Long-term organic crop rotation experiments for cereal production - yield development and dynamics.

A crop rotation experiment was established in 1996/97 at three locations representing different soil types and climates. Three factors were tested: i) crop rotation with different proportions of N2-fixing crops, ii) with and without a catch crop, and iii) with and without animal manure. A green manure crop increased yields in the following cereal crops, but at the rotational level, total yields were larger in crop rotations without a green manure crop. There were positive effects of animal manure and catch crops on yield. However, except for the coarse sandy soil, the yield effects of catch crops and animal manure decreased over time when a grass-clover green manure was included in the rotation. It appeared that the buffering effect of clover can counteract the positive yield effects of manure application and catch crop use. This shows the importance of assessing long-term effects in the evaluation of crop management measures.