Utilization of nitrogen (N) and phosphorus (P) in an organic dairy farming system in Norway

Inputs of N and P, flows through the soil–plant–animal pathway and removals by products were recorded for 3 years at the organically managed prototype dairy farm ‘Frydenhaug’ in Norway to assess the transfer efficiencies of N and P within and at the farm level. Nutrient balances and efficiency (N or P in products divided by N or P in inputs) were compared to data from other studies of dairy farm systems in Europe. Plant production on the farm covered nearly all the needs by the herd. However, about 10% of the plant production was sold as cash crop and about the same amount was bought as feed. At the farm level, ‘Frydenhaug’ realized annually and on average lower surpluses and higher N or P efficiencies than found in most studies concerned. On average, N and P surpluses were 41 and 0.6 kg ha−1 per year, the efficiencies were 0.30 and 0.85, and the surplus per animal produce was 2.4 and 0.2 kg kg−1, respectively. Despite relative high nutrient efficiencies at the farm level, there were considerable losses within the farm system. Nutrients were lost during harvesting, storage and feeding of home-grown crops. Thus, the intake of N and P by the herd was on average 62 and 59% of the harvestable N and P in field crops. The average apparent efficiency in the soil/plant component was 0.89 for N and 1.66 for P, and in the animal component 0.19 for N and 0.18 for P. The negative soil surface P balance (on average, 6.3 kg ha−1 per year) was not regarded as a problem on short-term, but it may limit the productivity of the system on the long-term. Improved forage quality through more frequent cuttings and a moderate concentrate level increased milk production and improved the N efficiency at the farm level without a negative effect on the N utilization in the animal component. This study illustrates the importance of including the internal nutrient flow in order to assess and improve the nutrient utilization in organic dairy farming.