Chlorsulfuron + metsulfuron applied in fallow is effective for long-term control of smooth scouringrush ( <i>Equisetum laevigatum</i> ) in wheat

Abstract Smooth scouringrush is a deep-rooted, rhizomatous, herbaceous perennial that has been difficult to control in dryland wheat-producing regions in the Pacific Northwest. Long-term field experiments were conducted at Omak, Edwall, and Steptoe, Washington, to evaluate applications of chlorsulfuron + metsulfuron (21.9 + 4.4 g ai ha −1 ), triasulfuron (29.5 g ai ha −1 ), or MCPA ester (780 or 1,122 g ae ha −1 ). The Omak trial (2017–2021) included two 2-yr cycles of a winter wheat/chemical summer fallow rotation. The Edwall and Steptoe trials (2019–2025) included two 3-yr cycles of winter wheat/spring wheat/chemical summer fallow rotation. Six preassigned herbicide sequences tested the efficacy of 1) chlorsulfuron + metsulfuron applied during one or both chemical fallow phases and 2) triasulfuron alone applied to wheat. MCPA ester was applied as a check treatment. Smooth scouringrush stem density was evaluated in each plot each year. Chlorsulfuron + metsulfuron applied in the first chemical summer fallow phase maintained relatively low stem density in all three trials through the first rotation cycle compared with the check treatment. Chlorsulfuron + metsulfuron applied in the second chemical summer fallow phase extended control through the second rotation cycle. However, in the 3-yr rotation trials, chlorsulfuron + metsulfuron was not as effective in the second rotation cycle at Steptoe where stem densities averaged only 6 stems m −2 at time of the second application. Triasulfuron applied to wheat was not effective in controlling smooth scouringrush. Smooth scouringrush reduced wheat yield in the MCPA ester check sequence at Edwall, but no differences were observed at Steptoe or Omak. Chlorsulfuron + metsulfuron applied in chemical fallow is effective for long-term control of smooth scouringrush in wheat-based cropping systems. A subsequent application in chemical fallow can be effective if an adequate number of stems are present to facilitate foliar uptake.