Weeds of the West

Kathy Parker
for Tri-State Livestock News
Canada thistle is one of the most problematic of the noxious weeds in the West. Courtesy photo.

Weeds compete for nutrients and water producers need for forage and hay production. A noxious weed is a non-native plant that has been introduced, accidentally or intentionally, into an environment and causes or is likely to cause environmental or economic harm, as in harming livestock.


Weed specialist Dr. Jane Mangold, Montana State University Associate Professor and Extension Invasive Plant Specialist, discussed weed control in her state.

“The most important weeds to control include those species which are toxic to livestock,” Mangold said.

“Keeping your desired vegetation healthy through sound management makes it less susceptible to invasion by weedy plants. Also, depending on what kinds of plants make up your forage base (grasses or broadleaved forms), the herbicides I’ve mentioned can cause injury to non-target species, so be sure to read the product label and apply at the appropriate rate and time. Dr. Jane Mangold, Montana State University associate professor and Extension invasive plant specialist

“Some of the more common species include hoary alyssum (Berteroa incana), poison hemlock (Conium maculatum), tall buttercup (Ranunculus acris), Russian knapweed (Acroptilon repens) and houndstongue (Cynoglossum offinale).

“All of these species are either state-listed or county-listed noxious weeds in Montana. Cheatgrass (Bromus tectorum) is also troublesome in forage and hay, although it is not toxic and can even provide some forage value if harvested or grazed prior to seed production. It a regulated plant in Montana.”

To control hoary alyssum and poison hemlock, products containing metsulfuron such as brandnames Escort, Opensight, Chaparral or Cimarron Max or 2-4D are most effective according to Mangold. These work best when applied from the rosette to pre-flowering stage of plant growth in spring to early summer.

Tall buttercup can be controlled with products containing aminopyralid such as Milestone, or dicamba such as Clarity. These chemicals should be applied during the leafy growth stage prior to bolting and flowering in the late spring.

“Bolting is the growth stage when a plant starts shooting up or extending its flowering stem,” Mangold said. As an example, she said spotted knapweed is a rosette or collection of basal leaves until its growth extends upward with a central stem on which flower buds form and eventually flower.

Mangold said Russian knapweed can be controlled with products such as Milestone or those containing clopyralid such as Transline or picloram such as Tordon 22K.

“The timing depends on which herbicide is used,” Mangold said, “but ranges from early bud stage or fall regrowth.”

Escort and Cimarron work well on houndstongue, along with 2-4D, which are most effectively applied at the rosette stage.

Products containing imazapic, such as Plateau, or glyphosate are effective herbicides for cheatgrass. Impazapic applications should be made in the fall shortly after cheatgrass seedling emergence. Glyphosate applications should be made early in spring when cheatgrass has started growing but forage plants are still dormant.

“With all of these weedy species, the key to management is maintaining a healthy and competitive stand of forage,” Mangold said.

“Keeping your desired vegetation healthy through sound management makes it less susceptible to invasion by weedy plants. Also, depending on what kinds of plants make up your forage base (grasses or broadleaved forms), the herbicides I’ve mentioned can cause injury to non-target species, so be sure to read the product label and apply at the appropriate rate and time.

“The only chemical I have mentioned which is a restricted use herbicide is picloram (Tordon 22K), so an applicator license is required to buy and apply this product.

“Some of these products have grazing or haying restrictions, meaning there may be a minimum waiting period between application and when the forage can be cut or consumed by livestock. Again, the product label should be consulted for such restrictions. Labels can be searched online at

Producers on tight budgets want to get the biggest bang for their buck, and Mangold said the best way to do that is proper care and maintenance of forage and hayfields. “That goes a long way to prevent weed problems,” Mangold said.

“Remember the saying ‘an ounce of prevention is worth a pound of cure.’ If herbicides are used, be sure to use them at the recommended rate and time to get the best results. Others methods like mechanical control, cutting at the right time, should be considered and integrated into a management plan.

“In cases where a field is overrun with weeds, renovation may be required.”

South Dakota

South Dakota State University Extension Weed Science Coordinator Dr. Paul Johnson discussed weed control in his state.

“We have them like anyone else, I guess,” Johnson said. The two worst noxious weeds are Canada thistle and leafy spurge, invading approximately 1.4 million and 330,000 acres, respectively.

Johnson said one of the most important factors in controlling weeds is to pick the right product for the situation.

To control leafy spurge in the pasture, Johnson said Tordon is most effective, but it often proves too expensive for producers to use alone. A mixture of Tordon and 2-4D can be used long term to eradicate the weed, but that treatment may take six years. If the combined chemicals are also too expensive, spraying 2-4D every year can keep leafy spurge at bay, but not eradicate it. Chemical application is most effective from mid May through June. When yellow bracts appear on the plant is the optimum time to apply.

Most every producer in South Dakota has a private chemical use license, since the law requires one for any farmer or rancher whose outfit can make $1,000. “That’s gross, not net,” Johnson said. “Anyone with two cows in the back yard can make that, so most everyone has a license.”

To control Canada thistle “we have a few more options,” Johnson said. Tordon works well, but Johnson said there is a carry over concern if it’s being used near fields. In that case, Milestone is a better option since it is not toxic to so many broadleaf species. Johnson said Prospective is a viable option if the surrounding area is all pasture land.

Chemicals should be applied ideally in the thistle’s bud stage. The plants may still be killed if they flower, but seven days after flowering the plants will have viable seed, Johnson said. So even if the current plant is killed, it may already have reseeded itself.

South Dakota has a certified weed free program. It is illegal to sell hay filled with noxious weeds. “You can feed it on your own place,” Johnson said, “but anyone who is reported for selling it can be quarantined and the hay can be destroyed on the spot.”

Biocontrol methods are also successful in South Dakota. “We’ve had good luck with flea beetles on leafy spurge,” Johnson said.

Aphthona beetle collections are made each summer from farms or ranches that have established populations. Producers who wish to use the beetles should call their weed supervisor who will set them up with a collection. The beetles and collections are free.

“They’ll go through cycles,” Johnson said of the beetles and the weed. “The spurge will get knocked back, then comes back. As long as you’re not in flooding areas or trees, the beetles with thrive.”


According to Wyoming Weed and Pest Control Council, noxious weeds are gaining a foothold on almost all private and public lands in the state. Left unchecked, the weeds will limit many uses on land now and for future generations.

Association literature says noxious weed species negatively alter water systems, wildlife habitat, agriculture and recreation areas forever. “The longer we ignore the problem, the more expensive it will be to manage and eradication becomes less and less likely.”

Common names of designated noxious weeds in Wyoming include field bindweed, Canada thistle, leafy spurge, perennial sowthistle, quackgrass, hoary cress, perennial pepperweed, ox-eye daisy, skeletonleaf bursage, Russian knapweed, yellow toadflax, Dalmatian toadflax, Scotch thistle, musk thistle, common burdock, plumeless thistle, dyer’s woad, houndstongue, spotted knapweed, diffuse knapweed, purple loosestrife, saltcedar, common St. Johnswort, common tansy, Russian olive and black henbane.

The Council is working with biological control for invasive plants in Wyoming.

Biological control is the use of living organisms to control weeds or pests, including insects, fungi, nematodes or diseases. Insects are most often used for invasive plant control.

Since livestock don’t normally eat the noxious plants freely, insects who do are released to damage and weaken the weed stand, giving the desirable forage or hay crop a foothold to outperform the weed.

Biocontrol agents are regulated by the United States Department of Agriculture Animal Plant Health Inspection Service and go through a rigorous scientific evaluation before they are approved for U.S. introduction. Agents which attack any native species will not be approved. Agents selected must only seek the weed of concern.

These control agents will not usually completely eradicate a weed, but control it by rising and falling with the amount of weeds in the pasture.

An example is Aphthona spp. insects used for leafy spurge management. The insect populations rise exponentially when there is a substantial source of leafy spurge, but as the leafy spurge population declines, so does the Aphthona beetle.

According to the Council, release of non-indigenous organisms doesn’t come without some level of risk. Wyoming Weed and Pest Districts utilize biocontrol as part of an integrated management plan. This means biocontrol is only considered for weed management of widespread weed populations or weeds in extremely challenging terrain. The goal of biocontrol is not eradication, but to even the playing field.

Currently, the Wyoming Biological Control Steering Committee is supporting research for potential agents to control whitetop, dyer’s woad, Russian olive, Russian knapweed,, yellow and Dalmatian toadflax, perennial pepperweed, houndstongue and ox-eye daisy. For more information contact the chairman of the Wyoming Biological Control Steering Committee, Aaron Foster, at (307) 733-8419.

Nebraska producers are required to control noxious weeds on their property pursuant to the Noxious Weed Control Act to “prevent establishment, provide eradication or reduce further propagation or dissemination of such weeds.”


The 2016 Nebraska Guide for Weed Management lists the following as state noxious weeds: saltcedar, purple loosestrife, phragmites, leafy spurge, Canada thistle, musk thistle, plumeless thistle, spotted and diffuse knapweed, Japanese knotweed and sericea lespedeza.

Nebraska has a weed free forage certification program which is required on public and provincial land including U.S. Forest Service, Bureau of Land Management, National Parks and Monuments, Bureau of Reclamation, Military Reservations, Tribal Lands and National Fish and Wildlife Refuges.

Producers committed to keeping land free of noxious weeds should purchase and promote certified forage, but it is required for forage buyers who transport forage products across national state or county boundaries. Hunters, sportsmen and outfitters using livestock for hunting, fishing or recreation must carry certified forage only. State and federal agencies which feed livestock or wildlife or initiate revegetation projects must use certified weed free forage or seed.

Forage is certified by a county weed superintendent’s visual inspection prior to harvest.