RESTRICTIONS AND BALANCES IN THE USE OF NITROGEN FOR GRASS SEED PRODUCTION IN FRANCE, THE NETHERLANDS AND DENMARK; A REVIEW WITH CASE STUDIES

R. Gislum¹, F. Deneufbourg², J. Hacquet², G. Borm³, W. vanDijk³ and B. Boelt^1
1DIAS, Department of Plant Biology, DK-4200 Slagelse, Denmark
²FNAMS, Impasse du Verger, 49800 BRAIN/L’AUTHION, France
³Applied Plant Research, P.O. Box 320, 8200 AK Lelystad, The Netherlands

Restrictions on the use of nitrogen to agricultural crops in Europe were introduced in order to reduce the environmental impact of N. The positive correlation between N available for growth and grass seed yield is indisputable. Therefore, N restrictions in combination with the positive correlation between N and the seed yield necessitates the need for a better understanding of the N dynamics in grass seed crops in order to utilise the N applied in the most optimum way. Establishment of N balances to identify knowledge gaps or dominate processes are valuable. However, currently no N balances are available for grass seed production under French, Dutch or Danish growing conditions. N balances were therefore developed for different grass species and showed a surplus ranging from 29 kg ha^-1 in perennial ryegrass to 81 kg ha^-1 in orchard grass. Establishment of an N balance is, however, encumbered by a substantial error, which encompasses e.g. different national N application rates and different seed yields. Furthermore, an important factor for the N balance is the effect of removing or leaving the straw in the system. It is concluded that N application strategies should be further developed and that substantiation of the discussed N balance must be made.