Keywords: nitrogen legislation, nitrate leaching, soil mineral nitrogen, undersowing, early sowing.
Catch crops have been an important tool for the reduction of nitrogen leaching in the Danish legislation since 1998. A great deal of experience with catch crops and alternatives to catch crops both from research, trials and practice has been gained. Catch crops are a cost effective tool to reduce nitrogen leaching if the alternative is bare soil without a crop in the autumn. If a farmer is forced to change the crop rotation from winter cereals to spring sown cereal in order to give space for catch crops, he will have a significant decrease of income, especially on pig production farms on soils with more than 10% clay.
To reduce nitrogen leaching in crop rotations with a high proportion of winter cereals the effect of catch crops of yellow mustard or fodder radish spread 2-4 weeks before expected harvest of cereals and ploughed into the soil after the 20th of September before sowing winter cereals has been tested. The effect of this system was found to be one half of that of a catch crop followed by a spring sown crop. Another possibility is to establish the winter cereal 2-4 weeks earlier than normal. Trials have shown a significant effect on the risk for nitrogen leaching, without yield loss.
Leif Knudsen, Knowledge Centre for Agriculture, Agro Food Park, 8200 Aarhus N, Denmark.
15 pages, 1 figure, 7 tables, 10 references.