Prescribed Grazing in Practice
The goal of prescribed grazing for invasive plant management is to manipulate patterns of defoliation and disturbance to place a target plant at a competitive disadvantage relative to other plants in the community (Walker et al. 1994). Achieving this goal requires extensive knowledge and solid understanding of how invasive and desirable plant populations within a particular ecosystem will respond to a particular herbivore's grazing behavior.
Not all ecosystems are compatible with grazing. Plant communities in two different ecosystems may respond very differently to the same grazing prescription. Likewise, grazing patterns and their influence on plant communities vary with different types of herbivores.
In weed-infested areas, grazing must be carefully managed to reduce rather than increase invasive plant establishment and spread. Ecologically-based grazing prescriptions pay careful attention to positively directing plant community change, not just removing the weedy species (Sheley et al. 1996). Grazing prescriptions may put target plants at a competitive disadvantage using two general approaches (Frost and Launchbaugh 2003):
- Use grazing management that harms the target plant species by grazing at a time and frequency when the target plant is most vulnerable.
- Modify the grazing behavior of animals to cause them to concentrate their grazing efforts on the target plant instead of the desirable vegetation.
Prescribed grazing tactics manipulate three basic variables—herbivore selection, seasonal timing, and intensity—to cause a predictable plant community response.
Herbivore Selection
Selecting an appropriate herbivore is an important step in designing a grazing prescription. Cattle, sheep, goats, and other livestock animals have different dietary preferences, foraging behaviors, and management requirements. Just as herbicides and biological control agents are specific to groups or species of plants, livestock animals are selective in which plants they eat, and how they eat them. Furthermore, livestock animals require access to water, protection from predators, and have variable needs in terms of containment and herd management.
General Dietary Preferences, Foraging Behavior, and Care Requirements for |
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(Frost and Launchbaugh 2003, Tu et al. 2001, Coffey 2001)
Grazing behavior can vary widely among breed, age, sex, body condition, hunger, and previous experience of individual animals (Frost and Launchbaugh 2003). Infestation density, growth stage, and availability of other foods also influence an animal's grazing behavior (Olson 1999b). Research is ongoing to identify predictable grazing patterns, modify foraging behaviors, and even breed inherent dietary preferences.
Multispecies grazing takes advantage of the inherent grazing preferences among different classes of livestock (Walker 1994). Multispecies grazing is the use of two or more herbivores to graze a common resource. The grazing species can be wild or domestic animals. They can graze an area simultaneously or at different times (Coffey 2001).
Benefits of multispecies grazing |
Challenges of multispecies grazing |
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Timing
By introducing grazing animals into an area at the proper time, land managers can interrupt invasive plant reproductive cycles and take advantage of differential vulnerability and palatability of target versus nontarget plants.
- Animals should be brought into an infested area at a time when they will be most likely to damage the target invasive plant species without significantly impacting desirable species.
- Grazing timed to remove developing flowers or seedheads can reduce seed production for that year (Williams and Prather 2006, Olson et al. 1997, Thomsen et al. 1996). However, grazing weeds during seed set may not be advisable because of risk of livestock spreading seeds.
Intensity
Grazing intensity is typically described as "low," "moderate," or "high" and can be manipulated to control the level of defoliation and impact produced by livestock. Grazing intensity levels are a factor of how many (stocking rate), how long (duration), and how often (frequency) livestock animals are allowed to graze in an area.
- Light stocking rates can be used to take advantage of an animal's inherent dietary preferences in sparse infestations of relatively palatable plants. Heavy stocking rates can be used to force more even use of forage in dense infestations or less palatable plants (Frost and Launchbaugh 2003).
- Grazing intensity should be closely monitored and the animals promptly removed when the proper amount of defoliation or impact has been achieved. As a rule, livestock should not be allowed to graze more than 60% of the desirable plant foliage and consumption of target plants should significantly exceed consumption of desirable plants (Olson 1999a).
- The interval between grazing periods allows recovery of grazed plants. One-time defoliation, even at a plant's most susceptible stage has a minimal impact on most weedy forbs. Therefore, repeated grazing within one season for several years is often required to control some plants.
A critical step in developing an effective and ecologically sound grazing prescription is establishing criteria by which the prescription’s implementation and effectiveness will be measured. By collecting quantitative data over time, one is better equipped to detect trends toward or away from the desired effects of grazing treatments. Furthermore, monitoring during grazing treatments will help to determine whether grazing treatments are applied at the appropriate season, duration, frequency, and intensity.
In general, monitoring activities should determine whether treatments are being applied as prescribed, meeting invasive plant management objectives, and having desired effects on the plant community.
Prescribed grazing can be used to implement a number of invasive plant management options.
| Prevention
With proper management, livestock can be used to develop and maintain desirable vegetation conditions and help prevent invasive plants from establishing. Grazing at the early stages of plant invasion can help reduce colonization and slow the rate of invasion. |
Suppression
Moderate densities of invasive plants may be suppressed through prescribed grazing. Selective grazing applied over the long term can gradually reduce the invasive plant’s competitive ability within the plant community. |
| Containment
Grazing applied at the appropriate growth stage can prevent flower and seed production, thereby containing plant populations that spread by seed. Once a plant community is dominated by an invasive plant, realistic grazing goals may be to use the invasive plant as forage while taking care to prevent expansion of infestations. |
Eradication
Eradicating invasive plants is rarely, if ever, a realistic strategy when using livestock grazing alone. |
Prescribed grazing has advantages over other management methods: |
Prescribed grazing has disadvantages that may preclude its use: |
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Prescribed grazing may be applied alone or combined with other invasive plant management methods. When applied alone, the effects of prescribed grazing can be very slow to appear and often require repeated treatments over a number of years to effectively suppress most target plant species. Integrating prescribed grazing with other invasive plant management methods can mutually enhance each method and can produce more rapid, long-term effects than the same methods used alone.
Prescribed Grazing and Physical Methods
Grazing can be used to clear vegetation and facilitate access to a site, or remove excess canopy vegetation to aid in locating low-growing target species for treatment with physical methods.
Prescribed Grazing and Chemical Methods
Grazing may be applied either before or after herbicide treatments to enhance the effectiveness of either treatment. On spotted knapweed- (Centaurea maculosa)-infested rangelands in western Montana, spring-applied herbicide treatments enhanced sheep grazing by shifting knapweed populations from mature, less-palatable plants to juvenile plants preferred by sheep (Sheley et al 2004). In North Dakota, moderate sheep grazing followed by a fall-applied herbicide treatment resulted in more rapid and long-term leafy spurge (Euphorbia esula) control than either method used alone (Lym et al 1997). Grazing leafy spurge during spring and summer can reduce canopy and stimulate shoots to grow in the fall. A fall application of an appropriate herbicide then acts on the rapidly developing regrowth.
Prescribed Grazing and Biological Control
Positive and negative interactions between grazing and biological control agents have been documented. In Montana, three years of sheep grazing and flea beetles (Aphthona nigriscutis) reduced densities of leafy spurge more than either sheep grazing or flea beetles alone (Hansen 1993). However, in Colorado, goat grazing resulted in fewer Dalmatian toadflax (Linaria dalmatica) biological control insects (Brachypterolus pulicarius) per flowering plant compared to ungrazed plots (Norton and Hufbauer, unpublished data).
Prescribed Grazing and Prescribed Burning
Grazing can be used to manipulate fire intensity by deferring (to increase fuel load) or increasing defoliation (to decrease fuel load) (DiTomaso and Johnson 2006). Fire can be used to stimulate regrowth and increase palatability of some invasive plants. However, desirable plant populations may also be more vulnerable to the negative effects of grazing following a burn event.
Prescribed Grazing and Restoration/Revegetation
Livestock animals can be used to restore degraded lands by breaking up the soil surface and incorporating seeds of desirable plants.
Domestic livestock grazing is a predominant land use in the western United States. Use of livestock for invasive plant management is fairly well documented in this region (Olson and Lacey 1994, Walker et al. 1994, Olson et al. 1997a, Olson et al. 1997b, Olson and Wallander 1998, Thomsen et al. 1996). Examples from the eastern United States are becoming more common as goats, sheep, and cattle gain popularity as an “environmentally friendly” control method (Luginbuhl et al. 1996, Luginbuhl 2000, Escobar et al. 1998, Hart 2000, Tesauro 2001).
Researchers and managers are finding creative ways to use goats, sheep, and cattle to control invasive plants in a variety of environments. Although non-domesticated herbivores (i.e., grass carp [Ctenopharyngodon idella]) have been introduced to control aquatic plants, current knowledge and practice of prescribed grazing is generally limited to terrestrial habitats.