With glyphosate-resistant Palmer amaranth (pigweed) rapidly spreading north and glyphosate-resistant waterhemp wreaking havoc on many acres across the Midwest, retailers and growers alike are looking for weed-management solutions.

Herbicide-resistant weeds have long been a headache for growers; and over the years, university and Syngenta scientists have pooled resources to crack the resistance code, using specially designed modeling software that can examine hundreds of management scenarios to reveal each scenario’s impact on resistance evolution.

“Unlike most research, which is done on a reactive basis, the computer model enables us to get a clear understanding of what factors contribute to resistance and what behaviors help mitigate resistance before the problem explodes,” said Paul Neve, weed scientist at the University of Warwick in England and the model’s primary developer.

Results from the model have reaffirmed that diversifying modes of action, herbicide-tolerant traits and crop rotation all play important roles in mitigating the evolution of glyphosate resistance. In the case of Palmer amaranth, preventing the weed from setting seed early in the season has been the most important factor.

Furthermore, the research has shown that residual herbicides applied early in the season had a greater impact on reducing the risk of resistance evolution than those applied later, and rotating to an alternate-traited crop further delayed resistance development.

The temptation to delay residual herbicide treatments to save money early in the season can actually result in much greater costs down the road and give weeds the upper hand on several levels, noted Les Glasgow, head of weed management strategies at Syngenta.

First, making post-emergence treatments on weeds that exceed a pre-mix’s maximum height recommendation increases selection pressure and raises the risk of resistance developing. Second, mature weeds set seeds for more competition the following year. And finally, early-emerging weeds actually produce more seeds than those that emerge after the crop and, therefore, have a higher chance of carrying a resistance trait to one or more active ingredients.

To avoid these resistance-related problems, Glasgow recommends:

• Burning down existing weeds in no-till scenarios to ensure fields are clean at planting
• Using a pre-plant or pre-emergence residual herbicide to prevent weed emergence
• Overlapping an early post-emergence residual for continued control
• Removing weed escapes from fields before seed set.

Glasgow believes the results of modeling demonstrate the need for proactive weed management by growers.

“The results of the model demonstrate the importance of early-season weed management and the need to diversify crops, herbicides and traits. If acted upon properly, the risk for resistance evolution will drop substantially.”