Controlling Grassy Weeds in Horse Pastures

Last year’s record rainfall caused significant damage to pastures across the southeastern U.S. Farm owners reported stunted growth in established stands as well as new seeding failure in cool-season grasses.

Muddy conditions also compounded hoof and vehicle traffic’s effects on pastures, further damaging established grasses. In addition, many farm owners and managers were unable to apply fertilizers and herbicides, simply because it was too wet.

All these factors resulted in increased bare soil across many horse pastures, which will encourage weed growth this spring and summer.

Because we often think of weeds as being broadleaf, such as dandelion and ragweed, Kentucky farm managers might overlook grassy weeds that can be prominent in summer pastures, such as large crabgrass, yellow foxtail, and nimblewill.

What determines if a grass is considered a weed? Simply, it’s considered a weed if it’s not desired in a pasture. For example, crabgrass is palatable to horses and cattle and typically has good quality, but it has a narrow growth window in Kentucky and throughout the transition zone, making it a less desirable addition to pasture.

For broodmare farms, tall fescue is considered a weed if it is infected with toxic endophyte and, thus, could cause fescue toxicosis. And animals typically don’t graze foxtail and nimblewill, so managers rarely desire them in pastures.

Before we can manage or control any of these grasses, we first need to understand their life cycles and growth habits. Warm-season grasses prefer to grow during summer, while cool-season grasses grow best in the spring and fall when temperatures are milder.

Annual plants reproduce from seeds that germinate in the spring (warm-season) or fall (cool-season). Perennials reproduce from seeds and vegetatively by rhizomes or stolons; these species typically go dormant during cold weather and survive to the following year.

Prevention

The best way to control grassy weeds is to maintain a thick stand of desirable grasses, such as orchardgrass and Kentucky bluegrass. Annual grassy weeds need open space to germinate, while perennials need this same open space to spread. And if pastures are filled with other desirable grasses, weedy grasses can’t become established or spread.

Good pasture management includes:

  • Soil testing every two to three years and applying needed nutrients;
  • Applying nitrogen in the fall;
  • Maintaining a minimum 3” grazing/mowing height by not overstocking pastures and using rotational grazing;
  • Using broadleaf herbicides to control weeds;
  • Overseeding thin stands in the fall; and
  • Reducing traffic during periods of wet weather.

All of these steps will help prevent future grassy weed spread, but they’ll do little to reduce their presence in pastures after significant damage, like what many farm managers experienced last year.

Control

Controlling grassy weeds in a pasture is much harder than controlling broadleaves. There are a few options available, but they all come with challenges.

Controlling Grassy Weeds in Horse Pastures

Tall Fescue

Plateau (imazapic) has been shown to remove tall fescue in cool-season pastures without seriously harming orchardgrass or Kentucky bluegrass (the latter grasses might yellow at first but stands generally rebound). Because tall fescue often covers a significant portion of a pasture, removing it can leave large bare areas, welcoming other weeds and grasses. Pastures must be overseeded at least 10 weeks after application, so timing is critical and should be well-planned.

Another herbicide, Chaparral (metsulfuron+aminopyralid), can suppress seedhead development in tall fescue.

Both products are labeled for use in cool-season pastures in Kentucky and have no grazing restrictions for horses. If you’re using them for other crops, livestock or at farms in different states, consult the product labels before use.

Nimblewill

There are no herbicides that control nimblewill labeled for use in pastures. Some turf or seed-field herbicides can provide limited control but applying them in pastures is considered off-label use (as there has been no testing to determine their effects on animals) and is not advised.

Controlling Grassy Weeds in Horse Pastures

Currently, the best way to remove nimblewill from pastures is to completely kill the existing stand using glyphosate (Roundup and many other products) in July and again in August, followed by reestablishing the field with desirable grasses. While this process can seem overwhelming, many central Kentucky farms have found success with this process to remove nimblewill, toxic tall fescue, and other weeds and establish highly productive and safe pastures.

Foxtail and Crabgrass

A common herbicide in crops and turf recently received a supplemental label for use in Kentucky pastures. Prowl H2O (pendimethalin) has shown excellent ability to control foxtail in perennial grass stands without damaging Kentucky bluegrass, orchardgrass, and tall fescue. It is a pre-emergent herbicide, which means it must be applied early in the growing season, so timing is key in this herbicide’s success.

University of Kentucky (UK) experts agree that, to effectively control foxtail in pastures, the herbicide must be applied before the soil warms enough for seeds to germinate. Late April or early May is likely the best time Kentucky, but the exact timing will depend on spring temperatures and rainfall. Pastures might also benefit from a secondary spray in the summer.

A significant question remains with this herbicide, and it focuses on how soon pastures can be reseeded after application. The label states four months for wheat and 10 months for other crops, but experts believe pasture grasses can possibly be seeded sooner.

Finally, Prowl H2O impacts many grasses, including crabgrass. While many farm managers would enjoy having pastures cleared of crabgrass, they likely underestimate the amount of crabgrass in their horses’ summer diet. As such, successful application could reduce summer grazing in pastures, and managers should be prepared to feed extra hay or concentrate when it traditionally isn’t required.

The UK Forage Extension team will work with Bill Witt, PhD (UK professor emeritus) on Prowl H2O farm trials this summer. The researchers will conduct these trials on Central Kentucky horse farms and hope to determine application and reseeding windows with Prowl H2O alone and in combination with other herbicides.

Summary

Good pasture management and new herbicides offer many options for controlling weedy grasses. Your local county extension agent can help you determine the best ways to improve your pastures this summer.

For more information on herbicides, visit the UK Weed Science website at weedscience.ca.uky.edu and consult the herbicide safety publication Practicing Good Stewardship When Applying Herbicides for Pasture Weed Control. More information on pasture management or establishment can be found at forages.ca.uky.edu.

And, regardless of which herbicides and other products you choose for your pastures, always read and follow all label instructions.

Bill Witt, PhD professor emeritus; Krista Lea, MS, research analyst and coordinator of UK’s Horse Pasture Evaluation Program; and Ray Smith, PhD, professor and forage extension specialist. All three are from UK’s Department of Plant and Soil Sciences, provided this information.


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