Russian thistle (Salsola tragus) post-harvest control and plant dispersal

Russian thistle is one of the most important broadleaf weeds in the semi-arid US Pacific Northwest (PNW). It consumes soil water after wheat harvest, compromising the yield of the following crop. The objectives of this work were to determine the impact of post wheat harvest herbicide application timing on Russian thistle control, and stubble height on Russian thistle post-harvest control and plant dispersal. For the first objective, experiments were conducted at the Columbia Basin Agricultural Research Center (CBARC, Adams, OR) and the Lind Dryland Research Station (LDRS, Lind, WA) in 2020 and 2021. Herbicides evaluated included paraquat, glyphosate, and either bromoxynil + pyrasulfotole (CBARC) or bromoxynil + metribuzin (LDRS). The different post wheat harvest application timings were 24 h, 1, 2, and 3 wk after harvest. For the second objective, two stubble heights (short and tall) were compared for their impact on control at CBARC and in a production field near Ione, OR. Paraquat provided the greatest control in all scenarios, with no differences in application timings or stubble height. Impacts of application timings were not clear for glyphosate or bromoxynil mixtures. For glyphosate treatments, control in short stubble was 11% greater than in tall stubble in both years. Control was also greater in short stubble for the bromoxynil + pyrasulfotole in 2020. However, Russian thistle plant dispersal was greater in short stubble at both locations. At CBARC, plant dispersal in short stubble was 58% compared to 18% in tall stubble. Near Ione, plant dispersal in flattened stubble was 88% compared to 43% in non-flattened short stubble. Leaving tall stubble at harvest should be considered to reduce Russian thistle plant dispersal if the infestation is going to be left untreated after harvest. Otherwise, short stubble might result in better Russian thistle control when using systemic herbicides, such as glyphosate.