Using Herbicides for Horse Pasture Weed Control

Good stewardship and management practices are required to ensure you’re getting the most benefit from an herbicide application.

Photo: iStock

Property owners can use various methods and strategies to combat weed problems in pastures. These include mechanical and cultural practices such as mowing or clipping fields, maintaining a good soil fertility program, grazing methods, and other management practices that promote desirable forage grass growth, which in turn competes against weeds.

Herbicides are one of the best ways to effectively control several troublesome broadleaf weeds. However, good stewardship and management practices are required to ensure you’re getting the most benefit from an herbicide application. Stewardship includes proper spray applications to minimize the potential for off-site movement of herbicides that might damage nearby sensitive crops and vegetation. Furthermore, consider reseeding and a field’s future uses before applying an herbicide.

Below are some important tips to consider when choosing and applying an herbicide product.

Tip 1: Select the appropriate product

It’s important to choose the right herbicide for the specific weed(s) you want to control. Most pasture herbicide products selectively target broadleaf weeds, but certain products are better for controlling specific weed species. Consult the product label and other resources such as university weed control guides (e.g., Weed Management in Grass Pastures, Hayfields, and Other Farmstead Sites, AGR-172 or Broadleaf Weeds of Kentucky Pastures, AGR-207) to determine if an herbicide will control the weeds of greatest concern. It is also important to determine if an herbicide product is approved for application on grazed pasture fields. Many herbicides that are registered for use to control weeds in lawn or turf areas contain active ingredients that are not EPA-approved for application on pastures to be grazed by animals.

Tip 2: Apply at the right time of the year

Herbicide products often work best on younger, actively growing weeds. Therefore, you must also consider the weeds’ size and growth stage. As annual weeds grow larger and mature, an herbicide’s effectiveness often decreases. Furthermore, herbicides will provide little long-term control of weeds that have begun to flower and produce new seed.

Target cool-season weeds, such as buttercup, biennial thistles, and poison hemlock, in the early spring (March-April) or after they begin to emerge in the fall (October-November). Treat summer annual weeds, such as common ragweed, spiny amaranth, and cocklebur, with an herbicide in early summer (June) when these plants begin to emerge as seedlings. The preferred time to treat many perennial broadleaf weeds, such as curly dock, tall ironweed, and Canada thistle, is in the late summer (August-early September). A mid-summer mowing followed by herbicide treatment of the regrowth works best for perennial weeds such as tall ironweed. Late summer applications will often result in more herbicide movement into perennial plants’ root systems.

Therefore, it’s important to know problem weeds’ life cycles and reproductive characteristics when determining the right time of the year for herbicide treatment. In addition, applying synthetic auxin (i.e., plant growth regulator-type) herbicides (e.g. the active ingredients 2,4-D, dicamba, triclopyr, and aminopyralid) at certain times of the year, particularly during the early and mid-summer months, has a higher potential to expose sensitive plants to off-target movement of spray particles.

Tip 3: Spray attention

Herbicides are chemical compounds designed and intended to kill undesirable plants. When applied appropriately, they selectively kill broadleaf weeds in grass pastures and not the desirable forage grasses. At times an herbicide treatment’s success depends on the applicator. The applicator should be knowledgeable of the herbicide being used and the conditions whereby the application will be made. He or she must be familiar with the proper use of the spray equipment to ensure adequate coverage on the weeds to be controlled.

Tip 4: Minimize the potential for off-site movement

When herbicides move off-target, they can damage or kill nearby sensitive vegetation. Thus, applicators should follow some important guidelines to ensure good spray coverage of weeds and minimize off-target spray movement.

Be aware of your surroundings and know what your neighbor is growing. Look around and determine whether sensitive crops or plants such as tobacco, soybeans, grapes, vegetables, home gardens, or landscape trees are growing nearby, particularly if they are within a half-mile of the proposed application area.

Become familiar with the herbicide you plan to use. In addition to the potential for physical particle drift, does the product contain active ingredients that can potentially convert into a gas or vaporize when temperatures are high and humidity is low? Herbicides containing 2,4-D are particularly at risk for volatility, and different formulations of 2,4-D respond differently to temperature. For example, 2,4-D LV Ester formulations are more likely to volatilize at lower temperatures than a 2,4-D Amine-formulated herbicide. Therefore, consult the label of any herbicide product to determine what steps the manufacturer recommends to minimize off-target spraying, including recommended buffer distances or label requirements for set-back distances to downwind sensitive plants.

Spray nozzles are designed to operate within a specified range of spray pressures to deliver the spray solution at various volumes and spray droplet sizes.

Photo: iStock

Be aware of wind speed and direction. The wind speed should range between 3 and 10 mph for optimum performance. When winds rise above 10 mph, spray patterns might become inconsistent, and some herbicide labels prohibit applications under these conditions. In general, avoid applications when wind speed exceeds 10 mph to reduce the potential for downwind spray particle movement. On the other hand, when the wind is still (less than 2 mph) a temperature inversion might exist, which can permit fine spray particles or vapors to move long distances.

Use the right nozzles and spray pressure. Spray nozzles are designed to operate within a specified range of spray pressures to deliver the spray solution at various volumes and spray droplet sizes. Nozzles that produce coarse to ultra-coarse droplets (>400 microns) are the desired output for most pasture applications. When fine and very fine droplets are produced, they are likely to move herbicide several hundred feet away from the target area. For example, spray tips that produce fine and very fine spray droplets (150 microns or smaller in diameter) can travel long distances (600+ feet) from the target in a 4 mph wind compared to less than 10 feet with medium to coarse spray droplets.

Select the appropriate spray volume. Another practice to produce larger droplets is to use spray tips designed for spray volumes of 15 or more gallons of water per treated acre. But avoid increasing spray pressure to achieve larger spray volumes, which in turn could result in finer droplets with some spray tips.

Control sprayer boom height. Keep the spray boom as close to the target as possible, preferably no more than 24 inches above the canopy. Choosing spray nozzles with wider angles (e.g., 110-degree spray angle) will allow the boom to be placed lower to the vegetative canopy while maintaining the right spray pattern across the spray boom.

Tip 5: Reseed fields and future crop uses

After applying an herbicide, how long should you wait before interseeding new forage grasses or legumes into the treated pasture? This often depends on the specific herbicide used. As a general rule of thumb, do not reseed immediately after treatment with selective broadleaf herbicides, including 2,4-D. Some herbicide products require a waiting period of several weeks (or months) after treatment. Thus, read the herbicide label to determine the minimum waiting period before reseeding grass or legume forages or other crops.

In some situations it might be desirable to convert a field that has been treated with a pasture herbicide to agricultural cropland or other uses. However, certain crops are highly sensitive to herbicides that remain in the soil for extended periods. For example, aminopyralid-containing products (e.g., Chaparral, ForeFront, GrazonNext, and Milestone) can damage tobacco and other sensitive broadleaf crops even if they’re planted into herbicide treated fields two to three years after an application. These products require a successful field bioassay before an alternative sensitive crop should be planted. Also, composted manure from animals that have been fed hay produced from fields treated with aminopyralid can harm sensitive plants. Always consult the product label for specific restrictions and guidelines.

J.D. Green, PhD, is a researcher in the Department of Plant and Soil Sciences.

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