PURPOSES OF COVER CROPS
Cover crops help to control weeds through two mechanisms: competing for resources, and the allelopathic or biochemical effects some cover crops have on weeds. If a vigorous cover crop is grown, few weeds will have a chance to occupy the soil. The farmer is left with the comparatively simple task of killing the cover crop, which is similar to dealing with only one weed species in a field. The suppression of weeds by some biochemicals produced by cover crops is receiving increased attention. The biochemicals that suppress the weeds are not yet well characterized, so it is difficult to select cover crops or cover crop varieties for these properties. Research has shown, however, that hairy vetch, rye, wheat, oat, and certain brassica species inhibit the growth of certain weeds. In one study, rye drastically reduced a yellow nutsedge infestation. Unfortunately, this allelopathic effect can also reduce the growth of the following crop. To keep this from happening, producers should kill cover crops in a timely manner. In general, allelopathy has not been a dependable tactic for managing weeds. A well-planned weed management strategy should still be used even when using cover crops in the rotation, and any impact of the cover crop on weed management should be considered a bonus.