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• Palmer amaranth, also known as pigweed, is the most commonly discussed or just plain cussed weed across the Sunbelt, but a host of others, nearly 100 separate species, show resistance to one or more classes of herbicides.
Specialists also have some different recommendations, depending on location, crop choices and management options.
York said direct sprayers and hooded sprayers may help take out herbicide resistant weeds. He already noted hand weeding and some tillage.
“Shifting varieties is also a possibility,” he said. “Include some non-glyphosate tolerant options. We’re seeing more Liberty Link cotton and soybeans.”
But he cautions growers not to abuse that or any other herbicide. “Don’t over-depend on any one product. Use other chemistries.”
Crop rotation also offers resistance management opportunities. Tobacco may work well for some North Carolina growers. “It’s a high value crop that takes a lot of management but will reduce weed pressure.”
Grain sorghum may be an option for Southwest cotton growers. Baumann said Roundup Ready sorghum is not available, “and probably never will be. Sorghum gives us an opportunity to use a different mode of action.”
And Metcalf said canola may be a good alternative to continuous wheat to reduce pressure from cheat and resistant Italian ryegrass. Growers do have the option of planting Roundup Ready canola, however, and should be cautious about overuse.
Baumann said growers should do all they can to “get rid of early-season competition.” Killing weeds early not only takes pressure off post-emergence herbicides, such as glyphosate, but it also gives the crop a better opportunity for early growth. “Farmers may think they can control weeds later,” he said, “but by then, it has already picked your pocket every day it’s allowed to stay in the field.”
Clean fields, he said, are essential. “Controlling 85 percent to 90 percent of weed populations is not enough.
He said hooded sprayers may be useful for post-emergence treatments.
“Always use the full rate of the proper herbicide,” Osborne said. “Lowering rates is the fastest way to get into trouble. Also, don’t forget to control weeds on fence-rows, ditches and turn-rows. Be aware of what you might bring into a field on equipment, in seed or hay. Those are all avenues for herbicide resistant seed.”
Specialists all agree that weed size and application timing are key factors in control success.
And diversifying production practices may become necessary. “Don’t rule out rotation or tillage,” Osborne said. “Can I recommend tillage at a no-till conference?”
Apparently so. Metcalf also suggested tillage as one possibility. Several other specialists argue that if resistant weed populations become unmanageable in no-till systems, tillage may be necessary until the fields are cleaned up.
They all agree that the best option is to prevent herbicide resistance and avoid the expense, aggravation and the reduced yields that will come with heavy populations of weeds they can’t control.
“Take steps to prevent resistance,” Baumann said. “That means employing a preventive program now. And farmers will get the added advantages of getting rid of weeds early.”
Waiting for resistance to develop before taking action, he said, is asking for trouble, lots of trouble. One waterhemp or pigweed plant may shed as many as 400,000 seeds. If just a few of those survive, the potential for “an overwhelming” infestation is significant.
An ounce of prevention, as the old saw goes…
Glyphosate resistance means Arizona cotton farmers need to change production practices. McCloskey recommends mechanical tillage at pre-plant, in-crop cultivation, and post harvest, plus hand rouging before seed set, and multiple herbicide use with different modes of action.
“The top pre-season pigweed control option I recommend is a dinitroaniline (DNA) herbicide application of Prowl, Treflan, or generics,” McCloskey said. “The best way to apply a DNA herbicide is on the flat with a boom on a disk or field cultivator to incorporate the product.”
Forrest Laws and Cary Blake contributed to this article
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