Resistance Fighters Start With Rotation

March 11, 2011 08:03 PM
Resistance Fighters Start  With Rotation

Malcom Haigwood never dreamed he’d be reminiscing about the good old days so soon. "The ’90s were so easy," says the Newport, Ark., farmer. "We sprayed glyphosate every Monday morning through canopy closure. It worked for a lot of years."

Along came glyphosate-resistant Palmer amaranth. And marestail. Johnsongrass got ugly, too. Haigwood says it’s hard to exaggerate the economic impact these weeds have had. "We have had to till up Roundup Ready cotton because we couldn’t control the Palmer," he admits.

Consider yourself lucky—or good, depending on your weed management program—if similar horror stories haven’t yet emerged in your fields. Whatever you do, just don’t consider yourself immune.

Southern states have been the hotbed for resistance issues, but weeds are putting up a fight across the landscape.

Cheap, but costly. When glyphosate prices dropped nearly 50% this past year, Greg Kerber of Gibson City, Ill., winced but held steady to his plan to rotate chemistries beyond glyphosate.

"There’s nothing easier than the Roundup Ready system," Kerber says. "That’s why it’s so important to protect it. If we are going to be good stewards of the technology, we have to look at other options."

University of Illinois Extension weed specialist Aaron Hager says when glyphosate starts to disappoint, farmers often reach for postemergence herbicide tank-mix partners.

weed piegraph"Control of tough weed species, such as waterhemp, giant ragweed, horseweed and morningglory, really needs to begin with a soil-applied herbicide," Hager says. "Residuals also provide another herbicide mode of action to reduce the selection pressure and delay the onset of or manage existing glyphosate-resistant weeds."

Kerber used a burndown residual on every acre this past year and plans to follow suit in 2011.
"Residuals allow me to put a different mode of action into the system to give my post product a chance against weeds such as waterhemp," he notes.

He admits it is more difficult to shell out the $8 to $12 per acre cost of a residual for soybeans versus corn. "In terms of bushel cost, though, it isn’t much of an expense," he says. He uses a full rate of all residual products.

Crop rotation and herbicide tolerant trait rotation further diversify his weed management program. He depends on Roundup Ready corn traits and alternates herbicide-tolerant traits by planting LibertyLink soybeans.

Kerber is still penciling out how many corn-on-corn acres he’ll have this year, but he is considering herbicide-tolerant stacked hybrids for those acres since he can control any volunteer [glyphosate tolerant] corn with glufosinate (Ignite). Stacks will also go on acres where he suspects more
waterhemp, too.

"Volunteer corn is one of our biggest issues, and I have to be careful not to work myself into a corner on my herbicide trait rotation by planting stacks," he says. "In soybeans, I can control [glyphosate tolerant] volunteer corn with Ignite and that saves me the $6 to $7 per acre for a selective herbicide."

First signs. Haigwood says that weed control has become his No. 1 crop priority over the past five years. "We can see now that we should have been better stewards of the glyphosate technology," he admits. "However, I don’t think southern growers have handled it any different than other regions. Our warmer climate and longer growing season have made resistance issues appear here first. Keep using the same herbicide program over and over and the weeds will win, no matter where you live."

Haigwood has an advantage in the ability to rotate between corn, cotton, milo, rice, soybeans and wheat. In early spring, he heads to the field with burndown that contains three different modes of action. He’s also gone to using overlapping residual products since Palmer amaranth tends to germinate all summer long.

"The first day you wonder if you should increase rates is the day resistance starts. At that point, you are two years away from major problems unless you take aggressive action," Haigwood says.

University of Arkansas weed scientist Ken Smith notes that there are currently 20 different weed biotypes resistant to herbicides in his state. He thinks farmers have a new respect for rotation, but it still makes him cringe to hear a grower say, "I think I can hold out one more year."

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