Battling Resistance

The message was clear for attendees: The weeds are getting more adaptive, so you must be more proactive in how you deal with them.

During the winter months, CropLife magazine, with financial support from the Monsanto Co. and the help of various ag retailers, held a series of eight Total Weed Management Answer Forums throughout the Midwest and South. These were attended by hundreds of growers who came to hear experts on the topic of weed resistance and discuss ways to control the problem.

Southern Perspective

In Dothan, AL, growers got to hear about the extreme difficulties that with which Georgia is dealing with in its battle against Palmer amaranth (called Palmer pigweed in the north). Stanley Culpepper, Extension weed specialist from the University of Georgia, warned the southwest Alabama grower audience about Palmer amaranth’stenacious competitiveness and prolific seed production capabilities.

Palmer amaranth is unique in that the species features a male plant and a female plant, and research has shown that the traits travel with the pollen. “If you have a sensitive female plant 1 meter away from a glyphosate-resistant male plant, 70% of that progeny will be resistant to glyphosate,” says Culpepper. “If you move that distance out to 250 meters distance, 20% of progeny resistant to glyphosate are the female plant.” With a reproductive capacity of 450,000 seeds per plant, resistance is a significant threat to spread.

“Managing the seed bank,” or ensuring more Palmer amaranth is coming out than going back into the field, is key to managing resistance. And with truly new weed management technology not due to come to the Cotton Belt for another eight years, judicious use of existing technology is key.

“If we make it to the next wave of technology, it will be completely dependent on the PPO herbicides,” says Culpepper. “And the two tools of greatest importance to the cotton grower are Reflex and Valor. If we get resistance to these two chemistries in the next few years we will never make it.”

Midwest Matters

At the Total Weed Management Answer Forums held in Omaha, NE, and Bettendorf, IA, Dr. Rick Cole, technology development manager for Monsanto, told attendees that there currently are eight confirmed glyphosate-resistant weeds in the U.S. — rigid ryegrass, hairy fleabane, marestail, common ragweed, Palmer amaranth, waterhemp, giant ragweed, and Italian ryegrass. There are, however, several other weed species with suspected glyphosate resistance including common lambsquarters and kochia.

“In all these cases, there are several common factors that appear in the resistance cases,” said Cole. “These include limited or no crop rotation, limited or no tillage practices, a high dependency on glyphosate and limited use of other herbicides, and reduced use rates of glyphosate.”

To appreciate just how deep the problem extends within the ag community, Dr. Mike Owen, professor and Extension agronomist at Iowa State University, told attendees about the results of a survey taken at the ag-chem dealer meetings and ICM Conference during 2007. In this survey, 40% of respondents said that fields were “more weedy” and 57% said that more glyphosate was needed for control.

Owen warned that no easy fix is on the horizon when it comes to glyphosate-resistant weeds. “While companies continue to develop new products, no new mode of actions have been discovered,” he said. “In this case, stewardship continues to be critically important.”


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