Investigating the Efficacy of Selected Very-Long-Chain Fatty Acid-Inhibiting Herbicides on Iowa Waterhemp (Amaranthus tuberculatus) Populations with Evolved Multiple Herbicide Resistances

Eric A L Jones,Micheal D K Owen

doi:10.3390/agronomy11030595

Abstract

Very long chain fatty acid (VLCFA)-inhibiting herbicides (Herbicide group (HG) 15) have been applied to corn and soybean fields in Iowa since the 1960s. The VLCFA-inhibiting herbicides are now applied more frequently to control multiple herbicide-resistant (MHR) waterhemp (Amaranthus tuberculatus Moq. J.D. Sauer) populations that are ubiquitous across the Midwest United States as resistance to the VLCFA-inhibiting herbicides is not widespread. Waterhemp has evolved multiple resistances to herbicides from seven sites of action (HG 2, 4, 5, 9, 14, 15, and 27), and six-way herbicide-resistant populations have been confirmed. Thus, the objective of this study was to determine if selected Iowa waterhemp populations are less sensitive to VLCFA-inhibiting herbicides when additional herbicide resistance traits have evolved within the selected population. Dose–response assays were conducted in a germination chamber to determine the efficacy of three selected VLCFA-inhibiting herbicides (acetochlor, S-metolachlor, and flufenacet) on selected Iowa MHR waterhemp populations. An herbicide-susceptible, three-way, four-way, and five-way herbicide-resistant waterhemp population responded to the herbicide treatments differently; however, several of the four-way and five-way herbicide-resistant populations exhibited resistance ratios greater than 1 when treated with acetochlor and S-metolachlor. Selected four-way herbicide-resistant waterhemp populations from Iowa were subjected to a dose–response assay in the field using the same VLCFA-inhibiting herbicides, and all herbicides achieved control greater than 80% at the maximum labeled rate. The results of the experiments provide evidence that some MHR waterhemp populations may exhibit decreased susceptibility the VLCFA-inhibiting herbicides, but generally, these herbicides remain efficacious on Iowa MHR waterhemp populations.

Highlights

Very-long-chain fatty acids (VLCFA) inhibiting herbicides (herbicide group (HG) 15)interfere with the elongation of C18 fatty acid chains [1,2]
Selected four-way herbicide-resistant waterhemp populations from Iowa were subjected to a dose–response assay in the field using the same VLCFAinhibiting herbicides, and all herbicides achieved control greater than 80% at the maximum labeled rate
Very long chain fatty acidinhibiting herbicides are generally considered more efficacious on monocotyledous weeds, but they are active on some small-seeded dicotyledonous weeds [3,4]

Summary

Introduction

Very-long-chain fatty acids (VLCFA) inhibiting herbicides (herbicide group (HG) 15). Interfere with the elongation of C18 fatty acid chains [1,2]. Very long chain fatty acidinhibiting herbicides are generally considered more efficacious on monocotyledous weeds, but they are active on some small-seeded dicotyledonous weeds [3,4]. The VLCFAinhibiting herbicides affect several elongases and mutations to the elongases that would provide resistance to the VLCFA-inhibiting herbicides can be lethal; resistance to these herbicides is extremely rare [5,6]. Prior to 2019, resistance to VLCFA-inhibiting herbicides had been confirmed in five grass species globally: blackgrass Resistance to the VCLFA-inhibiting herbicides evolved in two broadleaf species, waterhemp (Amaranthus tuberculatus Moq. J.D. Sauer) and Palmer amaranth

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Results