Confirmation of Glyphosate-Resistant Horseweed (Conyza canadensis) in Montana Cereal Production and Response to POST Herbicides

Abstract

In recent years, horseweed has become an increasing problem in Montana. To confirm and characterize the level of glyphosate resistance, seeds were collected from putative glyphosate-resistant (GR) horseweed (GR-MT) plants in a wheat–fallow field in McCone County, MT. Known GR (GR-NE) and glyphosate-susceptible (GS-NE) horseweed accessions from Lincoln, NE, were included for comparison in dose–response and shikimate accumulation studies. Whole-plant glyphosate dose–response experiments conducted at the early- (5- to 8-cm diameter) and late- (12- to 15-cm diameter) rosette stages of horseweed indicated that GR-MT accessions had a 2.5- to 4.0-fold level of resistance to glyphosate relative to the GS-NE accession, on the basis of shoot dry weight (GR50values). The level of resistance was 3.1- to 7.9-fold on the basis of visually assessed injury estimates (I50values). At the whole-plant level, about 2.1- to 4.5-fold higher shikimate accumulation was observed in the GS-NE accession compared with the GR-MT and GR-NE accessions over a 10-d period after glyphosate was applied at 1,260 g ae ha−1. In a separate greenhouse study, all three horseweed accessions were also screened with alternate POST herbicides registered for use in wheat–fallow rotations. The majority of the tested herbicides provided ≥90% injury at the field-use rates for all three horseweed accessions 3 wk after treatment. This is the first published report on the occurrence of GR horseweed in Montana cereal production. Increased awareness and adoption of best management practices, including the use of diversified (based on multiple sites of action) herbicide programs highlighted in this study, would aid in mitigating the further spread of GR horseweed in the cereal production fields of the U.S. Great Plains.

Horseweed (Conyza canadensis) management in Oklahoma winter wheat

“…All herbicide applications that included halauxifen plus florasulam and thifensulfuron plus fluroxypyr plus dicamba were in the top-performing group of treatments for controlling horseweed that ranged from 5 to 20 cm at time of application. Similarly, in a greenhouse study conducted by Kumar et al (2017) using seed collected from various glyphosate-resistant and -susceptible horseweed populations from Nebraska and Montana, 93% control of glyphosatesusceptible plants from Nebraska was achieved with halauxifen plus florasulam applied at 5.25 g ae ha −1 and 5.25 g ai ha −1 , respectively-the same rates used in this experiment. Kumar et al (2017) also observed similar results when halauxifen plus florasulam was tested on glyphosate-resistant horseweed from Nebraska; however, efficacy decreased to 85% in glyphosate-resistant horseweed from Montana.…”

2,4‐Dichlorophenoxyacetic‐N‐methylmethanamine and haloxyfop‐P‐methyl interaction: Sequential and interval applications to effectively control sourgrass and fleabane

“…In soybean crop, the average management costs with glyphosate‐resistant sourgrass are 150% higher and situations worsen with co‐existing sourgrass and fleabane populations (Adegas et al., 2017). Glyphosate‐resistant sourgrass is primarily controlled with acetyl‐CoA carboxylase (ACCase)‐inhibiting herbicide (Correia, Acra, & Balieiro, 2015; Gilo, Mendonça, Santo, & Teodoro, 2016), whereas fleabane is controlled by 2,4‐Dichlorophenoxyacetic acid‐N‐methylmethanamine (2,4‐D) (Kumar, Jha, & Jhala, 2017; Osipe, Oliveira, Constantin, Takano, & Biffe, 2017; Santos et al., 2015). However, a little is known about the efficacy of sequential or tank‐mix application of these herbicides to control coexisting fleabane and sourgrass populations.…”

Integrating cover crops for weed management in the semiarid U.S. Great Plains: opportunities and challenges

“…Glyphosate is the most common nonselective herbicide used for its control in NT production systems across the region. Resistance to glyphosate in C. canadensis has been reported in Montana, Nebraska, Kansas, and Oklahoma (Crose et al 2019;Heap 2019;Kumar et al 2017b). In addition to glyphosate resistance, C. canadensis populations with resistance to other herbicide SOAs, including photosystem I (PSI) inhibitors, ALS inhibitors, and PSII inhibitors, have been reported (Gadamski et al 2000;Heap 2019;Mueller et al 2003;Smisek et al 1998).…”