Green manuring with clover and ryegrass catch crops undersown in small grains: Crop development and yields

In three field trials in southern Norway, spring wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) monoculture was compared with crops undersown with Italian ryegrass (Lolium multiflorum Lam.), white clover (Trifolium repens L.) and subterranean clover (T. subterraneum L.) at three levels of N fertilizer. The catch crops were ploughed in in late October, and the after-effect on succeeding spring wheat or barley crops was determined. The catch crops were more competitive in 1986, when rainfall in spring was abundant, than in 1985, when it was low. Ryegrass was stimulated by N fertilizer, whereas the clovers were repressed owing to increased shading. At ploughing, catch-crop biomass N ranged from 17 to 94, 37 to 155 and 41 to 243 kg N ha(-1) for ryegrass, white clover and subterranean clover, respectively. Grain yields (mean of N-fertilized plots), respectively, ranged from 83 to 94, 84 to 110 and 84 to 100% of yields in monoculture. The catch crops reduced weeds. The fertilizer replacement value of ploughed-in white clover was 50-60 kg N ha(-1) on loam soil without N fertilizer. Fertilizer N given either of the years decreased the grain yield response to white clover. Yet, the grain yield with white clover plus 60 kg N ha(-1) exceeded that on plots receiving 120 kg N ha(-1) as mineral fertilizer only. On sandy loam, the aftereffect was much smaller, most likely owing to N loss during wet conditions in the spring. The effect of ryegrass was neutral and that of subterranean clover intermediate.