A Flood of Resistance

October 28, 2011 11:07 AM
A Flood of Resistance

The U.S. hosts 138 herbicide-resistant weed species—and the time to control them is now

When it comes to confronting herbicide-resistant weeds, farmers have two choices: pay now to prevent them or pay later to remove them.

Weed control experts say that this harsh reality exists because it’s not a matter of if a farmer will face the scourge of resistant weeds but when.

In roughly the past decade, the U.S. has gone from having a handful of resistant weed problems to hosting 138 different resistant species in 45 states, according to the International Survey of Herbicide Resistant Weeds, a collaborative effort between weed scientists in more than 80 countries. That’s about two-and-a-half times more resistant weeds than Australia, which has the next highest population, with 57 resistant weed species.

While some of the factors contributing to the spread of resistant weed infestations can be avoided, others are beyond even the most diligent farmer’s control, says Ken Ferrie, Farm Journal Field Agronomist.

"The reality is you can’t control what comes across your farm in water, and it could be resistant-weed seed," he says.

Ferrie references the historic Mississippi River flood this past spring as an example of a factor beyond farmers’ control. The river, along with its tributaries, washed dirt, debris and weed seed onto thousands of crop acres from Missouri south to the Louisiana Gulf Coast. Farmers in other parts of the country had similar experiences.

Floodwaters washed away 400 acres of Morris Heitman’s corn and soybean crops near Mound City, Mo., this past spring when the Army Corps of Engineers released water from reservoirs on the upper Missouri River in the Dakotas and Montana.

Portions of the same acreage also flooded in 1993. That year, the receding water left behind willow and cottonwood seed.

"We ended up using a moldboard plow to get rid of the seedlings that fall," Heitman says.
Richard Zollinger, Extension weed specialist, North Dakota State University, explains that weeds will readily germinate and thrive where the crop has not developed a canopy and covered the soil.

Fewer options. This time around, Heitman is worried about johnsongrass. "I know it’s up and down the river; I don’t have any at the present time and don’t want any either," he says.
The same goes for resistant weed species. So far, Heitman’s weed control program has kept them at bay.

"We stay away from continuous glyphosate and include other chemistries in corn and beans," he says.

Heitman, who serves as a board member for the Missouri Corn Merchandising Council, says stewardship of glyphosate and other available chemistries is important for minimizing weed resistance.

"I know resistance is moving from the southern part of the state north, and that does concern me," he says.

Three states bordering Heitman’s home state of Missouri—Illinois, Arkansas and Tennessee—have some of the worst herbicide-resistant weed problems in the country.

Tennessee has more than 2 million crop acres infested with at least one resistant weed, according to Larry Steckel, University of Tennessee row-crop weed specialist.

"The late planting we had this year caused us a lot of problems in cotton and soybeans. We had to deal with more resistant Palmer amaranth than ever before and at higher concentrations," he reports.

"We’re now spending an additional $50 an acre in cotton for Palmer amaranth alone," he adds.
Some cotton fields were so infested that Steckel told farmers they needed to tear out their crop and start over.

Farther north, Aaron Hager, University of Illinois weed scientist, has documented resistance to five different herbicide site-of-action families that are specific to herbicide-resistant
waterhemp. Some populations of waterhemp are resistant to glyphosate, some postemergence HPPD inhibitor herbicides, triazine, PPO inhibitors and ALS inhibitors.

That lack-of-control nightmare is one that Daniel Stephenson, weed control specialist with Louisiana State University AgCenter, is trying to help all producers avoid.

Glyphosate-resistant johnsongrass, a perennial weed species, concerns him more than even herbicide-resistant Palmer amaranth, which is reportedly the No. 1 resistant-weed problem in many states.

"When you’re fighting a perennial, you’re demanding the herbicide to translocate into the roots of the weed to provide control, and that can be a challenge," Stephenson says.

Mechanical weed control offers little help and can even contribute to a greater problem.

"If you chop up the rhizome, you’re probably spreading the weed in your field," Stephenson adds.
He says farmers must use multiple practices to control resistant weeds. No single practice is adequate.

"Mowing, tillage, crop rotation, burndown treatments, pre-emergence and post herbicides with residual control need to be used," he says.

"Don’t be chintzy with herbicide," adds Mark Baer, manager of Sun Ag Supply Inc. of Tremont, Ill. "Applying reduced herbicide rates multiple times over multiple years to save money has helped create this monster we’re dealing with today."

Ferrie agrees with Baer. Ferrie says he sees farmers put a half rate of a pre-emergence herbicide on problem fields and then come back to clean up escapes if needed with glyphosate, creating resistance to the pre-emergence herbicide and possibly glyphosate in the process. "You have to use full rates to control weeds correctly," he says.

For some farmers, that will mean going with a $30 or more per acre weed control program, for the product only, versus one that’s $15 an acre.

"It’s tough to take a preventive approach if you don’t think you have a resistant weed issue, but today it’s necessary," Ferrie adds.

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