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Casebook: Grassy Weed Gaffe

Two weeks after he sprayed his wheat field with a grass and broadleaf herbicide mixture, Jim discovered the wild oats in his wheat crop were still thriving. “There must be something wrong with the chemical,” he told me when I visited his farm to look at the problem.

We walked through Jim’s field, scouting for both broadleaf and grassy weed control. The broadleaf weeds were showing signs of herbicide activity — the plants were wilting and dying off, and the leaves had turned white with burnt edges. However, the wild oats were showing few to no signs of herbicide activity. For example, the meristems were white and healthy, there were no signs of wilting, and the leaves were still a nice green colour, although some had a touch of yellowing. The wild oats were scattered throughout the field.

Clearly, the field had been sprayed with chemical because control of broadleaf weeds was uniform throughout, so we weren’t addressing a spray miss. However, the wild oats were not dying.

I have witnessed an increasing incidence of Group 1 herbicide resistance in the surrounding area over the past few years. Jim, who farms 2,000 acres of wheat, barley and canola east of Fort Saskatchewan, Alta., could have herbicide-resistant wild oats in his field. However, resistant oats usually grow through the herbicide application in patches and would not be spread uniformly across the field, and some of the non-herbicide-resistant wild oats would still be affected by the herbicide. Jim has also implemented a good rotation strategy and had not overused Group 1 herbicides; therefore, herbicide resistance was probably not the issue.

We were also not dealing with a staging problem—the wild oats were close to the same stage as the wheat, which had been sprayed around the five-leaf stage, the appropriate time at which to spray wild oats. The problem had to be linked to the chemical controlling grassy weeds.

We checked Jim’s records. He had applied a Group 27 and Group 6 herbicide for broadleaf weed control and a registered Group 1 tank-mix partner for grassy weed control. However, something in those records caught my eye.

“You may have added the correct herbicide tank-mix partner, but we still have a tank-mix issue,” I said.

“I think I know what the problem is, too,” he said, with a sheepish grin.

Why are the wild oats in Jim’s field escaping control? What is wrong with his tank-mix? Send your diagnosis to Grainews, Box 9800, Winnipeg, MB, R3C 3K7; email [email protected] or fax 204-944-5416 c/o Crop Advisor’s Casebook. Best suggestions will be pooled and one winner will be drawn for a chance to win a Grainews cap and a one-year subscription to the magazine. The solution will appear in the next Crop Advisor’s Solution File. †

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