Tolerance of Maize (<i>Zea mays</i> L.) and Soybean [<i>Glycine max</i> (L.) Merr.] to Late Applications of Postemergence Herbicides

Abstract

Seven maize (Zea mays L.) and three soybean [Glycine max (L.) Merr.] field experiments were conducted from 2006 to 2009 at various locations in southern Ontario, Canada to determine the tolerance of these crops to late applications of the maximum labeled herbicide dose. Single and sequential (simulating a spray overlap) applications were evaluated for visible injury, plant height, and crop yield in the absence of weed competition. Maize exhibited excellent tolerance to herbicides applied at the 9-to 10-leaf growth stage as visible injury levels for almost all tested herbicides was similar to the untreated control 7 days after treatment (DAT). However, the sequential application of dicamba/diflufenzopyr or foramsulfuron caused 6 and 8% injury 7 DAT and 8 and 14% reduction in maize height 28 DAT, respectively. The observed injury and stunting were transient as there were no differences in yield at harvest. Soybean displayed good tolerance to most herbicides applied at the 7 th trifoliate leaf growth stage as visible injury levels were similar to the untreated control. However, thifensulfuron-methyl was injurious regardless of application and imazethapyr was injurious with sequential applications. For example, single thifensulfuron-methyl, sequential thifensulfuron-methyl, and sequential imazethapyr application treatments caused 35, 48, and 25% injury 7 DAT, respectively. Sequential thifensulfuron-methyl treatments also caused a 28 and 17% reduction in soybean height 14 and 28 DAT, respectively. Visual injury continued to be detected up to 56 DAT for single thifensulfuron-methyl, sequential thifensulfuronmethyl, and sequential imazethapyr treatments. But, soybean yields were reduced by 10% for only sequential thifensulfuron-methyl application treatments. For all other herbicides tested, the yields at harvest were similar to the untreated control. This research demonstrated that maize had exceptional tolerance to all the herbicides used in this study whereas soybean was tolerant to most of the herbicides used in this study.

Comparison of Glyphosate Formulations for Weed Control and Tolerance in Maize (<i>Zea mays</i> L.) and Soybean [<i>Glycine max</i> (L.) Merr.]

“…Using similarly high doses to those in the current study, other research demonstrated that an isopropylamine salt formulation was more injurious than a potassium salt formulation [27] and was equally injurious as a trimethylsulfonium salt formulation [28]. Conversely, related research in Ontario reported no injury in soybean treated with 3600 g·ae·ha −1 of glyphosate formulated as WeatherMAX [11], contrary to the results of the current study ( Table 6). Yet, visible soybean injury was transient.…”

“…Recently, research in Ontario has explored the tolerance of glyphosate-resistant maize and soybean to glyphosate formulated as Roundup WeatherMAX ® [10] (hereafter referred to as WeatherMAX) at various growth stages [1] [11]. However, Ontario maize and soybean growers would like to be confident about the efficacy and crop safety of commercially available, low-cost glyphosate products as well.…”