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Corn+Soybean Digest

Resistance Relief

Ricky Belk knows he's dodging bullets. His weed-control strategy is on borrowed time. And when glyphosate resistance finally arrives, he's ready to replace lady luck with LibertyLink if needed.

Belk is one of the fortunate southern growers who hasn't been hit with glyphosate herbicide resistance. The Greenwood, MS, corn, soybean and cotton grower has been able to handle pigweed and other pesky plants with a glyphosate program.

But for how long? He can't say. He and other growers who have escaped resistance problems know that pigweed, horseweed (marestail), morningglory and other weeds that now include even johnsongrass will eventually grow right through a traditional glyphosate application.

“Everything is revolving around resistance,” says Ken Smith, University of Arkansas Extension agronomist. “It's driving our whole train. Glyphosate resistance in corn is not a bad problem. But with the switch from cotton to corn and soybeans, no one knows which way we'll go.”

Belk credits a strict rotation of corn, cotton and soybeans with helping keep resistance at bay. But the economics of growing more grain and less cotton is changing his program.

“The market has screwed up every rotation,” says Belk. “I normally had 2,200-2,500 acres of cotton. With the higher corn and soybean prices, I now have about 400. Some fields have been three years continuous corn. I know I'll have to do something to prevent resistance.”

He has more Roundup Ready corn to prevent damage from glyphosate drift on beans and cotton. “I apply all herbicides on corn and soybeans with a Miller Nitro sprayer with a 90-ft. boom sprayer,” says Belk, noting that aerial applications are made if fields are too muddy.

He combines his glyphosate program with Bicep and Atrazine on corn to make sure weeds are burned down. He never skimps on application rates, which has caused some resistance problems for others.

“I was a custom applicator before, and all the resistance problems I saw were from growers cutting rates,” says Belk. “But if I still develop resistance, I'll go with LibertyLink if needed.”

THE LIBERTYLINK SYSTEM, in which plants contain a gene that protects them from damage from Ignite herbicide, is now available for soybeans. Smith is eager for it to hit the market.

“LibertyLink beans are a bright spot,” says the Arkansas weed specialist. “We may see a number of acres in Arkansas and other states.”

He reminds growers that LibertyLink is not like glyphosate. “It's a whole different bug,” he says. “It's a whole different mode of action. Hopefully it will take care of pigweed and other weeds that we expect to have continued problems in soybeans and eventually corn.”

Johnsongrass resistance to glyphosate has shown up in a few Arkansas acres and other parts of the South. But Smith still fears pigweed as the primary menace now and in coming years.

Pigweed resistance first showed up in 2005. “Now it's apparent in virtually every soybean production area,” says Smith. “Its resistance to glyphosate is just like the pattern pigweed developed in obtaining ALS herbicide resistance.”

He says the decision to control resistant pigweed should be made before planting. “Once it's up and growing and you can't control it with Roundup, coming back with Roundup and a mix of other herbicide is not going to work.”

One program that may work for resistant weeds is an application of Reflex or Prefix herbicides at planting. “They have worked well,” says Smith. “You can then come over the top with Roundup.”

So far barnyardgrass, another huge weed in the South, is being contained by glyphosate. “We're in trouble if barnyardgrass develops resistance,” says Smith.

“Growers should consider 19-in. rows over 38-in. rows with beans to guard against weeds. The 19-in. centers provide much quicker canopy,” he says. “Residual herbicides don't have to last as long as on 38s.”

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