Horseweed and giant ragweed are competitive, annual weeds that can negatively impact crop yield. Biotypes of glyphosate-resistant (GR) giant ragweed and horseweed were first reported in 2008 and 2010 in Ontario, respectively. GR horseweed has spread throughout the southern portion of the province. The presence of GR biotypes poses new challenges for soybean producers in Canada and the United States. Halauxifen-methyl is a recently registered selective herbicide against broadleaf weeds for preplant use in corn and soybean. There is limited literature on the efficacy of halauxifen-methyl on GR horseweed and giant ragweed when combined with currently registered products in Canada. The purpose of this study was to determine the effectiveness of halauxifen-methyl applied alone and tank-mixed to control GR giant ragweed and GR horseweed in glyphosate and dicamba-resistant (GDR) soybean in southwestern Ontario. Six field experiments were conducted separately for each weed species over 2018 and 2019. Halauxifen-methyl applied alone offered 72% control of GR horseweed at 8 wk after application (WAA). Control was improved to >91% when halauxifen-methyl applied in combination with metribuzin, saflufenacil, chlorimuron-ethyl + metribuzin, and saflufenacil + metribuzin. At 8 WAA, halauxifen-methyl provided 11% control of GR giant ragweed, and 76% to 88% control when glyphosate/2,4-D choline, glyphosate/dicamba, glyphosate/2,4-D choline + halauxifen-methyl, and glyphosate/dicamba + halauxifen-methyl were used. We conclude that halauxifen-methyl applied preplant in a tank-mixture can provide effective control of GR giant ragweed and horseweed in GDR soybean.

In 2005, the existence of glyphosate-resistance in Palmer amaranth wasconfirmed at a single 250 ha field site in Macon County, Georgia. Currently,all cotton producing counties in Georgia are infested, to some degree, withglyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys wereadministered to Georgia growers and extension agents to determine how thedevelopment of glyphosate-resistance has affected weed management in cotton.According to respondents, the numbers of cotton acres that were treated withparaquat, glufosinate and residual herbicides effective against Palmeramaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosateuse declined between 2000 to 2005 and 2006 to 2010 although, on average, theactive ingredient was still applied to a majority of cotton acres. Althoughgrower herbicide input costs have more than doubled following the evolutionand spread of glyphosate resistance, chemically-based control of Palmeramaranth is still not adequate. As a consequence, Georgia cotton growershand weeded 52% of the crop at an average cost of $57 per hand-weeded ha;this represents a cost increase of at least 475% as compared to the yearsprior to resistance. In addition to increased herbicide use and handweeding, growers in Georgia are also using mechanical, in-crop cultivation(44% of acres), tillage for the incorporation of preplant herbicides (20% ofthe acres), and post-harvest deep-turning (19% of the acres every threeyears) for weed control. Current weed management systems are more diverse,complex and expensive than those employed only a decade ago, but areeffective at controlling glyphosate-resistant Palmer amaranth inglyphosate-resistant cotton. The success of these programs may be related toproducers improved knowledge about herbicide resistance, and the biologicalattributes that make Palmer amaranth so challenging, as well as theirability to implement their management programs in a timely manner.

Root colonization by soil microorganisms has been shown to increase theactivity of glyphosate in resistant and susceptible biotypes of giantragweed and a susceptible common lambsquarters biotype, but not in horseweedbiotypes. The objective of this study was to investigate the colonization ofroots in glyphosate-resistant and -susceptible giant ragweed and horseweedbiotypes, and glyphosate-tolerant and -susceptible biotypes of commonlambsquarters after a sublethal glyphosate application. The three weedspecies were grown separately in sterile and unsterile field soil andtreated with glyphosate at two sublethal rates. Soil microbes were isolatedfrom the roots onto sterile media 3 d after the glyphosate treatment. Thesusceptible biotypes of giant ragweed and horseweed grown in unsterile soilwere colonized by more soil microbes at the higher rate of glyphosate,compared to the resistant biotype grown in unsterile soil. Oomycetes wereisolated separately on a selective media and they were also more prevalentin the roots of the susceptible biotypes of each weed species grown in theunsterile soil when glyphosate was applied at the highest rate. Therefore,the ability of these three weed species to tolerate a glyphosate applicationmay involve differences in the susceptibility to soil microbialcolonization, especially oomycetes.

In previous research conducted on nonweed species, the efficacy of glyphosate was shown to be greater in unsterile soils compared to sterile soils and soil microorganisms were found to play an important role in glyphosate efficacy. Conducting greenhouse studies in microbe-free soil may therefore produce unreliable data, leading to erroneous conclusions. The objective of this study was to determine the effect of soil microorganisms on the response of glyphosate-resistant and -susceptible biotypes of three problematic weeds of the midwestern United States: giant ragweed, horseweed, and common lambsquarters. A greenhouse dose–response study was conducted on each of the three weed species grown in sterile and unsterile field soil, and the dry weight response of roots and shoots was measured. The three weed species responded differently to glyphosate when grown in the sterile and unsterile soil; that is, in the presence and absence of soil microbes. Soil microbes influenced the response of the susceptible and resistant giant ragweed biotypes and the susceptible common lambsquarters, but not the tolerant common lambsquarters or either horseweed biotype. The different responses of the three species to glyphosate in the presence and absence of soil microbes demonstrates that rhizosphere interactions are fundamental to the mode of action of glyphosate. These findings suggest that the range of tolerance to glyphosate observed in weeds and the evolution of resistance in weed biotypes may also be influenced by rhizosphere interactions. The soil media used in dose–response screenings to identify susceptible and resistant weed biotypes is very important. Unsterile field soil should be incorporated into growth media when conducting dose–response screenings to avoid false-positive results. In addition, researchers performing glyphosate dose–response assays should be aware of these findings.