Determining the reason for herbicide failure on your farm is more important than you might think, as one Manitoba farmer experienced first-hand after some broadleaf weeds in his wheat field appeared to be growing uncontrolled after spraying. Peter, who farms 2,500 acres of wheat, canola and oats at Eden, called me late last June to diagnose the problem — he’d found wild buckwheat, redroot pigweed and hemp-nettle plants growing in his wheat field three weeks after spraying a tank mix containing Group 1 and Group 2 herbicides. “The herbicide didn’t work on some of my broadleaf weeds,” he told me.
At first, when I walked through Peter’s wheat field, I thought there was good control of all weeds. Although there were some green foxtail plants, as well as wild buckwheat and redroot pigweed in the cotyledon stage, these weeds were small, and I concluded they had emerged after herbicide application. I was more concerned about several patches of hemp-nettle I found at the four-to six-leaf stage. These plants were too advanced to be second flush germinations. These weeds appeared to have escaped control — yet other species of weeds in the field showed symptoms of herbicide injury.
There were a few possibilities for the failure of the herbicide to control the hemp-nettle: Peter may have made an error when making up his tank mix, or if he’d sprayed under adverse weather conditions, the herbicide could have been rendered ineffective.
One last consideration involved an alarming trend I’d been witnessing more often on farms in our area and across Manitoba. In recent years, the number of herbicide- resistant weeds has been increasing. Hemp-nettle is one of several weeds becoming herbicide resistant.
Records indicated Peter’s tank mix was correct, and since all other broadleaf weeds appeared to be controlled, he had not forgotten to add the Group 2 herbicide to the mix. Also, weather conditions had been excellent at the time of spraying — warm, sunny and little wind — so right away we were able to determine that was also not the cause for herbicide failure.
Finally, test results confirmed my suspicion — the hemp-nettle growing in Peter’s wheat field was Group 2 resistant.
The issue of herbicide-resistant weeds is gaining importance for producers across Western Canada. Over the last few years, I’ve seen a growing number of cases like this one. Weeds can become resistant to herbicide by repeated use of the same herbicide group on a field.
Hemp-nettle plants produce small seeds that are difficult to clean from grain, and the seeds can stay dormant for long periods of time, making the weeds hard to manage once they become established in a field.
Peter’s records indicated he’d used a Group 2 herbicide on that field two out of the last three years—one year he grew glyphosate-tolerant canola. In the past, the Group 2 herbicide had worked well to control the weed population, including hemp-nettle. Now that the population of hemp-nettle in Peter’s field was resistant to Group 2 herbicides, he had to change his game plan.
For example, Peter should rotate the herbicide groups, or modes of action, he chooses for control of weeds by tank mixing multiple modes of action to target weeds. It is important for Peter to avoid using only Group 2 herbicides in the tank mix. By keeping accurate and current records to track the herbicide modes of action used on a field, effective groupings are easier to determine for targeting weeds in the future.
Another essential step in minimizing the risk of developing herbicide-resistant weeds is to thoroughly scout fields early in the season in order to identify patches of these weeds.
The use of cultural control methods, such as higher seeding rates, can also help reduce the onset of herbicide resistance. This reduces the selection for resistant weeds.
Do not allow herbicide-resistant weeds to produce seeds and monitor known patches of resistant weeds each year to prevent their spread to the entire field—it is easier to manage a few patches than an entire field, or farm, of resistant weeds. Spot spray or manually rogue patches of resistant weeds.
Peter’s wheat crop and the hemp-nettle patches were too advanced for emergency application of in-crop herbicide. I recommended a pre-harvest application of glyphosate to reduce the viability of the hemp-nettle seeds or a manual rogue of the patches.
Keeping a careful eye on his tank mixes and following the above suggestions resulted in a reduction of the herbicide-resistant hemp-nettle population in Peter’s field. Now that he knew what to look for, and how to continue to manage herbicide-resistant weeds, Peter could go on controlling weeds in the future with confidence.