Crop advisor casebook: Herbicide not doing its job in this farmer’s flax field

A Crop Advisor's Solution from the March 3, 2020 issue of Grainews

Dennis Juba.
photo: Supplied

Back in June, I got a call from Earnest, who grows 2,500 acres of soybeans, canola, oats and flax on his farm near Starbuck, Man. He told me he’d recently sprayed his flax field with a Group 1 herbicide to eliminate green foxtail in his crop, but the treatment didn’t appear to be having any affect.

“It’s been 12 days and the green foxtail is still going strong,” Earnest said. “The crop is suffering because of the weed competition, and the herbicide isn’t working, what should I do? Do you think I should respray?”

I told Earnest I’d have to come out to the farm and have a look for myself before I could offer an opinion.

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Once I arrived at the flax field, I could see plenty of green foxtail plants that were between the two- and six-leaf growth stage. While I scouted the field, I searched for herbicide injury, but all of the weeds appeared to be actively growing and none exhibited any of the symptoms typical of the kind of herbicide the grower had applied.

Earnest informed me he’d sprayed the field with a commonly used Group 1 herbicide containing clethodim. I was aware it sometimes takes larger weeds a bit longer to show the effects of herbicide injury, but even so, the green foxtail plants should have been showing at least some degree of herbicide injury by now, so I knew something was amiss.

I asked Earnest about his herbicide application practices and what the conditions were like on the day he’d sprayed to see if this could provide some clues. I learned he’d applied the herbicide using the labelled rate and an acceptable water volume, and he’d maintained a constant speed of eight miles per hour while spraying the field.

Earnest sprayed the field with a commonly used Group 1 herbicide containing clethodim. The green foxtail plants should have been showing some degree of herbicide injury.
photo: Supplied

Earnest recalled conditions that day hadn’t been out of the ordinary for the kind of weather we were seeing that spring, which had been moderately warm and very dry in the area.

As I considered all the facts I had in hand, I began to clue in to what could be causing the problem.

Crop Advisor’s Solution: Herbicide resistance caused the problem in Earnest’s flax crop

The product Earnest had selected had also been applied correctly according to the label specifications and the herbicide was registered for use on green foxtail, which the grower was trying to control. After discussing herbicide-use history, I began to think herbicide resistance could be the problem.

I talked to Earnest about how we could try rescuing the field with another herbicide. The options were limited, as Group 1 products are the only graminicides registered for post-emergent grass control in flax. I knew switching to a different active ingredient within the Group 1 herbicide category had shown to be effective against herbicide-resistant plants in some instances.

As a test, we decided to spray some areas of the field with another Group 1 product that contained the active ingredient quizalofop. A few days later, however, there were no visible symptoms of the herbicide injury in any of the green foxtail plants, so it was likely these weeds were resistant to multiple Group 1 chemistries.

At this point, there wasn’t much Earnest could do to improve the state of his crop. The grower was able to harvest his crop, but he had the added expense of having to clean the flax prior to marketing.

I told Earnest if he wanted to continue growing flax in the affected field, he would need to change some of his practices. First of all, he could apply a pre-emergent herbicide, such as a Group 8 product containing triallate. Another option would be to apply a Group 3 product containing trifluralin to the field in the fall prior to it being seeded with flax the following spring.

I also let Earnest know the wisest solution would be to manage his green foxtail by adding some additional crops to his rotation, as this would provide new chemistry options. I suggested he could try wheat as a new crop in his rotation, since this would allow him to incorporate new Group 2 herbicides into his weed management plan for the field. Canola was also a good option for adding a Group 9 herbicide into the mix.

In addition to managing his crop rotation and herbicide groups, the last thing Earnest could do is ensure he evaluates herbicide efficacy after every application. If herbicide resistance is not caught early, it can be very difficult to manage. With these strategies in mind, Earnest is well on his way to sustainably managing herbicide resistance on his farm.

Dennis Juba, CCA, works for Richardson Pioneer Ltd. in Starbuck, Man.

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