Perennial crops undersown in cereals provide ground cover from harvest of the cereal crop to sowing of the next crop. Such cover crops can e.g. reduce soil erosion, nutrient leaching and N fertiliser requirements of the succeeding crop. The objective of this thesis was to develop guidelines on how to prevent grain yield losses due to competition from the perennial crops or to increase the yield of the main crops. The effects of species and time of undersowing of perennial crops in spring cereals, and the management of an intercropping system in which winter oilseed rape or consecutive crops of winter wheat were established in a remaining crop of white clover, were studied. The biomass of undersown cover crops by the time of the harvest of spring barley was significantly reduced with each delay in the undersowing, but the increase in biomass during autumn was generally not affected. Italian ryegrass (Lolium multiflorum Lam) and perennial ryegrass (Lolium perenne L.) reduced the grain yield by 6 and 1%, respectively, but may both be suitable as cover crops with the appropriate main crop, time of undersowing and seed rate. Undersown white clover (Trifolium repens L.)/perennial ryegrass mixtures kept soil mineral N as low as pure ryegrass and improved the residual effect. This suggests that clover in the cover crops may reduce the N fertiliser requirements of the succeeding crop without increased N leaching. Grain yields were smaller in the wheat/clover system than with wheat alone when the wheat was direct drilled and larger when sown after stubble cultivation. Direct drilled wheat and rape yielded more with clover varieties less adapted to the cold climate than with varieties in common use in the area. Grain yield increased, weeds were efficiently controlled and the white clover crop maintained by applying herbicides that mainly act as germination inhibitors when a third consecutive wheat crop had 1–2 leaves. It was possible to conclude that (1) the tillage performed in conjunction with the sowing of wheat, (2) the weed control practice, and (3) the choice of clover variety, have large effects on the yield of the winter annuals in the intercropping system
