Weeds often plague horse pastures, especially areas that are overgrazed or trampled by horse traffic, such as near water sources, shade or gates. When desirable plants are compromised, weeds waste no time taking over horse pastures. It doesn’t take much grazing pressure to change the dynamics of a plant population within a horse pasture, especially during drought.

Horse pasture management requires constant vigilance to balance grazing use with grass growth, making adjustments depending on weather (rainfall or drought) or availability of irrigation water. Poisonous plants, problem weeds, and vegetation that crowds out desirable forage plants should be eliminated from horse pastures.

Locoweed looks more like a wildflower than a weed, but it’s toxic to horses, so best to eliminate it.

Toxic Plants
Every geographic region has different problem plants, though some—like poison hemlock or wild cherry (chokecherry)—grow almost everywhere. To know which ones are in your area, check with your vet, county extension agent, or weed specialist.

Dr. J.D. Green, an extension weed science specialist at the University of Kentucky, says buttercup, usually found in overgrazed pastures, can be a problem in his state in the spring. "Horses tend to avoid it, but may eat it when they are short of grass," he says. "Plants we see in summer that are known to be toxic include hemp dogbane and butterfly milkweed."

Dr. Anthony Knight of Colorado State University says toxic plants in the West include locoweed, Russian knapweed, yellow star thistle, water hemlock, houndstongue, some senecios or grounsels, and narrow leafed milkweed. "Other plants that can cause problems for horses in our area include sagebrush species (such as sand sage and fringe sage)," says Knight, "especially on winter pastures when snow covers the grass and hungry horses eat sagebrush."

Just because the plants are there doesn’t mean they will be a problem, unless horses get short of feed or are the type to nibble strange plants. Some horses won’t touch potentially toxic plants, but others will try them.

If hay is your horse’s only source of food, he’s more apt to eat a harmful plant because he can’t be as selective as he might be in a pasture full of grass. Hemp dogbane and milkweed, for example, are still toxic when dry, so when they are harvested in hay, it’s a definite problem. If a bale of hay contains strange weeds, don’t feed it. Even if the weeds aren’t toxic, they may spread seeds that end up on your horse pasture. If you are feeding hay out in the pasture, this will also be the area trampled more, inhibiting the grass plants and making it easier for weeds to get started.

Some trees and shrubs can be toxic if a horse eats the leaves, or if leaves or branches fall into the pasture from along the fence line. Red maple and yew (an ornamental evergreen) are highly toxic. Leaves of wild black cherry or chokecherry are toxic when wilted (after a frost or on branches that blow down in the wind).

Become familiar with the plants in your pasture (or that can be reached through the fence) to know which ones are toxic. If you find a plant you don’t recognize, get help from your county extension weed specialist or send it to your state or county weed superintendent’s office. (See "Collecting a Weed for Identification.")

"Horse owners often become concerned at finding a strange plant," says Knight. "But rarely will a single plant cause poisoning (except water hemlock or yew). Poisoning is usually dose-related; a horse must eat a certain amount before it becomes toxic."

Bindweed chokes out grass and other forage. Unfortunately, it’s hard to kill as it stores a 2-3 year supply of nutrients in its roots and the seeds can remain viable for up to 40 years.

Weed Whackers

• Become familiar with the toxic plants in your area so that you can be on the lookout for them in pasture and hay. • Get rid of nuisance weeds because their burrs can irritate a horse’s skin or create sores and abscesses in the mouth. • Regular mowing of pastures can control many tall weeds and keep seeds from spreading. • When using an herbicide, spray in the mornings and evenings when there is less wind to minimize its spread to neighboring plants. • Biological control methods for weed control include fire, goats, sheep, and insects.

Nuisance Plants
Burdock and cocklebur produce prickly seed heads that stick to manes, tails and fetlock hair. Ripe burrs from the burdock plant have microscopic sharp "slivers" that may float in the air if a burr is shattered. These burrs may also get caught under an eyelid, causing irritation and infection.

Foxtail, downy brome, and other nuisance grasses have sharp awns that may puncture the mouth tissues, creating sores or abscesses. The ulcers in the mouth, and drooling due to this irritation, must be differentiated from vesicular stomatitis , an insect-transmitted disease that causes mouth ulcers in horses and cattle.

Other weeds, such as thistles or crabgrass, may proliferate and take over a pasture. Exotic plants like knapweed or leafy spurge may invade many acres of pasture.

"Some of the biggest weed problems in well-maintained horse pastures are weeds that grow close to the ground and escape the effects of mowing," says Green. "Curly dock, plantain, chicory, dandelions, etc. are some of the common, low-growing broadleaf plants in these pastures. We also have perennial broadleaf weeds like horse nettle and trumpet creeper.

"Some of the less desirable grasses, like crabgrass, foxtail and goose grass, can be a problem in horse pastures. One of the biggest grass problems in central Kentucky is nimblewill. Horses (and cattle) don’t graze it readily, so it multiplies."

Crabgrass and goose grass are summer annuals. Weedy grasses are harder to deal with than a broadleaf plant because they can’t be selectively controlled with herbicide.

Researchers are looking at newer herbicide options, but currently the only reliable way to get rid of an unwanted grass in a pasture is total renovation. This usually involves a non-selective herbicide spray, which kills everything, then replanting with more desirable grasses.

Despite its name, Canadian thistle is a huge problem in U.S. pastures. Mowing and spraying in fall and spring is a good first step in getting it under control.

Control Strategies
To control a certain weed, learn as much about it as you can, including its life cycle. That way you’ll have a better idea how to get rid of it—the best time of year to mow it, dig it up, or spray it.

You can often control tall weeds simply by mowing. Knowing when to mow a pasture before that weed goes to seed will help keep it from spreading.

"Mowing won’t totally prevent new seed production," says Green, "because you’ll get some re-growth and new blooms on plants such as thistles. But the amount of new seed produced will be a lot lower than if you didn’t mow or if you wait until the plants are fully mature."

Mowing horse pastures every two or three weeks will control biennial thistles, except for those around the fence line. Musk thistle, Scotch thistle, bull thistle, and even spotted knapweed can be well controlled by mowing (because they spread mostly from seeds). However, Canada thistle also spreads via its root system, so mowing won’t work. You need to use a selective broadleaf herbicide to get rid of Canada thistle.

Timing is crucial for good weed control. "Many of our worst weed problems are the ones that are not recognized until after they are beyond control—already going to seed," says Green. And if you mow too soon (or spray at the wrong time of year to get a kill on that plant), you don’t make much headway.

"There’s not one specific time of year that will take care of all weed problems," says Green. "Some weeds are summer annuals, winter annuals, or perennial species that reproduce at different times of year."

Discuss a weed-control plan with someone in your area who can advise you on proper timing.

Safe Use of Herbicides
Applying herbicide in the fall is often the best time when dealing with perennial broadleaf weeds, or even a biennial such as thistles or burdock. Herbicide travels readily into the root in the fall, since nutrient movement is directed downward as the plant tries to store energy reserves for winter.

"Another advantage to spraying in the fall is that most of the desirable vegetation in the area has completed its growth cycle for the year and tends to be less vulnerable to off-site drift of spray droplets," says Green. Leaves are ready to drop and plant growth is done for the year, whereas in the spring new leaves are growing and more likely to be vulnerable to the herbicide.

Leafy spurge contains a milky latex that can be damaging to eyes and sensitive skin. Some horses get an allergic reaction to it. Goats are a good biologic control.

Collecting a Weed for Identification

Plants are identified by their flowers, leaves, rosettes, stems, roots, seedlings, etc. When trying to collect a plant for identification, pick a sample that has as many of these identifiers as possible. Several plants are better than one. If you are sending it to your state or county weed supervisor, place the specimen(s) in a plastic bag between two paper towels without pressing or adding moisture. Close the bag and store it in a refrigerator until mailing or taking it in. If mailing, do it early in the week so it won’t sit in a post office or mailbox over the weekend.

Annuals such as spiny pigweed and cocklebur should be sprayed in summer, when the plants are fairly small but actively growing. "The difficulty is that many of these plants are not noticed or recognized until after they are mature and producing seed," says Green. "And then it’s too late."

If you decide to use herbicides, check your county regulations to see which ones are allowed in your areas. Become familiar with the product, its precautions, limitations and how to use it properly. Follow label directions. University extension publications and web sites can also give guidelines for proper use of herbicides.

Make sure the product is labeled for pasture use where animals are grazing. There may be a statement regarding the length of time you should keep animals off the pasture after application of the herbicide, or how long to wait before harvesting it for hay.

Pay attention to other vegetation in the area, and make sure to consider environmental conditions. Choose a day with little or no wind. It’s often best to spray early in the morning or in the evening because there’s usually less breeze. Use the proper rate for optimum effectiveness. There’s always some potential risk to other plants, animals and wetlands, so it’s best to use chemicals only if necessary.

Although dandelions are thought to have medicinal properties and horses will eat them, they can quickly crowd out pasture grass, and their parachute-like achenes allow the seeds to be carried for miles.

Herbicides merely treat the symptom, not the problem, but you may need them to get rid of an existing weed problem and then try to keep the weeds out with good pasture management.

If you have small pastures, it may take more time to set up the equipment than to spray, and it might be safer to chop or mow the weeds than have to worry about nearby plants and neighbors’ gardens that might be affected. Part of good weed control may be to let the horses eat the palatable weeds, (most weeds have some nutrient value when young and growing), and utilize more frequent mowing to control the rest.

Biological Control
Some weeds can be controlled without herbicides. Fire can be used in certain situations, such as along ditch banks or fences (especially a metal fence that won’t burn). Flocks of sheep and goats can be used on large weed patches to keep the plants from going to seed and to prevent spreading. But you have to repeat this graze-off every spring and hope that after a time the other grasses can re-establish themselves.

Insects can be used on large, rough land areas that can’t be easily mowed or sprayed. Keep in mind, however, that insects are very species-specific. The thistle weevil that controls musk thistle will not control yellow thistle or Canada thistle. It makes no sense to put out weevils to control one type of thistle and still have to use herbicide on the others.

"There are only a few insects that have been well-tested in North America," says Knight. "One is a beetle that can effectively control leafy spurge, and you can get these from the state insectary through your county extension office. Others are being tried, such as the weevil to control houndstongue and a knapweed fly, but these are still experimental and have not yet been released for public use."

Most of all, recognize that no one method of weed control works for every horse pasture. Take stock of your own pasture and then decide what best suits your horses, grass, budget, and the surrounding environment.

Weed Removal Tips

If a weed might be new to your area, record its location to enable adequate examination of the site later. If no flowers or seeds are present, pull the weed (and try to get the root) and leave it on the ground to dry out. If flowers or seeds are present, pull the weed carefully to prevent them from falling off. Place the weed in a plastic bag so you won’t lose the seeds, and dispose of it by burning or taking it in a closed garbage bag to a sanitary landfill.

Pasture Proactivity
Good pasture management is the best way to keep weeds from getting started. A thriving stand of grass helps keep weeds out.

Make sure you have proper fertility and soil pH to promote growth of desirable plants. Your county extension agent can check your soils and plants. If you use herbicides to kill problem plants, this leaves bare spots and weeds will return. Herbicides are just the first step in reclaiming a piece of pasture. After you kill a weed patch, you must re-seed it.

"With a difficult weed like Russian knapweed, you need a good herbicide that kills all vegetation, then re-seed the pasture with appropriate grass species for your area," says Dr. Anthony Knight of Colorado State University. "You must get a vigorous pasture grass growing to provide competition for weeds and suppress recurrence of knapweed."

If seeds are blown or carried in (by animals or vehicles), knapweed will re-establish. It soon becomes a monoculture because it secretes chemicals into the soil that kill off other plants; nothing grows under it.

Part of good pasture management is managing grazing animals so they don’t overuse it. You may need to rotate the horses to different areas so that each part of the pasture has a chance to recover. This takes constant monitoring. The length of time horses can spend in each portion will differ every year or season, due to variations in weather and growing conditions.

Herbicides

Some of the most commonly used herbicides include: 2,4-D, Weedmaster (premix of dicamba plus 2,4-D), Redeem R&P (mix of triclopyr plus clopyralid), Cimarron (metsulfuron)—Controls broadleaf plants in many types of grass pastures but may injure fescue pastures, Roundup (glyphosate), Remedy (triclopyr), Crossbow (2,4-D plus triclopyr), Banvel (dicamba), Milestone (aminopyralid)—Registered by the EPA for use on biennial thistles, Canada thistle, tall ironweed, and other difficult-to-control broadleaf weeds.

There may always be a few sacrifice areas in certain pastures, where horses tend to congregate and tramp out the grass. Even if you re-seed, you still may have trouble maintaining desirable plants in those areas. But if you rotate pasture use and don’t allow grazing when plants are most vulnerable, you can keep most of the pasture in good shape.

Keep the horses in a dry lot in early spring or any time the pastures need to recover. Don’t forget to control the weeds that become established in dry lots or barnyard areas, since they often provide a seed source that might spread to pastures.

Take note of any weed patches in the fall, and plan to control them earlier the next year, when spray or mowing will be more effective. Learn to recognize weeds when they are young, and where the patches are located in your pastures. Spread grass seed in the fall so it can get a good start next spring and compete against weedy plants.

For Further Information
You can obtain additional information on toxic/noxious weeds from your county or state extension weed specialist. Most states have publications or websites that describe and picture various weeds in that state and how to control them. Some examples: The booklet Idaho’s Noxious Weeds (University of Idaho Extension publication) pictures all weeds on the official state list, maps showing their regional distribution, and includes a Noxious Weeds Control Guide. Colorado State University has a website created by Dr. Anthony Knight , and his book A Guide to Plant Poisoning of Animals in North America is also very useful. The Cornell University Poisonous Plants Informational Database can be seen at http://www.ansci.cornell.edu/plants/horselist.html