Winter cereal cover crop termination timing to optimize trade-off between weed suppression and impact on sugar beet yield

Field experiments were conducted in 2021 and 2022 to evaluate cereal cover crops and herbicide programs for weed control in sugar beet (Beta vulgaris). The experiment was set up in a split-split-plot randomized complete block design with four replications. The main plot was two fall-planted cereal cover crops (cereal rye and winter wheat) and no cover crop; the split-plot factor was five cover crop termination timings (14 and 7 days before planting, day of planting, and 14 and 7 days after planting), and the split-split-plot factor was three herbicide programs (dimethenamid-p + glyphosate at four leaf sugar beet; dimethenamid-p + glyphosate at four leaf sugar beet followed by glyphosate at 10 leaf stage; and glyphosate at four leaf sugar beet followed by glyphosate at 10 leaf stage). Cereal rye produced more biomass and thus provided up to 50% better weed suppression than winter wheat. Terminating cover crops after sugar beet was planted reduced common lambsquarters and kochia density by 28%–90% at the time of the first postemergence herbicide application. One-time application of dimethenamid-p + glyphosate had similar weed control as glyphosate applied twice. Delaying cover crop termination until 7 or 14 days after planting reduced sugar beet stand density and reduced root yield by 8%–28%. Terminating cereal cover crops with herbicides at planting or up to 7 days prior provided optimal sugar beet yield in this study.