Beating Glyphosate-Resistant Palmer Amaranth Comes Down to Timing and Strategy
28 February 2012 RALEIGH, N.C. – With an estimated 1.3 million acres of cotton in the US mid-South taken over by glyphosate-resistant Palmer amaranth – commonly known as pigweed – growers are returning to tried-and-true integrated weed management programs in an effort to prevent costly damage.
“Achieving control before the weed reaches 1 to 2 inches tall is feasible, but once it gets beyond that, control measures can be costly, burdensome and many times less effective,” says James Whitehead, senior development technical lead for MANA Crop Protection in Oxford, Miss. “Experts are advising growers to approach every field proactively by using the right residual herbicide at the right timing to gain better results.”
With glyphosate-resistant weeds now a fact of life for cotton production, Dr. Kenneth Smith, extension weed scientist at the University of Arkansas in Monticello, says the rules of the road for weed management require a whole new approach.
“In cotton, we don’t have an over-the-top herbicide that will kill emerged Palmer amaranth, so we must go with residuals to keep them from germinating in Roundup Ready cotton,” says Smith. “In Arkansas, we’ve adopted the philosophy that glyphosate is now a tool for cleanup, not to kill the weeds.”
Studies show residual herbicides hold for an average of about three weeks, according to Smith, who recommends treating every two weeks – which signals a shift in strategy since the mid-2000s. “Back in 2004, we had almost gotten away from residual herbicides,” he says. “If we got the crop up, applied glyphosate with a residual herbicide, got a rain and it worked, great. We would come back with another treatment only if we had to. But that is no longer the case.”
Early-season use of residual herbicides – Smith recommends starting prior to planting – as part of an integrated systematic weed management strategy is critical to stopping glyphosate-resistant pigweed. Growers prevent not only early weeds, but can also address signs of or confirmed resistance.
After the crop emerges, he recommends growers follow up with an over-the-top treatment with metolachlor at the one- to two-leaf stage followed by another at the five- to six-leaf stage, and then scout closely at the 10- to12-leaf stage. “If weed seedlings are visible at the 10 leaf stage, a post-directed application of Cotton Pro® + MSMA is in order,” he says. “If the cotton is clean at the 10-leaf stage, we prepare to layby at the 12- to13-leaf stage with Direx + MSMA or Valor + MSMA.” Critical for success, he says, is rotating herbicides with different modes of action.
Research and on-farm experience has shown that Palmer amaranth can be effectively controlled by using residual herbicides. Most effective control is achieved by overlapping residual herbicides in a systematic approach starting at preplant/pre-emergence timings followed by post-emergence applications as needed through layby. Using residual herbicides with multiple modes of action at the appropriate timings provides extra protection and prevents Palmer amaranth from becoming established during the season.
While cultivation is a reliable technique to remove Palmer amaranth from young cotton, directed post-emergence applications can be successful after cotton reaches the six- to eight-leaf stage. “Careful scouting can target escapes in residual herbicides and allow needed time for effective post-directed treatment while the weeds are still young and have a height differential with the cotton,” Whitehead adds. “For layby, we’re seeing some growers use a post-directed application of residual herbicides to help remove small and late-germinating pigweed which helps to keep fields clean up until harvest.”
Smith and Whitehead both believe that growers should never rely on a resistance-management strategy solely focused on herbicides. A well-rounded program includes residual herbicides; rotation of different modes of action; weed control on surrounding non-crop ground; crop rotation; and use of good tillage and cultivation practices.
“Glyphosate-resistant weeds are expected to be an ongoing problem, but it’s a problem that farmers can manage,” says Whitehead. “It requires more management to avoid yield losses due to weed competition which means growers must pay more attention to product selection and timing.”
Source: MANA Crop Protection; Edited by Managing Editor Jaclyn Sindrich
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