Features Weeds
Glyphosate-resistant downy brome confirmed in Alberta

Add another weed to glyphosate resistance on the Prairies. There are 54 weed species in 30 countries worldwide that are resistant to Group 9 glyphosate, and glyphosate-resistant kochia has also been confirmed on the Prairies. Researchers at Agriculture and Agri-Food Canada (AAFC) have now confirmed the first case of glyphosate-resistant downy brome in southern Alberta.

“Based on how widespread downy brome was in the field, it was obvious to the agronomist that something was going on with glyphosate resistance,” says weed research scientist Charles Geddes at AAFC Lethbridge, Alta. 

The field in question in the Municipal District of Taber was brought to Geddes’ attention during the summer of 2021. Downy brome had survived four applications of glyphosate at the label rate of 700 ml/ac (540 g ai/litre concentration) (typical burndown application rate). Glyphosate had been applied twice pre-emergent and twice post-emergent in Roundup Ready canola. In 2020, glyphosate had also been applied pre- and post-harvest. The field also had a history of winter cereals and chemfallow.

Downy brome weed seed was collected from approximately 100 randomly chosen plants in the field in early July, 2021. The seed was combined into a single sample. Similarly, seed was also collected from two known susceptible populations near Lethbridge. Seed from each of the three populations were planted and grown in the greenhouse at Lethbridge.

Farmers and agronomists can submit samples of weed species with suspected resistance that have not previously been confirmed on the Prairies to Geddes’s research lab.

At the two to four leaf stage of downy brome seedlings, glyphosate was applied at typical field rates at 900 g ae/ha (equivalent to 0.67 l/ac of Roundup WeatherMax) in the single-dose response trial. The results found 100 per cent of the downy brome seedlings suspected of resistance survived 21 days after treatment. All susceptible plants were dead or near dead.

An additional dose-response trial was conducted to look at downy brome plant survival, visible control, and shoot biomass determined 21 days after treatment. Glyphosate was applied at 10 rates equivalent to 0, 0.063, 0.125, 0.25, 0.5, 1, 2, 4, 8, and 16 times the typical glyphosate field application rate. The suspected glyphosate-resistant population was 10.6- to 11.9-times resistant to glyphosate based on plant survival, 7.7- to 8.7-times based on visible control, 7.8- to 8.8-times based on biomass fresh weight, and 8.3- to 9.5-times resistant to glyphosate based on biomass dry weight, compared with two susceptible populations 21 days after treatment. Geddes says the estimated glyphosate rates for 80 per cent control of this population ranged from 2,795 to 4,511 g ae/ha compared to common usage rates of 900 g ae/ha.

“The risk of spreading of the population is based on biology of the weed. It won’t be outcrossing and spreading like kochia, but the confirmed resistance is certainly something to be concerned about,” says Geddes.

This glyphosate-resistant downy brome population is the first confirmed case in Canada. However, glyphosate-resistant downy brome was confirmed in Washington State prior to 2020. These populations had 88- to 165-times resistance to glyphosate compared with a susceptible biotype. 

Management practices
Downy brome is typically a problem weed in winter cereal crops because its growth habit mimics winter cereals. It can compete in the fall with emerging winter cereals, where densities of five to 10 plants per square foot (50 to 100 plants/m2) reduced yields by 30 to 40 per cent.

In the spring, early downy brome emergence can reduce winter wheat yield by 10 per cent. AAFC research in southern Alberta and Scott, Sask., found that several herbicides can provide commercial control of downy brome. Field trials found Focus (pyroxasulfone), and Fierce (pyroxasulfone + flumioxazin) provided acceptable control when applied as a pre-plant application three to five days prior to seeding winter wheat. Additional, Simplicity (pyroxsulam) applied post-emergent to winter wheat in late September/early October, or post-emergent in the spring at the two to three leaf stage of downy brome provided acceptable control.

Geddes is currently assembling a list of herbicides that research has shown will provide either suppression or control of downy brome on the Prairies. 

“We want to provide tools to farmers for controlling downy brome if there are glyphosate-resistant populations present,” says Geddes. “There are other herbicide options that are still effective; however other non-chemical weed management practices will be increasingly important moving forward.”