Invasive grasses such as cheatgrass and foxtail barley are dangerous to horse health—their barbed seed heads work their way into animals’ ears, teeth, and skin, causing pain, infection, and abscesses. But controlling a grass weed is difficult in a grass pasture or hay field, because any herbicide that controls the grassy weed will also damage the rest of the desirable pasture grasses.

Identifying Weedy Grass Species

The first step toward controlling a weed is correctly identifying it. “Out West, farmers and ranchers tend to call weedy barley species such as hare barley and foxtail barley ‘foxtail,’” says Mike Stafford, PhD, rangeland entomologist and sales manager with J.R. Simplot, an international food and agricultural company headquartered in Boise, Idaho. “As a scientist, most of us consider ‘foxtail’ any grass in the genus Setaria, such as green, yellow, bristly foxtails. These are weedy annuals and are easily controlled with (the herbicide) Prowl. In Idaho and southeastern Oregon, cheatgrass—aka downy brome (Bromus tectorum L.) —foxtail barley (H. jubatum L.), and hare barley (Hordeum leporinum) are major pasture weeds and are very difficult to manage.”

Getting Rid of Weedy Grass Species in Horse Pastures

Cultural practices, pasture management, proper grazing, and applying soil nutrients are a horse owner’s strongest tools for controlling these weedy grasses. “If you manage your pasture correctly and fertilize and irrigate it appropriately, the desirable grasses will get really dense and you are not going to get cheatgrass and foxtail barley,” says Stafford. The weedy species move in when a pasture gets overgrazed and pulverized by hooves, creating bare spots in the ground. “It’s okay to graze it down to a few inches, grazing intensively, but then pull them off and let the grass regrow.”

Grass plants need green leaves to manufacture their food—the longer the leaf blade, the more the plants photosynthesize, Stafford says. The power to the plant comes from sunlight, and the more photosynthetic material you have, the healthier the root system will be. “If you have 6 inches of photosynthetic area, then the roots will respond, and you get a more robust root system,” he says. By the same token, if sunlight hits bare ground it’s likely to germinate a weed seed lying dormant there and begin a cycle of weed growth.

Weed science program specialist Marcelo Zimmer, from Purdue University, in Lafayette, Indiana, agrees that poor cultural practices are what create weed problems in horse pastures. “Problematic grassy weeds of pastures in the Midwest include barnyardgrass (Echinochloa crus-galli P. Beauv.), cheat (Bromus secalinus L.), downy brome (Bromus tectorum L.), large crabgrass (Digitaria sanguinalis fall panicum (Panicum dichotomiflorum Michx.), foxtails (Setaria spp), quackgrass (Elymus repens (L.) Gould), and Johnsongrass (Sorghum halepense (L.) Pers.),” says Zimmer. “All of these species can become problematic in Indiana fields with low fertility that are being overgrazed. If infestations with these grasses are not addressed early enough, many fields end up needing to be terminated with a glyphosate application and reseeded to start over.”

Management strategies for grass weeds generally involve correcting soil fertility and pH, rotational grazing, and overseeding pastures with desirable forages. “These are the ‘big hammers’ for weed management in pastures, and these practices will make the forages more competitive with the weeds,” Zimmer says.

Low-fertility soils will have more bare spots where annual weeds can emerge and thrive. This is especially the case in drought-affected or overgrazed pastures. “Growing a stand of healthy forage will shade the soil surface and reduce the germination of annual weeds such as foxtail barley, cheatgrass, and others (which need sunlight to germinate),” he says. “In addition, weeds are generally more adapted and competitive in these low-fertility environments.” A pasture with high-fertility soils and a thick, tall stand of grass that is being rotationally grazed will help you avoid overgrazing and the introduction of weeds.

Herbicides for Grassy Weed Control

Herbicide options are limited. “For grass pastures, herbicides are generally not the preferred strategy,” says Zimmer. “In this context, herbicides are what we call ‘little hammers.’ Herbicides can help, but only if we are (already) growing a healthy stand of forage.”

Because most of these grassy weeds are annuals, and an annual plant needs to reseed itself every year, preemergence herbicides can successfully be applied to small weedy patches. You can use nonselective herbicides (e.g., glyphosate) to spot-spray weedy areas (such as along pasture borders) or to wipe out an entire field and start over.

“A rope wick (an applicator consisting of an absorbent rope plumbed into a reservoir filled with herbicide) with a post-emergence grass herbicide can also be used to control grassy weeds that are taller (than the desirable) grasses,” Zimmer says. This application strategy requires a significant height differential between the grassy weeds and the rest of the pasture. “Some pasture injury may occur if the rope wick is leaking or if there is not enough height differential between the weeds and the pasture.”

Are Weed Control Herbicides Safe in Horse Pastures?

In terms of animal health, most herbicides labeled for use in pastures will be safe to apply to a horse pasture, even if the animals are currently in it. Our sources say it is important to check the label of the product being used for any exceptions. If you intend to sell hay, you might encounter harvest restrictions for some herbicides. Again, always read, understand, and follow product label directions when using herbicides.

“As a rule of thumb, horse owners should use herbicides to control weeds whenever the weed density is high enough to start interfering with the animal’s well-being and grazing behavior—or whenever toxic weeds are present in the pasture,” says Zimmer. “Johnsongrass, for example, can have high levels of prussic acid (cyanide), which is toxic to all grazing animals.”

For help identifying a plant or determining a pasture management strategy, contact your regional extension office, local conservation district, a licensed commercial herbicide applicator or crop advisor, or a well-trained farm and garden store professional.

“The best weed control always starts with good cultural practices,” Stafford says. Avoid grazing grass below 4 inches. Implement a rotational grazing strategy to allow pastures to rest and regrow between grazing periods. And fertilize or spread compost to provide nutrients to grass plants. “A thick pasture is the best defense against these invasive, non-native grasses. Herbicides are a tool. If you have correctly managed pastures, you won’t need them.”