Keywords: Manure, nitrogen, organic, residual effect, fertilizer, site specific management.

Summary:
Manures are an inevitable consequence of livestock in agriculture. They represent a valuable source of nutrients, including nitrogen (N), phosphorus and potassium. The industrial production of fertilizers containing these elements is associated with finite resources and this alone justifies a proper recycling of manures. The N fertilizer replacement value (NFRV) of manures is still insufficiently appreciated and exploited. Mineral fertilizer N supplementation is thus unnecessarily large, to the potential detriment of the environment. The present paper reviews methods to correctly assess the NFRV of manures and presents measures to increase the NFRV. When proper attention is given to the composition of manures and decisions on rates, timing and application methods are made accordingly, the relative NFRV of manures could rise from the present 20-50 percent to 80 percent. This could lead to a further reduction of mineral N fertilizer use, N surpluses and pollution. Under these conditions, 'manure', 'precision farming' and 'environment' should no longer be a contradiction in terms.

Jaap J Schröder, Agrosystems Research, Wageningen University and Research Center, P.O. Box 16, 6700 AA Wageningen, The Netherlands.

32 pages, 5 figures, 8 tables, 98 references.