Influence of cover crop use on soil microbial activity and fate of sulfentrazone, S-metolachlor, cloransulam-methyl, atrazine, and mesotrione

Residual herbicides are primarily degraded in the soil through microbial breakdown. Any practices that result in increased soil biological activity, such as cover cropping (between cash crop seasons), could lead to a reduced persistence of herbicides in the soil. Furthermore, cover crops can also interfere with herbicide fate by interception. Field trials were conducted between 2020 and 2023 in a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation to investigate the influence of cover crop (cereal rye [Secale cereale L.] and crimson clover [Trifolium incarnatum L.]) use on soil enzyme activities (beta-glucosidase [BG] and dehydrogenase [DHA]), its effect on the concentration of residual herbicides (sulfentrazone, S-metolachlor, cloransulam-methyl, atrazine, and mesotrione) in the soil, and the interception of herbicides by cover crop residue. The use of cover crops occasionally resulted in increased BG and DHA activities relative to the fallow treatment. However, even when there was an increase in the activity of these two enzymes, increased degradation of the residual herbicides was not observed. The initial concentrations of all residual herbicides in the soil were significantly reduced due to interception by cereal rye biomass. Nevertheless, significant reductions in early-season weed biomass were observed when residual herbicides were included in the tank mixture applied at cover crop termination relative to the application of glyphosate plus glufosinate. Results from this research suggest that the use of cereal rye or crimson clover as cover crops (between cash crop seasons) do not impact the persistence of residual herbicides in the soil or reduce their efficacy in controlling weeds early in the growing season.