Model-based estimates of combinations of cattle slurry and mineral fertilizer nitrogen in view of water quality requirements

Many factors determine which combinations of manure and mineral fertilizer are environmentally sound. We developed a simple model to handle this complexity. Simulations of dairy farming systems indicated that sound rates of manure with a typical composition, may range from 190 to 300 kg manure N per ha (including N excreted during grazing) when targeting at a N concentration in water of 11.3 mg nitrogen (N) per litre and a phosphorus (P) surplus of 0 kg per ha. Our calculations show that the rates of manure and fertilizer should be lower than the ones currently permitted unless dairy farms would succeed in changing the composition of manure. The EU standard of 170 kg manure-N per ha per year is in any case unnecessarily strict for most dairy farms if a N concentration standard of 11.3 mg per litre of water would suffice from an environmental point of view. Whatever the eventual standard, any measure contributing to a wider (available) N to P ratio of the manure will permit dairy farmers to cover a larger share of the crop N requirements via manure instead of mineral fertilizers.