Weed resistance challenges growers, agriculture industry

The barn door is open, the toothpaste is out of the tube, Pandora has left the building. Glyphosate resistance is now a fact of life.

And herbicide-resistant weeds will continue to challenge producers of cotton and other row crops to manage tools effectively to control resistant weeds and to prevent losing other weed control chemistries.

It will take a multi-pronged approach to bring agriculture back from the precipice of herbicide resistance, says David Shaw, Mississippi State University professor of weed science and past president of the Weed Science Society of America.

Shaw, addressing the opening session of the Beltwide Cotton Conferences in Atlanta, said growers do have options and that many agencies, organizations and individuals are working to find better weed control strategies to deal with resistant weed species.

It’s a problem many never anticipated. “Early predictions were that glyphosate resistance would not occur,” he said, “because of the material’s unique mode of action, the minimal occurrence of resistance in plants, the difficulty in selecting resistance for glyphosate-resistant crops and target site alterations lead to less fit plants.”

Oops. That assumption “ignored or could not foresee the intense selection pressure placed on agronomic systems,” Shaw said. “More than 90 percent of cotton and soybean acreage shifted to glyphosate-resistant crops. Price reductions made glyphosate the herbicide of choice in no-till systems and reduced rates of glyphosate became common.”

Which led to trouble.

Now, Shaw said, nine weed species in the United States have been confirmed as glyphosate resistant. These include: Palmer amaranth, common waterhemp, common ragweed, giant ragweed, hairy fleabane, horseweed, Italian ryegrass, rigid ryegrass and Johnsongrass.

A concerted effort is underway to identify how widespread the problem has become and what changes weed resistance is forcing farmers to make. “We are concerned that many farmers are going back to more tillage and away from conservation tillage systems,” Shaw said.

Consequently, the Weed Science Society of America, along with other partners, is developing education initiatives to help growers learn how to manage resistant weed species. They have developed a list of best management practices.

Crop rotation includes rotating to other chemistries and selecting crops with different growing seasons—wheat or canola instead of cotton, for instance.

Cultural practices such as changing planting dates or converting acreage to haying, grazing or burning may break the weed cycle.

"Start clean and stay clean,” Shaw said.

Rotating herbicide chemistry, including switching mode of action not just products, will reduce potential for resistant weeds to take over. “Also limit the number of applications for any mode of action in any one year.

"Using more than one herbicide chemistry in a given year is an extremely important practice in managing weed resistance,” Shaw said. “That may cost a little more than relying on only one product, but you get enough of a yield bump to get to break even. From a net return standpoint, a pro-active approach is better. Reactive tactics will be more costly.”

Shaw said herbicide mixtures help with resistance management. “Both chemistries must have a high level of activity on target weeds and they must have different modes of action,” he said. “Also, monitor fields within, and across years carefully.”

He said farmers should watch for weeds that are more difficult to control than usual or are of a species known to be resistant in other locations, even in other states.

Farmers are getting help. “Most Extension specialists have statewide efforts in place to educate growers, dealers, distributors, consultants, state agricultural agencies and the public.

"Extensive research efforts have been developed in states where resistance is occurring. Monitoring efforts are widespread and industry and universities are working to eradicate new outbreaks as much as possible.”

Shaw said efforts should be focused, targeting appropriate groups, including media, growers, dealers and distributors, and consultants. “Information must be accurate,” he said. “And the message must be consistent and urgent.”

He recommends mode of action labeling on herbicide containers to make producers aware of what they apply.

He said a double-handful of organizations is involved in developing educational materials including: the National Research Council, the National Cotton Council, the National Corn Growers Association, the American Soybean Association, National Association of Conservation Districts, Herbicide Resistance Action Committee, CropLife America, USDA-APHIS, USDA-NRCS, USDA-NIFA, and EPA.

"Glyphosate resistance is here to stay,” Shaw said. But he also acknowledged that the industry needs more science to understand the effectiveness of resistance management practices. “We also need to learn how to keep it from happening again.

'We have enough tools and knowledge to do what needs to be done.”

Source: Western farm press