The Mechanism for Weed Suppression by a Forage Radish Cover Crop

Little is known about the mechanism of winter annual weed suppression by forage radish (Raphanus sativus L. variety longipinnatus) winter cover crops. Previous studies suggest that allelopathy from decomposing residue and competition due to rapid canopy development contribute to weed suppression by other Brassica cover crops. Four contrasting experimental approaches were used to identify the mechanism of weed suppression by forage radish cover crops. Results of a field based cover crop residue-transfer experiment supported the hypothesis that fall cover crop weed competition is the dominant mechanism of weed suppression following forage radish cover crops. A high level of early spring weed suppression was observed where forage radish grew in the fall regardless of whether residues were left in place or removed. In contrast, there was limited weed suppression in bare soil treatments that received additions of forage radish tissues. Bioassays using cover crop amended soil or aqueous extracts of cover crop tissues and amended soil did not reveal any allelopathic activity limiting seed germination or seedling establishment. In a field-based weed seed bioassay, forage radish cover crops did not inhibit emergence of winter-planted weed seeds relative to a no cover crop control. Forage radish amended soils stimulated seedling growth of lettuce (Lactuca sativa L.) in all types of bioassays. The results of the four experiments in this study point to a common conclusion that fall weed competition is the dominant mechanism for early spring weed suppression following forage radish winter cover crops.