Controlling Pasture Weeds

Controlling Pasture Weeds

Photo: The Horse Staff

Prevent these noxious and nuisance weeds from overtaking desirable grasses.

Healthy pastures provide excellent nutrition for horses during the growing season; an abundance of pasture forage can mean you'll spend less on hay. But like any garden or lawn, a pasture must be maintained to flourish. Neglect a pasture, and you'll find yourself saddled with a long-term weed problem. A few weeds here and there can look innocent enough, but once they've taken hold, these plants compete for pasture space, pushing out desirable vegetation. Some weeds are even toxic to horses.

"Horse owners tend to think if there's green stuff out there and horses are eating, everything must be okay," says Karen Waite, MS, PhD, extension specialist at Michigan State University. "But they don't think about what that green stuff is."

William Witt, PhD, retired specialist in plant science at the University of Kentucky, says preventive action is key to dealing with weeds, but owners often neglect this side of horse keeping. "We get busy, we don't notice what's happening in the pasture, and a lot of people don't understand the magnitude of the problem," he adds.

Preventing Weeds

One of the main reasons weeds invade a pasture is overgrazing, says Witt. If grass is gone, weeds will take over that space. "In my experience, most problems with weeds can be solved by getting two-thirds more land or getting rid of two-thirds of your horses," Witt says. "Horse owners usually have two to three times more horses than their land can bear."

Grazing specialists and extension agents can help you determine carrying capacity, which is how many animals your property can realistically support. Their rule of thumb: Provide at least two acres per standard-sized horse (around 1,000 pounds) living on pasture; fewer for ponies and more for draft horses. So if you have 10 horses, you'll need at least 20 acres to prevent overgrazing and to ensure your fields offer ample nutritional value. If this amount of land is not available, you'll need to limit your horses' grazing access to a few hours daily. Also consider "rotating" your horses to a different pasture when they have grazed the forage down to three inches; don't return them to the previous pasture until it grows back to six inches.

If you notice bare patches, you can get ahead of weeds by sowing seed in late February and early March, or later in very cold climes. "Cast cool-season grasses by hand--such as orchardgrass and Kentucky bluegrass," Waite says. "The freezing and thawing process during that time will help the seed work its way into the cold soil."

Knock existing weeds out before they get a chance to spread. Witt says one or two plants might not seem like a big deal but, if left to go to seed, some plants can spread hundreds and thousands of seeds throughout the pasture. "If you don't mow early enough," he said, "and you let the weeds make seed, when you mow you'll scatter the seeds in all directions. Pigweed (page 44) may have a few thousand seeds per plant. The common ragweed (page 44) has hundreds if not thousands on a big plant."

Healthy vegetation can help keep weeds at bay, especially if you have hardy forage such as fescue, which has a particularly deep root system. "Fescue competes with weeds better than bluegrass or orchardgrass," Witt says. "Nimblewill (page 44) doesn't grow as well in tall fescue. If you have riding horses and you don't breed (as endophyte-infected tall fescue can be toxic in pregnant mares), growing the old Kentucky-31 fescue can be a good option. Nontoxic endophyte-friendly tall fescue isn't widely used because it's so expensive, but there's a new fescue (also nontoxic and endophyte-free) that has the potential to be excellent. Fescue breeders have done grazing trials with beef animals with it, and this spring we will do grazing trials with pregnant mares."

Removing Weeds

Weeds can't seed if you eliminate them in the first place. There are three ways to do this: Pull them out by hand, mow them, or spray them with herbicide. The method you choose depends upon the weed, how much space it's occupying, and its -maturity.

As a sidenote Witt says some weeds can be eaten when small and are nutritious, but once they grow and get woody they -become unpalatable to horses, as with buckhorn plantain, so it's still important to manage these intermediate weeds.

Plant identification can be tricky for the layman, but Witt says your local county agriculture extension agent can help you identify weeds and evaluate your pasture. "We have a pasture evaluation program for horses here in Kentucky," he says. "We go out to the (owner's farm) and take counts across the pasture. (The owner) gets back information that details the percentage of weeds, forage, and bare ground. It also identifies the weeds."

Owners might decide to hand-weed plants that are poisonous (e.g., poison hemlock) or low in number, but for the most part this removal method is not practical.

Mowing is not as useful for weed management in pastures as you might think, says Witt. "You have to kill low-growing weeds with low-mowing, which will kill good forage, too--you shouldn't mow a pasture below four inches. Mowing might stop some weeds from going to seed, but you won't get rid of the existing plants."

If many invasive weeds have taken hold, you might need to spray with an herbicide and kill off all vegetation. "Killing the entire pasture and starting over is something that no one wants to hear," says Waite. "But sometimes this is the only answer. Once a pasture is killed off and reseeded, you won't be able to let horses back on the pasture until the plants are established, which means one growing season."

Common Pasture Weeds Slideshow

See images and learn more with this photo slideshow.

Witt advises keeping horses away from cut or sprayed weeds, and to remove cut weeds from the pasture. "Do not let horses have access to a decaying plant," he says. "Once the plant is cut or sprayed, your horses might eat it when they wouldn't otherwise. (In the case of poisonous weeds or those sprayed using herbicides) the toxin is still in those plants while it's decaying."

Noxious and Nuisance Weeds

Our sources have selected nine weeds prevalent across North American to describe in further detail:

1. Thistle (Asteraceae family) This is the common name for a group of nontoxic plants (including Canada thistle, musk thistle, and plumeless thistle) that have spiny leaves. Thistles are very difficult to control once they've invaded a pasture, and mowing is ineffective. Their roots can be deep, and if you break one another plant will grow in its place. The only effective way to remove thistles is using herbicides.

Witt says that because thistles have spines, horses avoid grazing any good plants growing near or in a patch of thistles. Left alone, thistles can spread quickly and occupy 20% or more of your pasture.

2. Hoary alyssum (Berteroa incana) Hoary alyssum is widespread in the northern states and in Canada. It grows well in dry areas and flourishes during droughts. It is toxic to horses, causing depression and lower limb swelling. It might also cause founder and colic. "Hoary alyssum was a big problem for us in Michigan last summer because it's been so dry," says Waite. "Ordinarily horses will eat around hoary alyssum, but last year they didn't. For the most part horses can tolerate a little of the plant, but some horses might be more affected by it than others."

Hoary alyssum is toxic fresh or dried. It resembles alfalfa when dried and can be difficult to identify in hay. University of Minnesota Extension veterinarians recommend not feeding hay that's more than 30% hoary alyssum.

3. Buckhorn plantain (Plantago lanceolata) Widespread throughout the United States, buckhorn plantain thrives in agricultural and developed land. It's also drought-tolerant. Generally horses nibble on the plant, but they won't graze it fully unless they have no other plant choice. Buckthorn plantain falls into the nuisance weed category. Dig the weed up as you would a dandelion, removing the taproot. You can also use herbicides to eradicate this plant.

4. Hemp dogbane (Apocynum cannabinum) Hemp dogbane contains a white milky sap similar to the common pasture dweller milkweed, but while milkweed has leathery leaves and large seedpods, hemp dogbane has smaller leaves, a different branching habit, and reddish stems. It grows throughout the United States, especially in the mid-Atlantic region, and it has toxic properties. The plant's underground roots grow in plant communities similar to the thistle. Mowing and herbicides suppress hemp dogbane.

5. Poison hemlock (Conium maculatum) Poison hemlock is a tall, multistemmed biennial that grows in shaded wet areas across the United States. Poison hemlock resembles Queen Anne's lace, but Witt says the easy way to tell the two apart is to look at the stem. Poison hemlock, even in its immature state, has reddish or purplish blotches on the stem. Poison hemlock is very toxic to horses; it contains neurotoxins that cause tremors, incoordination, and respiratory failure. Hand-pull plants prior to flowering to remove them from the pasture. Multiple close mowings can kill the plant completely.

6. Nimblewill (Muhlenbergia schreberi) Nimblewill is a native perennial grass that forms a thick mat over the ground. It has short, narrow leaves and stolons (stems that form roots and buds, also called runners). Nimblewill resembles Bermuda grass, but it's not nearly as useful; in fact, horses refuse to touch it. Its stems and stolons die off in the fall, resulting in brown patches of pasture that used to be green.

"We have not found a way to control nimblewill," says Witt. "It's not poisonous, but nothing will eat it. It produces thousands of little tiny seeds, and anywhere there is a bare space it will germinate. It also runs along the soil. You can start over, and you'll keep it at bay for a few years, but it will grow back. In Kentucky it's our No. 1 problem in pastures."

7. Spiny amaranth/Spiny pigweed (Amaranthus spinosus) This summer annual is widespread in the eastern half of the United States and can take over an entire pasture. It thrives in compacted soil, particularly around watering or feeding stations, or along fence margins where horses walk, and each plant produces thousands of seeds. It's fairly easy to control with herbicides, but you must spray it early in the year. Horses avoid grazing near it because of the sharp spines at the base of the leaves.

"It's hard to see before it's mature," says Witt. "You can mow it but it's one that you have to mow close to the ground to kill. We don't do a lot of spot mowing in pastures, but this would be where we could do that and be fairly efficient. Mow when it's a foot tall."

8. Common ragweed (Ambrosia artemisifolia) Common ragweed tolerates dry conditions, thrives in bare areas, and competes with desirable forage. Although toxicologists say it might cause horses to accumulate harmful nitrites in their bodies, this is uncommon. Mowing is ineffective, as branches below the mowing line produce seed heads.

9. Black walnut trees (Juglans nigra) don't immediately spring to mind when considering pasture weeds, but Waite says not to discount them. "At MSU we often get calls about black walnut trees in pastures," she says. "Most people know the issue with black walnut shavings in bedding, but there have been cases of founder from horses standing on the roots. Black walnut trees are fascinating because the toxic principal, juglone, is really toxic. It kills everything growing around the tree."

Waite says that although wilted black walnut leaves can be an issue, the toxin of concern lies with the wood. "I always discourage people from planting black walnut trees on horse farms," she says. "Even if they aren't in the pastures, a storm can cause the trees to fall down or branches to blow into the pastures."

Take-Home Message

Our sources agree that weed-free pastures start with managing the good forage you have. "Soil test, reseed, and fertilize as appropriate, and don't let weeds go to seed," says Waite. "Mow before that happens. Impose rotational grazing strategies, and don't let pastures get grazed below three inches. All of these strategies will help keep weeds under control."

About the Author

Sharon Biggs Waller

Sharon Biggs Waller is a freelance writer for equine ­science and human interest publications. Her work has appeared in several publications and on several websites, and she is a classical dressage instructor.

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