The War Against Weeds

When it comes to pasture weed control, an ounce of prevention is worth hours of back-breaking cure.

In order to grow, weeds need to gain a foothold. While it can be harder for them to do so in thick, lush pastures, a variety of weeds can get established easily in an overgrazed field or drylot. The problem of weeds congregating in grass-deficient pastures, frequently caused by keeping too many horses on too little land, is one that's taken root across America, and it's growing all the time.

Pursuing Greener Pastures

According to William W. Witt, PhD, a researcher in Plant and Soil Sciences at the University of Kentucky, overgrazing is a national crisis, often linked to increased urbanization and equestrian land loss, and it's a main reason for weed growth. "Many of our pastures or paddocks are overgrazed," says Witt. "It's an issue literally across the board; everywhere I've driven over the past few years, overgrazing is the No. 1 problem."

Overgrazing and poor pasture condition can result from a number of multifaceted situations, according to the Oregon State University Extension Service's Managing Small Acreage Horse Farms report. Authors state that it's crucial to examine and manage the relationships between horses, grass, soil, manure, and water on any property because they're all interconnected. For property owners failing to manage those relationships suitably, poor pasture health and weeds can result.

"Anywhere you have a spot where the pasture grass goes out, that biological spot can become occupied by a weedy species," explains Witt. This is because there are anywhere from hundreds to millions of weed seeds naturally in the soil that can crop up when grass dies out.

Hooves--particularly those that are shod--also can cause soil and plant degradation, especially when horses are given free access to wet pastures; this damage provides weeds the toehold they need to get established. The Oregon State University Extension Service also points out that because horses are highly efficient grazers that are able to nibble plants to the soil, failing to manage grazing access and allow grass recovery time can literally kill off your pasture grass.

To resolve overgrazing as a weed control issue, Witt points out that it's primarily about the numbers. "Invariably an overgrazing problem can be resolved by either increasing your land by two to three times or by reducing your number of horses by half. It's that simple; either get more land or reduce your animal numbers," he says.

Rotational grazing is another method to reduce overgrazing and prevent stressing pastures, thus inviting weeds to take root. If you don't have an abundance of extra pasture space, subdivide existing pastures into smaller units with cost-efficient portable electric fencing. Remove horses from pastures when the grass is grazed down to about 3 to 4 inches, and turn them back out on a section when the "rested" pasture grass is 6 to 8 inches tall. Depending on time of year and weed abundance, these rested pastures might also require weed control methods such as herbicides.

Identify and Conquer

Witt points out that a plant's value, or lack thereof, simply depends on whether it's beneficial to us or causes harm. "The plants we call weeds are only so in the minds of humans," he says.

Toxic (poisonous) and invasive or rapidly spreading (noxious) weed species vary tremendously according to geographic region and climate. "In Kentucky we have extremely toxic plants like poison hemlock, one of the few that's poisonous to humans as well as to horses and beef cattle," says Witt. "Then we have buttercups, which are only somewhat toxic to horses."

To help horse owners identify harmful weeds and plants, Witt is developing an equine pasture and weed control website that will be available via the University of Kentucky's Equine Initiative at In the meantime, if you're not sure what's indigenous to your area, how to remove it effectively, or if there are weeds in your area that you can simply let grow, you can contact your local agricultural extension office, read poisonous plants books, or search reputable Internet sources.

The Only Good Weed is a Dead Weed

Traditional methods of both toxic and noxious weed control and removal include mowing, spraying, digging, and using biological agents such as insects. These present varying degrees of appeal and success, according to Witt.

"If you have a poisonous plant and you know it's poisonous, the simplest thing for small-acreage horse owners is to hand-remove and destroy it," he says.

For large-scale properties Witt recommends blocking off the affected area and spraying. He emphasizes that you should wait before returning horses to the area. "Horses might not graze on an unsprayed poisonous plant, but if you spray herbicide on it they might," he cautions. "Spraying might change the composition of sugars in the plant, enough that it could be detected by the horse. Plus, it could be several weeks to several months before it decays enough that it's no longer harmful." This is why immediate hand-removal is often recommended instead.

Nontoxic weed control methods are often a matter of scale and personal preference. "If you have a small pasture, digging and hand weeding are the way to go if you have the time and inclination," he says. "If you have five or more acres, or thousands of (noxious) dandelions, that's probably not going to be your choice."

Other weed control methods include:

Herbicides The advantages of these chemicals are that they're fast and effective, which means these can save farm owners a lot of time, labor, and money. They also are selective to weeds so they don't damage beneficial forages.

Herbicides registered with the Environmental Protection Agency have a "zero-day limit for removal," meaning that horses can remain in the pasture when you spray. Witt does recommend keeping horses off a pasture when you're spraying so they don't spook, but when to reintroduce them to the pasture is an individual decision.

"Some will put horses out that day, some will wait a week, others will wait several months. It depends on what you're comfortable with," says Witt, who feels that EPA-registered herbicides are safe for horses. "If they weren't, we'd have animals dying all over the countryside from exposure to them." As for when to spray, there are many variables to consider, including weed species, climate, and growing rates; however, general guidelines include spraying prior to weed maturation (when seeds will spread), and following all manufacturer guidelines. Following label guidelines will also help prevent environment or water contamination.

Mowing If you prefer not to introduce chemicals into the environment, you might choose mowing. However, Witt has concerns about this approach. "From a weed standpoint, mowing is generally not effective in pastures," he notes. "For example, you can mow all you want and never kill dandelions, which are a prime weed in many overgrazed pastures."

Mowing can be used to top off tall, upright plants before they go to seed, and it is sometimes used to cut down the "roughs," or areas where horses have defecated. Since horses generally don't graze in the roughs, plants tend to grow taller there. While many horse owners spread manure from roughs around a pasture using a dragging process, Witt believes this negatively impacts soil health. "Dragging can disturb the soil, making it easier for weed seeds to germinate and take hold, or it can spread that weed across a wider area," he says.

Owners might instead remove manure from pastures on a regular basis or rest the pasture after dragging and applying herbicides.

Biological agents Witt says that, unfortunately, there are few effective biological agents for weeds. "There's a thistlehead weevil that lays eggs on the musk thistle and the larvae eat the seeds, but that's about the only example of biological or insect control I know of for weeds," he says. Another downside to biological agents is they have to be host-specific in order to prevent unintended damage. "(For instance), we don't want to turn anything loose that's going to transfer from the thistle to the soybean (which is a crop that needs to remain productive)," says Witt.

Keeping the Grass Greener

In terms of weed control, your best defense is a well-executed offense. Keeping a vigorous grass cover thriving in your pasture is key to keeping the weeds out. "The biggest plant always wins; it has more biomass and takes up more water and nutrients in a root system," says Witt. Fencelines are also common weed habitats, along with the areas of manure mentioned prior; mowing these areas several times per year and combating weed growth when observed should help keep weeds under control.

A tall fescue might be an ideal pasture grass for many horses, with the exception of breeding stock. "Tall fescue is the single best pasture grass you can put in if you're a recreational horse owner; it's a big, robust grass, and it is very competitive against the weeds," says Witt. "However, it's the last thing you want if you have pregnant mares; there's a toxin that can greatly increase the chances of a last trimester abortion."

According to the authors of Managing Small Acreage Horse Farms, "turf" types of tall fescue grasses contain a fungus called an endophyte. Forage types of tall fescue don't have the endophytic (i.e., within the plant) fungus. Be sure to use endophyte-free tall fescue seed when planting; if you're concerned about existing pastures, having a local county extension agent run an endophyte test on the pasture grass can help you determine if there is any infection.

Other forage grasses, and ones more suitable for breeding stock, include orchardgrass, perennial ryegrass, and Kentucky bluegrass. However, your region's climate will determine what you can grow. Research your options and consult with representatives from your area extension office; these resources will point you in the right direction for cultivating thriving, weed-resistant pastures.

Take-Home Message

There's no one correct approach to weed control, but there are some recommended best practices that everyone can follow:

  • Knowing the common toxic and noxious weed species for your area;
  • Conducting regular walk-arounds of your pasture to look for weeds;
  • Implementing a proactive weed control strategy you feel comfortable with; and
  • Using appropriate safety precautions when applying herbicides.

Whether you're working with one acre or a thousand, you, your horses, and your land will benefit from a weed control strategy that's based on these suggestions.

About the Author

Lisa Kemp

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