPM) is an approach that emphasizes non-chemical methods of pest control. Chemicals are used only when necessary, and the goal is to reduce their use. The equivalent of IPM on rangelands is Integrated Brush Management Systems (IBMS). IBMS combine two or more brush management methods in the right sequence so that brush can be controlled effectively and economically while meeting other goals such as manipulating wildlife habitat. Such brush control programs make use of careful, long-term planning, so that the needs of both livestock and wildlife are met. IBMS extend the effectiveness of brush management over a longer period than single treatment methods. As a result, less herbicide is needed.
The non-chemical brush control methods used in IBMS are mechanical, biological and prescribed fire. An example of an IBMS would be burning pricklypear with a prescribed fire and then spraying cactus with herbicide. This approach should control more than 90 percent of the cactus with only 25 to 50 percent of the herbicide rate required to control unburned cactus (Fig. 5). Another example would be roller-chopping mixed brush and then browsing it with goats to maintain control without the use of herbicides. Each site and situation is different, so IBMS require careful planning.
The needs of wildlife should always be considered when planning brush control programs. Before using chemicals, ask the following questions:
Every herbicide user has a legal duty to use the chemical according to label directions. Using pesticides contrary to label directions jeopardizes wildlife and exposes the user to criminal prosecution. It may ultimately result in further restrictions on pesticides. Please do your part to see that herbicides are used in a responsible fashion according to label directions.
| Herbicide | Brand Name | Birds | Mammals | Fish |
|---|---|---|---|---|
| Clopyralid | Reclaim® | L | L | L |
| 2,4-D | Weedar 64®, Formula 40®, Hi-Dep®, Weedone LV4®, Esteron 99®, and others | L | L | M |
| Dicamba | Banvel® | L | L | L |
| Hexazinone | Velpar® | L | L | L |
| Metsulfuron methyl | Ally®, Escort® | L | L | L |
| Paraquat | Gramoxone®b | M | M | L |
| Picloram | Grazon PC® | L | L | L |
| Tebuthiuron | Spike 20P® | L | L | L |
| Triclopyr | Remedy® | L | L | L |
| H (Highly toxic) - LD50 less than 30 mg/kg and LC50 less than 500 ppm M (Moderately toxic) -LD50 greater than 30 but less than 100 mg/kg and/or LC50 greater than 500 but less than 1,000 ppm L (Low toxicity) - LD50 greater than 100 mg/kg and LC50 greater than 1,000 ppm b Not labeled for rangeland use |
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| Woody plants | Application technique | Forbs | Application technique | ||
|---|---|---|---|---|---|
| Foliar broadcast | Basalb | Soil-applied | Foliar broadcast | ||
| Mesquite | L-H | VH | H | Ragweed | H-VH |
| Coma, Chittam | L | H | M | Daisy | VH |
| Sumacs | H-VH | H | H | Crotons | VH | Acacias | L-M | VH | VH | Orange zexmenia | L-M |
| Hackberry | M-H | VH | VH | Snoutbean | VH |
| Oaks | L-H | VH | VH | Sunflower | VH |
| Juniper | L | VH | VH | Dayflower | VH |
| Lotebush | H | H | VH | Partridgepea | M-H |
| Guayucan | L | L | L | Mallows | VH |
| Pricklypear | H | M-H | L | ||
| Plum | VH | H-VH | VH | ||
| Cottonwood | VH | VH | VH | ||
| VH (Very high) - greater than 75% mortality H (High) - between 50 and 75% mortality M (Moderate) - between 25 and 50% mortality L (Low) - less than 25 percent mortality bBasal applications usually control target plants better, but don't usually affect other plants. |
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