| Hairy vetch (Vicia villosa Roth) and cowpea (Vigna unguiculata (L.) Walp), two leguminous cover crops, have been suspected of containing allelochemicals that allow them to suppress weeds and in some cases to affect the growth of vegetables. These studies were conducted to analyze species specific responses to the presence of residues of these two cover crops. Due to its suitability to Michigan's climate, hairy vetch served as the focus for our field study investigating the impact of hairy vetch residues on the weed community in pickling cucumber. The duration of hairy vetch phytotoxicity was examined by delaying cucumber planting dates after its incorporation. In the laboratory, the effect of hairy vetch and cowpea water, methanol, and ethyl acetate extracts on germination and radicle elongation of several vegetables and weeds was examined. Our results indicate that hairy vetch significantly reduces quackgrass populations in the field and consistently increases cucumber yield compared to no cover crop. Planting cucumbers 3 to 4 weeks after hairy vetch incorporation best maximizes yields. In the laboratory, vegetable and weed germination percentages varied in response to the extracts; however, the radicle lengths of most species tested were reduced. Occasional stimulation occurred at low extract concentrations in certain vegetable crops. Based on these findings it appears that both hairy vetch and cowpea possess allelopathic compounds which are able to help suppress weed growth when grown as a cover crop or applied as an extract. |
