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Corn+Soybean Digest

Resistant Ragweed?

Never having to hoe a single day … few if any trips through the field with a cultivator … limited chemical applications … Roundup Ready soybeans, cotton and corn have helped remove chores that were once the norm.

But there has been a consistent fear that weeds, both the most troublesome and those that crop up occasionally, would become resistant to glyphosate herbicides. A pesky ragweed in North Central Arkansas looks like it could be the next marestail, a weed that appears unaffected by Roundup applications.

“This is a ‘suspected’ resistance situation with ragweed,” says Arkansas Extension Weed Specialist Bob Scott. “We have seen the weed survive multiple applications. But until we are positive about laboratory and field tests, we can't be positive that this is a Roundup resistant weed.”

The fear is that there will be another marestail or “horseweed” that has invaded parts of Ohio and Indiana, as well as the south. Bob Hartzler, Iowa State University Extension weed specialist, adds that researchers in Missouri also are investigating another ragweed biotype showing some resistance to Roundup Ready crops.

Arkansas grower Gary Sitzer isn't taking any chances. He is a soybean/rice producer at Weiner, AR, about 20 miles from where the suspected ragweed was found. He intends to maintain a 50-50 soybean and rice rotation with minimum-till production. He is also working more conventional varieties into his some 750 acres of soybeans.

“The field that had the ragweed problems was in a no-till Roundup Ready program, which makes it more susceptible to any potential weed resistance,” he says. “We feel our minimum-till program should help insulate us against any ragweed resistance for a while. But if it definitely is a glyphosate-resistant weed, then it could show up in our fields eventually.”

The pesky weed was discovered in a Roundup Ready soybean field near Newport, AR, in the summer of 2004. The field in question was in a no-till program that had been continuous soybeans at least seven straight years, says the area county agent Randy Chlapecka. The ragweed had been sprayed with glyphosate herbicide several times to no avail.

Scott says that even though the ragweed survived multiple applications, questions still remain about its true resistance to glyphosate herbicide.

“We weren't sure about the resistance of the ragweed when it was first sprayed,” he says. “The ragweed was fairly large when we first got into the field, and it's more difficult to control at that stage.”

University of Arkansas researchers in Fayetteville are attempting to get seed from the rugged ragweed to germinate in a laboratory setting for further testing. “But it has a natural dormancy mechanism that has made germination difficult,” says Scott. “So it may be the next season before we can determine if the ragweed can withstand properly timed spray treatments.”

Sitzer has rarely grown continuous soybeans. His every-other-year rotation with rice helps prevent unwanted weed problems.

“This rotation helps promote better yields for both crops,” he says. “Weed control is also improved. Because of our rotation, we'll probably be among the last ones to see Roundup resistant weeds.”

Late Group IVs and early Group Vs are planted to make sure soybean harvest does not interfere with rice harvest, between Sept. 1 and Oct. 1. He is also planting more conventional beans. Sitzer now plants up to 20% of his crop in conventional varieties.

“Hopefully we have cleaned up some things with Roundup and can now get back in the ballpark with conventional varieties,” he says.

He worries, however, that if glyphosate-resistant weeds evolve in Roundup Ready fields, growers will face the expense of having to apply costly herbicide tankmixes.

“We may have to compromise on weed control to keep from having to spray three or four herbicides,” he says. “The beauty of Roundup Ready is its effectiveness and its flexibility over a wide window of time.”

Iowa's Hartzler says cultural practices that include alternative herbicides along with glyphosate can help prevent the emergence of unwanted weeds.

“Rotation with conventional crops is the best approach,” he says. “Also, in a glyphosate system, growers are probably better off going with a pre-emergence herbicide followed by a postemergence glyphosate applied by itself. It also is important to establish a good crop canopy as soon as possible.”

He says that other than minor incidents of apparent resistance, occasional problems with a few less-troublesome weeds having natural tolerance to glyphosate, Iowa has seen no significant resistance problems. “Things are looking pretty good,” he says. “We are getting a few weeds that can tolerate the current use rates of glyphosate. Asiatic dayflower has shown some tolerance, but is not an aggressive weed. Wild buckwheat has also become more of a problem in northern Iowa.”

What scares Scott and others is the potential for a major weed developing resistance. “The bigger issue than whether this one is resistant or not is what are we going to do when something like Palmer amaranth pigweed or any kind of pigweed becomes resistant,” he says. “First we had the horseweed and now the suspected ragweed. When are we going to hit on one that really causes us problems?”

He urges growers to monitor their fields closely for weed problems, then report them to their local weed specialists, county agents or consultants. “Growers must be very diligent,” he says.

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