Winter annual legumes for use as cover crops in row crops in northern regions. I. Field experiments

Weed suppression is one of several benefits achieved by including a cover crop in a cropping system. A requirement for developing cover crop systems is to find species and cultivars which are adapted for the local climate and latitude, prevent weed infestation, and do not compete with the main crop. Two experimental series were established at three locations in Norway to evaluate the ability of different species for use as cover crops in vegetable production. The first series of experiments was with subclover (Trifolium subterraneum L.) cultivars sown at different dates throughout the summer and the second with different winter annual legume species sown in late summer. Winter hardiness, life cycle, growth characteristics, and weed suppression ability of winter annual legumes sown in the autumn were studied. The experiments showed the following ranking of winter hardiness: hairy vetch (Vicia villosa Roth.) (best), black medic (Medicago lupulina L.), crimson clover (Trifolium incarnatum L.), and subclover (poorest). Time of flowering and senescence, factors related to competition in cover crop systems, showed that subclover was the most promising species followed by crimson clover and hairy vetch-black medic. The winter annual legume species showed the ability for natural reestablishment in the second autumn. Hairy vetch showed the highest biomass production, the lowest regrowth ability after mowing, and the best weed suppression. Subclover showed a very promising life cycle for the use as living mulch; however, more winter hardy cultivars or species with similar growth characteristic are needed before common commercial use in northern regions. In conclusion, until more suitable species or cultivars are found or developed through plant breeding, hairy vetch is the most promising winter annual cover crop under Norwegian conditions.