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Managing Herbicide-Resistant Pigweed in Xtend Soybeans

Dr. Larry Steckel, UT Extension Weed Specialist at Jackson, Tenn., has posted several articles that provide options for controlling multiple-resistant Palmer amaranth, including those in Xtend soybeans that are tolerant of dicamba herbicide. These articles are linked below along with a summary of their content.

“Recent Midsouth Studies Show Dicamba not very Effective on some Populations of Glyphosate/PPO-Resistant Palmer Amaranth.” Don’t assume that Engenia or Xtendimax dicamba herbicides will automatically control all Palmer amaranth. In Dr. Tom Barber’s (Arkansas Weed Scientist) plots that Dr. Steckel visited in Crittenden County Arkansas, it was obvious that dicamba applied to small Palmer amaranth provided less than optimal control. The only herbicides that appeared to work best in the viewed trials were atrazine and Liberty.

Following the viewing of these trials and noting their results, screenings were conducted on Palmer amaranth biotypes from Shelby County, Tenn. plus biotypes from Knox County, Tenn. These screenings produced the same results on the Shelby County biotypes as those viewed in the Arkansas trials, but interestingly, the Palmer amaranth samples from Knox County, Tenn. showed 95% control by dicamba compared to only 65% control of the West Tenn. biotypes.

Dr. Steckel’s conclusions are:

•   There is a good chance that some Palmer amaranth will escape control by Engenia and Xtendimax herbicides; scouting will be critical to ensure that dicamba applications have in fact controlled this targeted weed.

•   PRE herbicides/herbicide mixes that contain an efficacious component against Palmer amaranth must be used.

•   Herbicides with known resistance issues against targeted weeds should not be abandoned, but rather should be combined with other residuals that have good activity against Palmer amaranth.

“Controlling Multiple-Resistant Palmer Amaranth.” Results from a survey of West Tennessee soybean fields found that 83% of tested fields had Palmer amaranth populations that were resistant to fomesafen (PPO inhibitor–Group 14), and subsequent field research showed that these populations are also resistant to ALS herbicides (ALS inhibitors–Group 2) and glyphosate. Subsequent research was conducted to determine the effectiveness of various herbicides in controlling PPO-resistant Palmer amaranth compared to a location with Palmer amaranth populations that were still susceptible to PPO-inhibiting herbicides. Findings/determinations from this activity follow.

•   XtendiMax (dicamba) and Enlist Duo (2,4-D+glyphosate) herbicides provided 80% control at the PPO-resistant site and 100% control at the PPO-susceptible site. Liberty provided 90% and 100% control at the two respective sites, whereas atrazine provided 100% control at both sites. Flexstar (fomesafen) provided ~28% control at the PPO-resistant site and nearly 100% control at the PPO-susceptible site.

•   Interestingly, even though Flexstar applied alone did not control the PPO-resistant Palmer, it did control the Palmer that had escaped the initial application of dicamba. Thus, it appears that Flexstar is of value for controlling weakened Palmer following a dicamba application.

•   From a stewardship standpoint (reduced potential for offsite drift of auxin herbicides and optimum resistance management from applying 2 modes of action), applying an auxin herbicide (either dicamba or 2,4-D) as the first POST application followed 7-10 days later with Flexstar provides good control of any escapes and potentially removes the need for a second application of an auxin herbicide.

“Palmer Amaranth Management Strategies in Xtend Soybean”. Dr. Steckel provides weed control options for the following two scenarios.

1) Xtend soybeans are planted but there is no intention of applying POST dicamba. He strongly recommends planting soybeans in rows that are ≤20 inches wide in a field free of weeds, and applying a PRE herbicide with at least two modes of action (see example herbicides listed in linked article) that are effective against Palmer amaranth. If a premix that contains metribuzin is used, he recommends increasing the rate of metribuzin in the mix. This should be followed by POST applications of Prefix or Anthem (both Group 15+14) or Zidua (Group 15) herbicides no later than 10-14 days after the PRE application. If all components of this plan are included, the likelihood of controlling glyphosate, PPO-, and ALS-resistant Palmer amaranth is high.

2) Xtend soybeans are planted and either Engenia or XtendiMax dicamba herbicides will be applied POST. As in system 1) above, plant in narrow rows in a field free of weeds and apply PRE residual herbicides as indicated. Before Palmer amaranth reaches 4 inches tall, apply the dicamba herbicide POST. Follow that 7-10 days later with a POST application of a fomesafen-based herbicide like Prefix or Warrant Ultra to remove any pigweed escapes.

A final note. Under no circumstances should growers rely solely on POST-applied dicamba herbicide for weed management in soybeans just as they should not have relied solely on POST-applied glyphosate. This will lead to the same result–the end of the usefulness of the technology as an additional tool for weed control in soybeans. It is imperative that the PRE component of the weed control strategies outlined above be an integral part of weed management programs that use auxin-tolerant soybeans with auxin herbicides applied POST.

Click here for a White Paper that elucidates how to use herbicide mode of action (MOA) in soybean weed control strategies.

Composed by Larry G. Heatherly, June 2018,