Control of Glyphosate-Resistant Palmer Amaranth in Enlist Soybean

August 5, 2021

Introduction

Glyphosate-resistant Palmer amaranth is a problematic annual broadleaf weed in soybean production fields in Nebraska (Figure 1) and many other states in the United States. A glyphosate-resistant Palmer amaranth population was confirmed in a grower’s field in a continuous glyphosate-resistant corn-soybean rotation located in Thayer County, Nebraska. A subsequent greenhouse dose-response bioassay confirmed that Palmer amaranth was 37- to 40-fold resistant to glyphosate compared to a glyphosate-susceptible population (Chahal et al. 2017). Moreover, the mechanism of resistance was found to be amplification of EPSPS gene copies (32- to 105-fold) compared with a glyphosate-susceptible biotype (Chahal et al. 2017).

**Figure 1.** Glyphosate-resistant Palmer amaranth in soybean field in south-central Nebraska (Photo by Amit Jhala).

A new multiple HR soybean trait that exhibits resistance to 2,4-D choline, glufosinate and glyphosate has been developed by Corteva™ Agriscience and was commercialized in 2019 in the United States (Shyam et al. 2021). However, little scientific literature exists pertaining to the most effective herbicide programs for managing GR Palmer amaranth in this new soybean production system. Therefore, the objectives of this research were to investigate and compare the effect of three pre-formulated herbicide mixtures applied pre-emergence (PRE) alone or followed by a late-post-emergence (POST) application of either 2,4-D choline (Enlist ONE) or glufosinate (Liberty) or both, and sequential applications (early-POST fb late-POST) of Enlist ONE or Liberty on glyphosate-resistant Palmer amaranth control in a multi-year field study in Nebraska.

Materials and Methods

Field experiments were conducted in 2018 and 2019 in a grower’s field infested with glyphosate-resistant Palmer amaranth near Carleton in Thayer County, Nebraska. The field was rain-fed without supplementary irrigation. Palmer amaranth was the predominant weed species at the research site. The soil at the experimental site was silt loam with 63% silt, 19% sand, 18% clay, 2.63% organic matter and 4.8 pH. The previous crop was no-till soybean at the site. The experiment was arranged in a randomized complete block design with four replications. Enlist soybean with 2.5 maturity group was no-till planted at a rate of 13,000 seeds per acre on May 10, 2018 and May 6, 2019. Herbicide programs included PRE, PRE fb a late-post (LPOST), and early-POST (EPOST) fb LPOST with a total of 15 treatments, including a nontreated control.

Results

Herbicides applied PRE in this study provided 84% to 97% control of glyphosate-resistant Palmer amaranth at 14 days after PRE, and no differences in control were observed among the PRE herbicides (Table 3). Although statistically similar, Sonic provided 84% to 87% control, while Zidua PRO, and Trivence provided 87% to 97% control at 14 days after PRE (Table 1).
At 14 days after early POST, Liberty or Enlist ONE provided 88% and 65% control of Palmer amaranth, respectively (Table 1). Lower control by Enlist ONE could be attributed to variable Palmer amaranth height at the time of application.
At 14 days after late-POST, herbicides applied PRE without a follow-up POST herbicide failed to maintain Palmer amaranth control compared with PRE fb POST or early-POST followed by late-POST herbicide programs (Table 1).
The sequential application of Enlist ONE or Liberty provided 85% to 92% control of glyphosate-resistant Palmer amaranth 14 days after late POST (Table 1).

Table 1. Glyphosate-resistant Palmer amaranth control as affected by herbicide programs in Enlist soybean in field experiments conducted at Carleton, Nebraska ^a,b.
Herbicide and rate per acre	Timing^{^a}	Palmer amaranth control^c (%), 14 DA-PRE^a	Palmer amaranth control^c (%), 14 DA-EPOST	Palmer amaranth control^c (%), 14 DA-LPOST
Nontreated control		0	0	0
Sonic (4.5 oz)	PRE	85 a*	49 f	27 e
Sonic (4.5 oz) fb Enlist ONE (2 pt)	PRE fb LPOST	87 a	71 de	70 d
Sonic (4.5 oz) fb Liberty (32 fl oz)	PRE fb LPOST	85 a	81 b-d	88 bc
Sonic (4.5 oz) fb Enlist ONE (2 pt) + Liberty (32 fl oz)	PRE fb LPOST	84 a	75 c-e	96 ab
Zidue PRO (6 fl oz/A)	PRE	97 a	81 b-d	75 d
Zidue PRO (6 fl oz/A) fb Enlist ONE ()	PRE fb LPOST	95 a	89 ab	95 ab
Zidue PRO (6 fl oz/A) fb Liberty (32 fl oz/A)	PRE fb LPOST	95 a	92 ab	99 a
Zidue PRO (6 fl oz/A) fb Enlist ONE (1.5 pt/A) + Liberty (32 fl oz/A)	PRE fb LPOST	87 a	94 a	96 ab
Trivence (8 oz)	PRE	94 a	75 c-e	85 c
Trivence (8 oz) fb Enlist ONE (2 pt)	PRE fb LPOST	93 a	90 ab	93 a-c
Trivence (8 oz) fb Liberty (32 fl oz/A)	PRE fb LPOST	89 a	87 a-c	92 a-c
Trivence (8 oz) fb Enlist ONE (2 pt) + Liberty (32 fl oz/A)	PRE fb LPOST	97 a	92 ab	100 a
Liberty (32 fl oz/A) fb Liberty (32 fl oz/A)	EPOST fb LPOST	_____	88 ab	85 c
Enlist ONE (2 pt) fb Enlist ONE (2 pt)	EPOST fb LPOST	_____	65 e	92 a-c
P-value		0.2076	<0.0001	<0.0001
Contrast Analysis^d
PRE vs. PRE fb LPOST		_____	_____	62 vs. 92***
PRE fb LPOST vs. EPOST fb LPOST		_____	_____	92 vs. 88^NS

^aAbbreviations: DA-PRE, d after PRE herbicide application; DA-EPOST, d after early POST application; DA-LPOST, d after late POST application.
^bData presented in this table were pooled across both years (2018 and 2019).
^cMeans presented within each column with no common letter(s) are significantly different based on Fisher’s protected LSD test, where α = 0.05.
^dA priori orthogonal contrasts; ***=significant (P < 0.0001), NS= non-significant.

Take Home Message

Enlist soybean provides additional POST herbicide options such as Enlist ONE or Liberty to soybean growers for managing not only glyphosate-resistant but also multiple herbicide-resistant Palmer amaranth.
The adoption of integrated weed management practice is necessary to maintain the effectiveness of Enlist crop technology. For example, Enlist corn is available commercially but it should not be planted in rotation with Enlist soybean to avoid the use of the same POST herbicides in the same field.