If you see a Canada thistle look-alike in your alfalfa, it may be Russian knapweed, a noxious weed to tackle early on, says Earl Creech, University of Nevada extension weed specialist based in Fallon.
“Once established, Russian knapweed can create a problem forage producers could spend the rest of their lives dealing with,” he says. “It's a tough weed to get rid of because it's such a prolific producer of seed and roots.”
The weed, a deep-rooted perennial, originated in Eastern Europe and arrived here in the late 1800s. Although its spread was gradual for several decades, its expansion rate has picked up steam in recent years. The plant's classified as a noxious weed in most Western states and is heading east. It's found in Nebraska and half the counties in North and South Dakota.
“Depending on whom you ask and where you live, estimates of spread vary from 8% to 15% per year,” says Creech, who adds that the weed has a stronghold in well over a million acres. It can spread long distances when contaminated hay is moved from an infested area to another county or state.
Russian knapweed flourishes in alfalfa, grass, pasture mixes and rangeland and in a variety of climates.
“Here in Fallon, where we get just an average of 4” of precipitation per year, it thrives on non-irrigated soils. But when it does get irrigation, it grows even better and can even withstand some temporary flooding.”
The weed creates several problems:
It's poisonous to horses. “It causes chewing disease, which is a neurological disorder that attacks the part of the brain that controls fine motor movements, such as those in the mouth, lips and tongue,” says Creech. “If horses consume large quantities of Russian knapweed over a month or two, it affects their ability to chew and swallow, so they eventually die from dehydration and/or starvation.”
Fortunately, the weed is not toxic to other animals and has a bitter taste and unpleasant odor, making it unattractive to animals. But they'll eat Russian knapweed if they're hungry.
Intake may decline. “Beef and dairy cattle may eat less if the foul-tasting plants are mixed with other forages.”
It's very competitive. “It can crowd out alfalfa and other forages, decreasing their yield and value.”
Large populations may become allelopathic. “It produces compounds in its roots that can suppress the growth of nearby plants.”
There are no control options for Russian knapweed in alfalfa, Creech says. “Once an alfalfa stand has become infested, growers can rotate to corn or a small-grain crop, where a number of herbicides with Russian knapweed activity can be used. For grass hay, pastures and rangeland, there are several good options for chemical control, but repeat applications may be needed. Tillage, mowing and grazing are not effective control measures.”
Prevention is the key.
“Keep an eye on plants growing on your farm or ranch. If you see something you don't recognize, find out what it is. It could be a new invasive weed looking to gain a foothold,” he says. “Jumping on new weeds immediately can save a great deal of time and money down the road. A few dollars put toward prevention can be worth thousands or even millions of dollars worth of cure.”