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Crop Advisor’s Casebook – for Jun. 6, 2011

Weed control is an essential part of all crop production systems, and at this time of year it plays a major role in growers’ schedules. Peter, a producer from Eden, Man., who farms 2,500 acres of wheat, canola and oats, called me late last June after he found some weeds growing in his wheat field three weeks after spraying.

“The herbicide didn’t work on some of my broadleaf weeds,” he told me. He was convinced the Group 1 and Group 2 herbicides used in the tank mix he had applied the first week of June didn’t do the job they were supposed to do.

As I walked around Peter’s field the following day, at first I thought there was good control of all weeds, with the exception of some green foxtail just breaking through the ground as well as some wild buckwheat and redroot pigweed still in the cotyledon stage. I soon changed my mind though, as I found several patches of hemp-nettle throughout the field. These weeds appeared to have escaped control, yet other species of weeds nearby showed symptoms of herbicide injury.

I wondered why some weeds were being controlled while others were not? Had Peter forgotten to add part of the tank mix — the portion that controlled broadleaf weeds? Or perhaps Peter had sprayed during adverse weather conditions and the herbicide hadn’t worked. One other possibility could be the hemp-nettle plants I was finding were a second flush. But I didn’t think that was the case because the hemp-nettle plants were already at the four-to six-leaf stage and too advanced to be a second flush.

Peter said the weather conditions during and right after spraying had been perfect — warm and sunny with little wind. Thus, the weather was not to blame for the herbicide’s failure to control these broadleaf weeds.

The wild buckwheat and redroot pigweed I found were small; therefore, these weeds had emerged after herbicide application. Also, all of the other broadleaf weeds had been controlled with the only exception being the hemp-nettle, so the herbicide must have worked, I concluded.

There was only one other possibility, but I couldn’t be certain until I sent a sample of the hemp-nettle for testing. If I was right, what was happening on Peter’s farm was part of an alarming trend I’d noticed over the last few years.

Why are some weeds being

The Case Of The Still-Living Broadleaf Weeds

Scott Newton

controlled in Peter’s wheat field while others are not? Send your diagnosis toGrainews,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 aGrainewscap and a one-year subscription to the magazine. The best answer, along with the reasoning which solved the mystery, will appear in the next Crop Advisor’s Solution File.

ScottNewtonisasalesagronomistat RichardsonPioneerLtd.atMinnedosa,Man.

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