Wash-off potential from living and aged cover crop residues differs among soil-residual herbicide sorption properties

Delaying cover crop termination until planting (i.e., planting green) in no-till production systems is likely to mediate the fate of herbicides that provide soil-residual activity. There is currently limited knowledge of how the interaction between physiochemical properties of cover crop residues and sorption properties of herbicides influence the wash-off potential of residual herbicides from cover crop residues in a planting green scenario. We conducted field- and laboratory-based experiments using herbicide wash-off assay methods to evaluate the interaction between lignin (%) of cereal rye (Secale cereale L.) and herbicide lipophilicity (K ow) on wash-off potential across herbicide application timings. We contrasted herbicides with intermediate lipophilicity (atrazine, pyroxasulfone, and S-metolachlor) to less lipophilic (mesotrione) and highly lipophilic (pendimethalin) herbicides. Wash-off was greater for atrazine and pyroxasulfone than for mesotrione and S-metolachlor when applied into living cereal rye. Pendimethalin had the least wash-off potential. When pendimethalin was applied into fresh to aged cereal rye residues (0 to 84 d after termination), wash-off was below the detection threshold. Wash-off of mesotrione, pyroxasulfone, and atrazine declined as lignin (%) in cereal rye residues increased, whereas a positive relationship between S-metolachlor recovery and lignin (%) was observed. Results of our study partially support the hypotheses that (1) herbicide lipophilicity, measured via log K ow values, can be a useful indicator of wash-off potential among residual herbicides used in cover crop systems; and (2) wash-off potential declines as cover crop residues age within herbicide application windows.