In 1997 a crop rotation experiment was initiated at three sites in Denmark. The objective of the experiment was to explore the possibilities for both short-term and long-term increases in organic cereal production through manipulation of crop rotation design on different soil types. The experiment included three factors (proportion of grass-clover in the rotation, catch crops and manure application) in a randomised factorial design. The main indicators measured in the experiment were crop yield, nutrient leaching and weeds.
The crop rotation experiment has shown significant effects of location, crop rotation, catch crops and manure on yields, weed infestation, soil fertility and nutrient losses (Olesen et al., 2002; Schjønning et al., 2004; Askegaard et al., 2005a, b; Rasmussen et al., 2005, 2006). There were indications of changes of some of these effects over time as a result of the ‘buffering power’ of grass-clover, where the N2-fixation is affected by crop management and soil fertility. This buffering power will probably not only affect yields but also the risk of N losses, and there is a need to further investigate possibilities of controlling N supply to the crops better through improved use of green manure crops, catch crops and manure at the crop rotation level.
Perennial weeds (especially C. arvense and E. repens) have proliferated severely in the crop rotation experiment at two of the three locations during the experimental period, and these weeds may seriously reduce yields in organic arable farming. Direct control of perennial weeds is typically carried out by stubble cultivation in autumn, reducing the possibilities for growing catch crops. The energy use, as well as the elevated risk for nutrient leaching, jeopardizes the sustainability of this practice, and therefore cultivation should be avoided (Rasmussen et al., 2006). The crop rotation experiment has shown that the occurrence of these weeds is significantly affected by the choice of crops and catch crops, and by the addition of manure. However, this needs to be further substantiated, and the effects of weed infestation on yields, as well as the interaction with fertility level needs to be quantified.
Crop production in organic farming systems relies to a large extent on soil fertility for nutrient supply. The soil fertility must be maintained via choice of crop rotation and (green) manuring practices. Fertility building by such means requires a long-term integrated approach, rather than the short-term and targeted solutions common in conventional agriculture (Watson et al., 2002). It follows that, for studies of management effects on soil fertility, long-term cropping experiments are indispensable
