The discovery of glyphosate resistant kochia at two locations in southern Manitoba serves as an important reminder that extra care must be taken to limit the spread of herbicide resistant weeds.

That is one of the key messages stemming from a 2013 study that examined hundreds of Manitoba farm fields for the presence herbicide resistant kochia plants.

The study was jointly funded by the Western Grains Research Foundation and life science company BASF.

“There were two confirmed glyphosate resistant (kochia) populations out of 283 (populations) that were screened,” said Hugh Beckie, an Agriculture and Agri-Food Canada weed expert, who led the study along with researchers Rob Gulden from the University of Manitoba, Nasir Shaikh from Manitoba Agriculture, Food and Rural Development, and Scott Shirriff from AAFC.

“Certainly one recommendation that comes out of our experience  is that farmers should try, whenever possible, to tank mix glyphosate with a herbicide that uses another mode of action, whether that’s in the burn down phase or other timing,” Beckie added.

“For in-crop control, there’s certainly fewer opportunities to do that … but certainly, post-harvest might be another window where they can tank mix glyphosate.”

The discovery of glyphosate resistant (GR) kochia in southern Manitoba was not completely unexpected.

Resistant populations have already been recorded in Alberta and Saskatchewan, particularly in areas where chem-fallow is a common practice among farmers. In southern Manitoba, where chem-fallow is less common, GR kochia had not been previously documented, although researchers suspected that it may be present in areas where crop rotations often include glyphosate resistant canola and soybeans.

The WGRF survey confirms those suspicions and highlights the need for careful adherence to crop rotations, herbicide rotations and integrated weed management strategies. Farmers in affected areas should also monitor field borders closely and watch for the emergence of suspected GR kochia populations.

“Monitor any populations that are moving into your field and try to control them immediately,” Beckie said.?“Try to minimize the use of glyphosate or, if possible, tank mix glyphosate or follow it with another mode of action in the same year.”

The presence of herbicide resistant weeds is becoming a prominent concern in many parts of North America. Kochia is particularly worriesome. Kochia is an early-emerging tumbleweed that is tolerant to heat and drought. It matures later than many other weeds, produces up to 25,000 seeds per plant and continues to grow under hot, dry conditions. A mature kochia plant can distribute its seeds over a large area as it tumbles across the landscape.

Kochia has also developed resistance to Group 2 herbicides known as acetolactate synthase (ALS) inhibitors. Group 2 resistance has been documented across much of Western Canada and has been reported by Beckie to exist in roughly 90 percent of kochia populations found on the Canadian prairies. Resistance to dicamba, a Group 4 chemical, has also been reported in in the midwestern United States but so far not in Western Canada.

In the Manitoba survey, researchers randomly selected kochia plants from nearly 300 locations in 40 rural municipalities. Seeds from the plants were collected, threshed, dried and planted in controlled greenhouse conditions. Seedlings that emerged were sprayed with a standard rate of glyphosate to assess resistance. Of the 283 kochia populations that were grown out and sprayed, plants from two populations showed resistance.

According to Shaihk, an extension weed specialist with Manitoba Agriculture, the discovery of GR kochia did not come as a complete surprise. However, its presence highlights the need for extra vigilance. “We were expecting this glyphosate resistant kochia to show up in Manitoba eventually…,” he said. “GR kochia has been reported in Alberta, Saskatchewan and also south of the border in North Dakota, South Dakota and Minnesota,” he added.

Both of the resistant populations identified in the Manitoba study were located in the province’s Red River Valley. Shaihk said management of glyphosate resistant kochia populations is particularly important. Zero-till or minimum till crop production is a common practice in western Manitoba, Saskatchewan and Alberta. Glyphosate is one of the key herbicides in this system. “I think this is a matter of concern because already we have over 90 percent of kochia that is resistant to Group 2, so Group 2 herbicides are not an option,” he said. “I would say the key message here is to follow an integrated weed management program,” he added.

Producers should rotate herbicides, rotate crops, tank mix different herbicides, assess current adjust agronomic practices and, if necessary, consider tillage and manual weeding. Shaihk said Manitoba Agriculture would be contacting farmers in Manitoba and offering advice on steps that can be taken to control glyphosate resistant weeds. Fact sheets will be prepared and distributed this spring and information will be presented during Manitoba’s Crop Diagnostics School, beginning July 8.