Herbicide-Resistant Waterhemp (Amaranthus tuberculatus) in Minnesota: Profile, Distribution, and Management in Soybean-Based Crop Rotations

Abstract

Waterhemp (Amaranthus tuberculatus) is the most troublesome weed species in agronomic crops in Minnesota and other Midwestern United States. A system approach that integrates cultural, mechanical, and herbicide-based tactics along with the adoption of recently introduced multiple herbicide-resistant soybean can improve waterhemp control. The goal of this thesis was to facilitate waterhemp management by: 1) evalauting the extent and distribution of multiple herbicide-resistant waterhemp in Minnesota; 2) evaluating a multi-tactic approach for waterhemp control in soybean-sugar beet rotation; and 3) evaluating herbicide application timing and sequence for waterhemp control in Enlist E3® soybean (resistant to 2,4-D choline, glyphosate, and glufosinate). From whole plant bioassays conducted in greenhouse with 90 waterhemp accessions, 10 and 89% accessions were either moderate- or highly-resistant to 2,4-D and glyphosate, respectively. Two accessions were observed to be moderately resistant to dicamba. Waterhemp accessions resistant to 2,4-D and dicamba were from the southwestern part of the state. Two accessions from one from Lincoln County (accession A49) and other one from Lyon County (A50) were resistant to 2,4-D, atrazine, dicamba, fomesafen, glyphosate, imazamox, and mesotrione, leaving no effective herbicide site of action (SOAs) except for the glutamine synthetase inhibitor (glufosinate) for POST control. In a multi-site-year field experiments, for 90% green canopy cover (G90) narrow soybean row spacing (30- to 38-cm) required 158- to 400- less cumulative growing degree days (GDD) units than that for wide-row spacing (56-cm). Soybean row spacing did not impact in-season waterhemp control, density, biomass, and seed production. In soybean, the high-input herbicide program with flumioxazin applied as PRE followed by (fb) lactofen plus acetochlor as Early-POST (EPOST) fb Late-POST (LPOST) application of 2,4-D choline plus glyphosate resulted in ≥ 95% waterhemp control across all the sites and reduced waterhemp seed production to 0 seeds per m-2. The narrow row spacing in soybean resulted in 560 and 399 kg ha-1 more yield than that at wide row spacing at Franklin 2021 and Rosemount 2022 sites. In field experiments evaluating herbicide application timing and sequence, herbicide program with PRE followed by (by) Early-POST (EPOST) glufosinate application fb Mid-POST (MPOST) application of 2,4-D choline plus glyphosate without or without S-metolachlor provided ≥ 96% waterhemp control, highest soybean yield (2,930 kg ha-1 in Rosemount and 3,710 kg ha-1 in Franklin) associated economic returns (1,670 US$ ha-1 in Rosemount and 2,170 US$ ha-1 in Franklin). This thesis emphasizes the importance of diversified management tactics integrating non-chemical control measures with the effective herbicide use for waterhemp control.