1Agronomy & Range Science Department, UC Davis

Winter legume cover crops are important in four of the ten cropping systems at UC Davis's 100-year Long Term Research on Agricultural Systems experiment, now in its sixth year. Woollypod vetch produced significantly more biomass than purple vetch, bell beans, or peas in a replicated comparison in 1997. In the early years of the long-term experiment, a vetch-pea mixture grown as a green manure during the fallow year of a wheat-fallow rotation provided enough N to achieve the yield potential of the wheat in wet years with limited yield potential. Winter weed populations have been increasing in this system, however, relative to the wheat system with chemical fallow. Depending on the timing of winter rains, losses of N mineralized from a winter legume cover crop can be severe. The timing requirements of winter cover crops can limit yields and probably water-use efficiency of summer crops. It remains to be seen whether expected long-term benefits of green manures to soil quality will outweigh these problems.

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