By Boyd Kidwell
Progressive Farmer Contributing Editor
Jimmy Wilson's weed-control program for this year's cotton crop started before his 2013 crop was harvested. Wilson hired a local pilot to aerially broadcast triticale as a winter cover crop into cotton fields.
He has used the small-grain crop in combination with strip-till as a soil conservation tool for many years. During the last few years, winter cover crops have also become an important weapon in the battle against glyphosate-resistant weeds.
"We want to keep vegetation covering the ground all year," said Wilson, who farms with his brother, Bill, and son, Dan, near Scotland Neck, N.C. "Triticale has a large root system similar to rye but without the early flush of growth that makes a rye cover crop hard to manage at planting," Wilson said. "Keeping the ground covered until planting shades the soil and keeps resistant weeds from getting an early start."
A winter cover crop costs about $20 per acre to establish. It's just the first of several costs that have crept into Wilson's farming operation along with resistant weeds. Glyphosate-resistant Palmer amaranth pigweed is the primary weed problem on Wilson Farms. One escaped female pigweed can produce 450,000 seeds.
"Resistant pigweed can take a farm in two years," Wilson noted. "Roundup Ready crops were a godsend for us, and weed-control costs were approximately $20 per acre in the Roundup system. But now, we spend $50 to $60 per acre for residual herbicides in addition to our Roundup Ready program."
Cotton (1,450 acres) is the primary money crop at Wilson Farms, and Palmer amaranth can flourish before the crop produces a canopy to shade the ground.
"All a pigweed needs is a crack of daylight, and it takes off. When I drive around a field this time of the year, it's like the pigweeds are screaming at me," Wilson said. "I keep a man patrolling my fields on a four-wheeler. His job is to get out there and pull up any pigweeds that have escaped and are producing seed for next year."
In late winter, before cotton planting approaches, Wilson makes his first herbicide application of 2,4-D mixed with Valor. This tank mix controls winter weeds but doesn't kill the triticale cover crop. Just ahead of strip-till planting, he applies Gramoxone (or Touchdown) to kill the cover crop. At planting, he sprays Reflex (fomesafen).
At first true leaf, Wilson applies Touchdown plus an insecticide. As a lay-by treatment, he applies Sequence (a premix formulation of Touchdown and Dual Magnum.) "The key to weed control in my cotton is spraying Reflex at planting," Wilson said. "Reflex does a good job if we get any rain after planting."
Wilson's herbicide program uses multiple (five) sites of action, and, at times, he overlaps residual products. "We may overlap residuals, but we want to be on the safe side because we have no rescue treatment. Once these pigweeds grow shoe-top high, they've escaped," said the veteran row cropper.
Reflex can be tank-mixed with other preemergence herbicides or mixed with glyphosate products for postemergence control. However, Reflex should only be used preemergence on coarse-textured or sandy-loam soils. Reflex should not be applied preemergence to medium- or fine-textured soils, as crop injury may occur under cool and wet conditions.
Cotoran or Caparol, or a mixture of the two applied preemergence, is a better fit for Tennessee silt-loam soils, said Larry Steckel, weed specialist at The University of Tennessee. While Reflex provides up to 30 days of pigweed control after planting, growers expect only two weeks of control from Cotoran/Caparol.
About 80% of Tennessee's growers plant Liberty-tolerant cotton varieties and apply a tank mix of Liberty and Dual over-the-top at the one- to two-leaf growth stage, Steckel explained.
Controlling Palmer amaranth requires a complex system applying herbicides (usually five to seven sites of action) throughout the season plus other management approaches, such as hand weeding, tillage and cover crops, said Stanley Culpepper, weed specialist at The University of Georgia.
"We're fighting a biologically impressive plant, and we have to use a whole system to be successful," he said. "You can't just spray your way out of a Palmer amaranth problem."
Culpepper emphasized winter cover crops as part of a biological attack on this serious weed pest. An ideal cover crop keeps the soil shaded throughout the season and can prevent 70% to 90% of the pigweed from emerging.
Georgia researchers are experimenting with killing and rolling a rye cover crop (video at www.gaweed.com) to prevent weed seed germination between rows. Palmer amaranth fiourishes in sunlight and warm weather.
Culpepper's recommendation for early weed control in conventional tillage cotton is a preplant-incorporated mix of Reflex plus Prowl/Trefian within seven days of planting. In a conservation tillage system, he suggested Valor mixed with a burndown herbicide for residual control.
A mix of Warrant plus Reflex is his recommended preemergence treatment for both tillage systems. Culpepper urges growers to make this application after planting but within 24 hours after the seed is in the ground. Be sure herbicide rates are correct for each soil type.
Culpepper has been working to develop a weed-control program that helps growers become timely with their postemergence applications simply by using the calendar:
-- His early research in Roundup Ready cotton suggests making the first postemergence application 13 to 16 days after planting and the preemergence application. He recommended a shorter interval when planting after May 10.
-- He recommended a second postemergence application 14 to 16 days after the first postemergence application.
-- Finally, he advised a lay-by application 16 to 18 days later.
Culpepper emphasized these intervals are based on South Georgia growing-degree days and may differ in other areas. He continued to stress the importance of timeliness and the need for residual herbicides applied throughout the season.
Knowing the problems glyphosate-resistant weeds have caused, Wilson is presently holding LibertyLink cotton in reserve as his "ace in the hole" if the costs of planting Roundup Ready cotton continue to climb. He also rotates cotton acres that have serious pigweed problems into grain sorghum to take advantage of better weed-control options.
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