Smooth scouringrush (Equisetum laevigatum) control with glyphosate in eastern Washington

Abstract

Smooth scouringrush has invaded no-till production fields across the US Pacific Northwest. Equisetum species can take up and accumulate silica on the epidermis and in cell walls that may affect herbicide uptake. The objectives of this study were to measure the silica concentration in smooth scouringrush stems over time, and to determine how time of application affects the efficacy of glyphosate for smooth scouringrush control, with and without the addition of an organosilicone surfactant (OSS). Field studies were conducted at three sites in eastern Washington from 2019-2021. Three herbicide treatments (no herbicide, glyphosate, and glyphosate + OSS) were applied at four application times (May, June, July, and August) in 2019 fallow. The silica content of smooth scouringrush stems increased over the course of the 2019 growing season at all three sites. In 2020, smooth scouringrush stem densities were reduced when the 2019 herbicide treatments were applied in late June (12% of no herbicide density) compared to late July (24%) or August (30%). Smooth scouringrush stem densities at all three sites, in both 2020 and 2021, were reduced in the glyphosate + OSS treatment compared to glyphosate alone. In 2021, two years after herbicide application, there was no effect of application timing for the glyphosate treatment without OSS, but stem densities were reduced when glyphosate + OSS was applied in late June (1%) compared with late July (26%) or late August (21%) applications. It is not clear if the cause of reduced glyphosate efficacy with late July and late August applications is the result of increased silica content in smooth scouringrush stems over time. Maximum glyphosate efficacy on smooth scouringrush was achieved with a late-June application and with the addition of an OSS. Control of smooth scouringrush with glyphosate + OSS can be sustained for at least two years after application.

Smooth scouringrush (Equisetum laevigatum) control with glyphosate is affected by surfactant choice and application time

“…This may help explain the reduced herbicide efficacy for day-applied treatments containing OS1 or OS2 observed at Reardan (Table 4). In addition, silica content in smooth scouringrush stems increases during the growing season, which could potentially affect herbicide efficacy (Lyon and Thorne 2022; Sapei 2007). Silica on a stem surface can decrease transpiration and prevent excessive water loss when plants are exposed to dry conditions and can also interfere with cuticle uptake of herbicides.…”

“…In previous studies, glyphosate has been reported to provide limited control of Equisetum species (Bernards et al 2010; Coupland and Peabody 1981; Kerbs et al 2019) at rates up to 2.1 kg ae ha –1 . However, Lyon and Thorne (2022) found that high rates of glyphosate (3.78 kg ae ha –1 ) applied alone or with an organosilicone surfactant provided effective control of smooth scouringrush 1 yr after application compared to a nontreated check. Addition of an organosilicone surfactant improved glyphosate efficacy 1 and 2 yr after treatment (YAT) compared to glyphosate applied alone.…”