Cover crops can be used to reduce leaching and erosion, introduce variability into crop rotation and fix nitrogen (N) for use by the main crops, less is however known about effects on weeds. The effects on weed seed bank, weed growth and grain yield of 4 years of annual undersown clover and ryegrass alone and in combination, and one of the 4 years with clover or clover + grass as green manure, were studied in oat and spring wheat at two experimental sites in south-eastern Norway. These treatments were compared with no undersown crop (control) and with weed harrowing. In contrast to many results in the literature, the undersown clover in this study did not suppress annual weeds, but fertilized the weeds as well as the cereals. Undersown clover resulted in a statistically significant increase of grain yield at the two sites to 116% and 121% of control. During the 4-year period relative seed bank and density of emerged weed (dominated by Spergula arvensis) increased significantly about 4.5 and 10 times respectively in the undersown clover plots at Apelsvoll. At Kise both ryegrass alone and ryegrass mixed with clover significantly suppressed the weed biomass to 70% and 74% of control respectively. It is concluded that fertilization effects of undersown clover may have dominated and overriden the competitive effects. One whole-season clover green manure did not increase the mean yield, but resulted in a significant drop in seed bank size the following year, because of limited weed establishment in an established ley. Only a slight increase in average weed biomass was observed at one of the two experimental sites. The weed seed bank and the weed biomass were essentially kept at steady state during the experimental period in harrowed plots, but harrowing decreased grain yield significantly at both sites.
