Heather Smith Thomas

A weed is any plant growing where it is not desired—a plant that has no value in its present location, especially one that crowds out or damages the desired crop. If the crop is pasture forage, a weed is any invasive plant that crowds out more desirable forage or causes problems for livestock. It might be a poisonous plant like larkspur or hemlock, or plants that cause other health problems such as lump jaw (cheat grass or foxtail), or eye irritations (from burdock burr fragments). Some weeds have forage value at certain stages of growth but become unpalatable as they mature, taking up space that could be used by more beneficial forage. Some weeds are forbs or shrubs that crowd out grass, while others are undesirable grasses.

Dr. Don Morishita, University of Idaho Professor of Weed Science and Extension Specialist, says some weeds are a problem in dry areas like western rangelands while others are found in wetter areas, such as irrigated pastures. He gets a lot of questions from ranchers about foxtail barley (often called foxtail). “If someone allows irrigation water to stay too long in one area, weeds like foxtail can become established because they can tolerate saturated ground better than most desirable pasture grasses,” he says. The seedhead has long awns that can irritate and puncture the mouth tissues of a grazing animal, or an animal that eats hay containing foxtail.

“Once established, it's difficult to control this grass in a pasture because there isn't much you can use to selectively control it with herbicides, without injuring or killing out other grasses,” says Morishita. It is best controlled with management practices like preventing the over-watering that allows it to flourish while the desirable grasses suffer from too-wet conditions.

“Whenever you stress the desirable pasture and hay grasses this allows other species and invaders to come in and take over. If you have an area where the desirable species are stressed—either by too much water or by lack of water—an invader may become established because it is better able to tolerate those conditions,” he explains.

“When irrigating, manage your water so you don't end up with an area of saturated soil in low-lying areas, stressing the desirable grass plants and allowing species like foxtail barley or ‘slough' grasses to come in.” It's worth the extra effort to take care of any accumulating tail water or to drain swampy areas.

“In dry situations, common mullein, knapweed species, black henbane, certain types of thistles, etc. are plants that invade pastures. All of these are better able to tolerate dry conditions than are many desirable forage species,” he says.

Management tactics—whether controlling and reducing wet areas or providing water in a dry year—can often be the biggest help in weed management. “Don't think of herbicides as the only tool for weed control. Think about irrigation management and fertility management, too. Some soils with low fertility may also allow infestation of certain plants that can tolerate lower soil nutrient conditions than grass can,” says Morishita.

“For non-chemical or organic producers, you definitely need to use an integrated approach for weed control. The only difference is that you are not using herbicide as one of the tools,” he says. In some pastures, timely grazing and/or mowing can help control certain undesirable plants, grazing them when early stages of the weed are palatable, or mowing them when they become more mature but before they go to seed.

“Mowing some thistles can be effective, especially bull thistles, Scotch thistles and musk thistles, which can all become a problem in pastures. These are all biennial thistles, and if you keep them from going to seed for a couple of years this will take care of them. It's a little more difficult and challenging with perennials like Canada thistle,” he says. Mowing can still help because you can minimize seed production and spreading if you mow before they bloom and go to seed, but the plants will still keep regrowing.

Some people spot graze certain areas with sheep or goats to control invasion by species that cattle won't eat, such as leafy spurge. In rough rangeland where you can't drive a vehicle to mow or take a spray rig, selective grazing with herded sheep or goats can be helpful.

You can sometimes use herbicides in rough areas where it's hard to use any other kind of weed management strategy, as a means to keep small intrusions from spreading and becoming larger problems. “You might be able to spot spray a few plants to keep them from going beyond the rocky or dry area where they've started—keeping them from moving out into the rest of the pasture,” he says. You can back-pack enough herbicide to kill small patches of undesirable plants such as larkspur or knapweed.

“One of the best analogies for fighting weeds was a practice encouraged by Dr. Steve Dewey, a weed scientist at Utah State University (now retired). He said you should fight weeds just like you fight a fire. Often a forest fire or range fire starts from a small spot fire and if you can eliminate it, the problem is solved. In many cases a big fire will develop smaller fires around its perimeter (from sparks or hot embers blown by wind). These are the areas you need to tackle first. If it's a large weed invasion, determine the perimeter and how big it is. Once you know that, go beyond it and look for the smaller outcroppings so you can eliminate them and halt the advance of the invasion. Once you have the smaller spots taken care of you can focus on the big problem.” You need to look at the larger picture and attack it in a systematic way.

It's always good to consult your local county weed superintendent or extension agent to learn as much as you can about the plant you are trying to control. That person can give you the best advice regarding management strategies to attack it when it is most vulnerable in its life cycle, and which herbicide might work best if you choose to use that tool.

Management of pasture is a key factor. “Anyone who has run cattle for very long knows the effects of overgrazing, and that cattle use must be matched to the carrying capacity of the pasture. Many invading weeds take advantage and move in whenever the primary plants are overstressed—whether by drought or overgrazing,” says Morishita. Good grazing management can tip the balance in favor of the desired species.

There are many invasive species that become very difficult to control if you don't halt them before they become widespread. “Russian knapweed is a good example. It can overtake even a good stand of forage or a hayfield. You need to jump on some of these invaders as soon as you notice them,” he says.

If there are just a few plants you may be able to eliminate the problem by digging them up. But if you dig or pull them up, do it early in the year before they bloom and go to seed. Even the act of digging, pulling, chopping and hauling them off may spread a lot of seeds if they've matured. You may inadvertently spread them over a larger area by moving those plants.

If you have a serious weed problem you may need to become familiar with herbicides and their proper use, since this may be one of the tools you'll have to utilize in an integrated management program to reduce or eliminate that weed. “The worst mistake someone can make with herbicides, however, is thinking that this one tool can solve the problem, says Morishita. Best results are usually obtained when herbicides are judiciously used along with other management measures.


“There have been some biocontrol efforts with leafy spurge, and often times this works well,” says Morishita. Certain types of insects feed on these plants, and researchers imported some of these insects from areas of the world where leafy spurge is native.

“Scientists traced the DNA of leafy spurge back to its area of origin in eastern Europe and western Asia. The reason they traced it back was because when they released the insects that feed on leafy spurge, in many instances the insects did a great job and wiped out the plants, but when they released the insects in other areas, this control measure didn't work. For a long time they thought there must be a difference in the climate of areas with contrasting results. They thought there might be an environmental difference for leafy spurge growing in North Dakota, compared to leafy spurge growing in Colorado, Montana or Idaho. But they realized that these plants were often growing in the same kinds of climatic zones as we see in North Dakota. So someone started looking at DNA of leafy spurge populations across the country and found there were different biotypes of leafy spurge and the insects liked some of them better than others,” he says.

“So now, some of the biocontrol efforts have gone back to these regions of eastern Europe and western Asia to try to find the specific insects for those regions—the insects that like to eat that particular type of plant. We have a biocontrol entomologist at University of Idaho who is doing a lot of work on these projects,” says Morishita.

To best control an undesirable weed, learn as much as you can about it and how it grows and reproduces. Chopping it down, mowing, or spraying at the wrong time of year may be wasted effort and expense. You must attack it when it is most vulnerable.

Burdock is a perfect example. The tall burdock plant (a native of Eurasia, probably brought to this continent by seed burrs stuck to imported animals) puts forth clusters of round burrs that become caught in the hair coat of cattle and other animals, spreading seeds via these animals. Burdock flowers in late summer, producing a composite seedhead which matures by mid-August in southern areas and later in northern climates. When burrs are ripe they release hundreds of microscopic barbed slivers, and if these get into an animal's eye they cause severe irritation—especially if caught under an eyelid where they continually scrape the eyeball every time the animal blinks.

Burdock can be controlled by chopping it down before it's mature enough to make seeds. It can also be controlled with herbicides. Morishita says several broad-leaf herbicides will kill burdock, if done properly. Burdock is a biennial, which means it lives for two growing seasons. The first year, it doesn't grow tall stalks or bloom; it merely grows leaves and accumulates food reserves in its roots, like a carrot (also a biennial).

The second year of life it grows a long, deep taproot, and a tall stalk, producing flowers and burrs. Putting forth flowers and burrs exhausts food reserves in the root, says Morishita, and the plant dies after burrs are mature. After the stalk comes up, it is harder to kill with herbicides because the plant is sending food up from the roots instead of down.

Burdock plants are easiest to kill in early spring or in the fall. The first-year plant stays in a rosette stage that first summer (circular cluster of leaves, no tall stalk) and this is the easiest time to kill it. “Apply spray at a time when the plant is putting food into the root, since you have to get herbicide into the root to kill the plant. Use a broad-leaf herbicide like 2,4-D that's a translocated herbicide that can move down into the root. If you spray early in the spring you generally kill the new young sprouts and last year's rosettes (the plants that are trying to create more food reserves in their root for their big push to complete second year growth and make burrs). You have to spray very early to get the second year plant. After the stalk comes up it is harder to kill. If you spray in the fall you are killing this year's rosettes--the plants that would mature and create burrs next year,” he says.

"Fall is actually a good time to spray burdock, to kill young plants that are storing food reserves in their roots for next year's growth. The first hint of cold weather is a trigger to send food to the roots, so this is a good time to spray. By contrast, in the spring the second year plant is taking food from its roots to produce leaves and make the big push for a tall stalk and blooms. The food is going up, and it's harder to get the herbicide down into the root." Food reserves in the root are lowest when the plant starts to bloom.

The main thing to remember when using herbicides to kill burdock or other biennial and perennial weeds is to not overdo it. "If you use too much, it quickly kills the top growth leaves and doesn't get down into the taproot. The root survives, to regrow. This is a case in which more is not better. You want a slower kill so the leaves survive long enough to transfer the herbicide on down into the root, to kill the whole plant. Use the recommended rate (two to three percent solution of 2,4-D which is about .5 to 3.75 ounces per gallon of water) and spray the plants lightly--until they are barely wet but not dripping," he explains.

He also cautions against using anything other than broadleaf herbicides. "Burdock is a bare-ground plant; it doesn't grow well where there's a lot of grass cover or other competing plants. Don't use Roundup, since it kills everything--including the grass that tends to inhibit regrowth of burdock." He usually recommends 2,4-D for burdock because it is cheap, and also very effective when applied at the proper time. However, other herbicides like Milestone and Redeem R&P work very well, too.

Chopping plants down is also effective for control, but you must do it at the right time or the plant will regrow from the root. The best time to chop it is after the stalk is budding but before the burrs are ripe. At that point the food reserves are so low in the root that it cannot regrow, says Morishita.

Once you have burdock patches in a field or pasture it may take several years of diligent control to eradicate it, since the seeds can live a long time. Even though you chop or spray the plants, there may be viable seed in the ground--from earlier years--that will sprout and grow. Keep checking those patches, says Morishita, and get rid of new plants that grow up from old seeds.


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