Weeds: An Ongoing Battle

The fight to control yield-robbing weeds goes back thousands of years, to agriculture’s beginnings. For the nation’s ag retailers and their grower-customers, this presents both challenges and opportunities.

According to most analysts, weeds account for approximately $95 billion annually in lost global food production. In recent years, much of this decline can be traced back to an ever-expanding list of glyphosate-resistant weed species.

“Glyphosate-resistant weeds currently affect an estimated 7.5 million U.S. row crop acres in corn, soybeans, and cotton,” says Chuck Foresman, manager of weed resistance strategies for Syngenta Crop Protection. He adds that most observers predict this number will grow significantly by 2013, when one in four acres will be invested with glyphosate-resistant weeds.

To appreciate how this all adds up for the average grower-customer, says Gary Schmitz, technical service manager for BASF, consider the following. “According to one study from The Ohio State University, farmers experienced a 6% to 10% yield loss per acre when the weeds in their fields were 9- to 12-inches high,” says Schmitz. “In corn, results show that weeds as little as 4 inches tall have the potential to cause a 4.5 bushel-per-acre yield loss.”

This has even become a major problem outside the nation’s Corn Belt. “Pal­mer amaranth or pigweed has taken over many cotton fields in the South,” says Dan Westberg, technical marketing manager for BASF. “In many places, growers are now using workers in row gangs to take this weed out of their fields by hand because traditional crop protection products are not proving effective in providing any kind of control.”

Besides Palmer amaranth, there currently are several other glyphosate-resistant weed types in more than one dozen states. These include common ragweed, giant ragweed, hairy fleabane, common waterhemp, Italian ryegrass, and marestail (or horseweed), which was the first glyphosate-resistant weed species discovered back in 2000.

“When glyphosate was first introduced for weed control, its unique way of inhibiting protein synthesis and growth in plants led many to believe that resistance would not be an issue,” says David Shaw, president of the Weed Science Society of America. “Obviously, that prediction was wrong.

“Unfortunately, it is too late to prevent glyphosate resistance from developing,” Shaw continues. “It’s a problem that is already with us. The challenge now is to adopt effective management techniques that can keep resistance from spreading.”

Methods To Control Weeds

University scientists recommend growers rotate the types of herbicides used to control weeds, making it tougher for the plants to adapt. Jeff Carpenter, corn portfolio manager for DuPont Crop Protection, agrees with this recommendation.

“We are constantly trying to introduce products for weed control that offer multiple modes of action and blend exisiting products that offer more effective control,” says Carpenter. “Ultimately, the goal with any of these is to help prolong the life of the glyphosate system that growers have relied on for the longer term.” To this end, he adds, DuPont offers its brand Prequel, a pre-mix which combines the active ingredients rimsulfuron and isoxafutole.

According to BASF’s Westberg, certain weeds such as marestail must be controlled prior to the emergence of crops such as soybeans, or else growers will be dealing with “a complete mess” in their fields. The company’s herbicide saflufenacil (marketed under the Kixor technology brandname) can be used in these instances as a preplant burndown option. “Kixor also provides residual control, meaning they need fewer in-season postemergence applications,” he adds.

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