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Control of GR Palmer amaranth in Enlist soybeans

Weed management in soybeans has always been and is still a major concern for producers. Programs for controlling/managing weeds in soybeans are now exacerbated by herbicide resistance that has developed in major problematic weeds such as Palmer amaranth. The resistance that is especially problematic is glyphosate resistance (GR), especially since many weed populations in the U.S. are now GR. New weed control technologies such as Extend (dicamba-tolerant) and Enlist (2-4,D-tolerant) soybean varieties have allowed producers to continue to devise strategies to control these problematic weeds in the absence of glyphosate use, while continuing to control other weeds that are also present.

Since 2-4,D-resistant soybean varieties are the newest available weed control technology, much ongoing research has been/is being directed toward how best to use this transgenic weed control trait to keep Palmer amaranth from continuing its role as the number one weed problem for Midsouth soybean producers to manage. The Enlist varieties that are resistant to 2,4-D are especially important since they are also resistant to glufosinate and glyphosate. This allows producers to choose from and use a wide array of herbicides with different modes of action to manage in-season weeds.

A report titled Control of glyphosate-resistant Palmer amaranth in Enlist soybean presents results from research that was conducted in Nebraska to determine the best strategy for controlling GR Palmer amaranth in Enlist soybeans. The report was published on the UNL Cropwatch site on Aug. 5, 2021. A summary of the conduct of and results from that research follow.

•    A GR Palmer amaranth population was confirmed in a grower’s field that had been in a continuous GR corn-soybean rotation. This site was used for the field study.

•    The objectives of the research were to: 1) investigate and compare the effect of pre-mix herbicides applied PRE alone or applied PRE followed by (fb) late POST applications of 2,4-D choline (Enlist One) or glufosinate (Liberty), or both; and 2) determine the effectiveness of POST-only sequential applications (early POST fb late POST) of Enlist One or Liberty on GR Palmer amaranth control in soybeans.

•    The research was conducted in 2018 and 2019 without irrigation on a site with a silt loam soil with 2.63% organic matter and a pH of 4.8.

•    PRE herbicides were: 1) Sonic–Group 2 cloransulam-methyl (e.g. FirstRate) + Group 14 sulfentrazone (e.g. Spartan); 2) Zidua PRO–Group 2 imazethapyr (e.g. Pursuit) + Group 14 saflufenacil (e.g. Sharpen) + Group 15 pyroxasulfone (e.g. Zidua); and 3) Trivence–Group 2 chlorimuron ethyl (e.g. Classic) + Group 5 metribuzin + Group 14 flumioxazin (e.g. Valor).

•    POST herbicides were: 1) Enlist One–Group 4 2,4-D choline; and 2) Liberty–Group 10 glufosinate.

•    Zidua PRO and Trivence herbicides applied PRE with no following POST herbicide applications provided 94-97% early Palmer amaranth control, but control after the date of a late POST application of either Enlist One or Liberty in the PRE fb POST treatment dropped to 85% or lower.

•    The POST-only treatments of either Liberty (85% late-season control) or Enlist One (92% late-season control) were less than desired.

•    The PRE + POST treatment of Zidua PRO fb POST Liberty resulted in 99% late-season control of GR Palmer amaranth. The PRE + POST treatment of Trivence fb Enlist One + Liberty resulted in 100% control.

•    The greatest control of GR Palmer amaranth in this study was achieved with PRE Zidua fb late POST Liberty (99% control) and PRE Trivence fb late POST Enlist One + Liberty (100% control). Neither PRE nor POST herbicides alone provided acceptable long-term control.

•    Take Home Message. 1) Enlist soybean varieties provide POST herbicide options for managing not only GR weeds, but also weeds with multiple herbicide resistance. 2) An integrated weed management strategy is necessary to prolong the effectiveness of present or forthcoming weed control technology. This includes not following Enlist soybean varieties with Enlist corn hybrids so that the same POST herbicides are not used in the same field on an annual basis. 3) These results reinforce the weed management practice of using a combination of both PRE and POST herbicides to maintain season-long control of problematic weeds in soybeans.

Click here to access the CDMS site for up-to-date herbicide labels.

Composed by Larry G. Heatherly, Aug. 2021,