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ABSTRACT
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 leftin 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.
Abbreviations: BARC-NF, Beltsville Agricultural Research Center North Farm; BARC-SF, Beltsville Agricultural Research Center South Farm; DM, dry matter; EC, electrical conductivity; ITC, isothiocyanate.
In the Mid-Atlantic region of the United States, forage radish winter cover crops planted before 1 September suppress winter annual weeds from fall through early April (Lawley et al., 2011). This weed suppression may be used by farmers to provide pre-plant weed control for a subsequent crop while taking advantage of the soil quality and nutrient benefits of cover crops (Chen and Weil, 2010; Dean and Weil, 2009; Gruver et al., 2010; Weil and Kremen, 2007; White and Weil, 2011). In contrast to the highly repeatable pre-plant weed suppression observed following forage radish winter cover crops in the coastal plain of Maryland (Lawley et al., 2011), researchers report that...