Group 2 resistant-kochia has widened its range from the red dirt in the U.S. Midwest to the outer reaches of the Prairies’ black soil zone.
In the 1970s, prairie weed surveys showed kochia was a weed found mainly in south-central Saskatchewan. In the 1980s, the persistent tumbleweed spread to Alberta and then expanded its range north from south-central Manitoba in the 1990s.
“It made a steady move from the brown soil zone and into the black,” said Agriculture Canada weed scientist Hugh Beckie.
At the same time that kochia was first identified as a weed of economic interest in Saskatchewan, it was already developing resistance to Group 2 herbicides in Kansas.
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Today, the annual shrub that was once an ornamental has developed resistance to multiple herbicides in some parts of the United States. So far in Canada, it is only resistant to Group 2 products.
Ken Sapsford of the University of Saskatchewan told producers attending the Farm Tech conference in Edmonton last month that Group 2 herbicide resistant-kochia is everywhere on the Canadian Prairies.
“If you have kochia, it’s likely got some Group 2 (resistant strains),” he said.
The drought tolerant weed seems ideally adapted to reduced tillage systems. With its 50,000 seeds per plant and its tumbling distribution system, it can rapidly spread any new genetic adaptations it develops.
Kochia is obligate outcrossing, which means that each plant is unique, and when exposed to extreme selection pressure such as herbicide applications, it can quickly adapt to threats.
Sapsford said producers that encounter the weed should either not apply Group 2 herbicides, or apply them along with chemicals from other groups with different modes of action and in quantities that will ensure control.
Defining where resistance is taking place on the Prairies has become less important than dealing with the weed.
In 2004, to obtain a clearer picture of Group 2 resistance, Manitoba Agriculture staff studied live kochia plants. Staff travelling around the province would find the weed growing, scoop it up, put it in a Styrofoam cup and take it back to the lab where it was grown in pots. In that season agrologists found that when sprayed with the Group 2 herbicide Refine Extra, many of the plants survived.
Before these live tests Manitoba had only identified a few cases of herbicide tolerant kochia. The informal tests showed it was in as many as 80 percent of samples.
The following year the Manitoba agronomists tested for other Group 2 herbicides. They found kochia was resistant to every Group 2 chemical tried, including Ally, Pursuit and Odyssey.
“Manitoba’s experience is similar to Alberta and Saskatchewan. It’s pretty much everywhere,” said Sapsford.
Testing showed even products that are packaged with Group 2 herbicides fail to control the weed.
“You have to imagine that the Group 2 isn’t even being applied. In those premixed products you don’t get enough of the Group 4 to provide control.”
The exception to this is Triton K, a mix of Express SG, 2,4-D Ester and Banvell II. Sapsford said that specific Group 2 and 4 combination is effective against kochia. However, the product is only registered for use with spring wheat and barley.
Dicamba, Attain, Prestige and glyphosate remain the most effective ways of controlling the weed in cereals and in some cases canaryseed, under minor use label expansion, he said.
“There is no (herbicide based) method of control in crops like peas,” he said.
Neil Harker of Agriculture Canada in Lacombe, Alta., said producers have choices to control the weed beyond shifting to herbicides with other modes of action.
“We need more than herbicides, tank mixes or not. Chemical control just delays resistance developing in this weed.”
Harker said because kochia is a summer annual and has poor seed life in the soil, it can be controlled with cultural approaches such as crop rotation.
“Grow a winter annual in rotation with it. It is a poor competitor. Go to narrow (seed row) spacing. Add an annual forage to the rotation,” he said.
