A SENSITIVITY ANALYSIS OF NITROGEN LOSSES FROM DAIRY FARMS

International attention has focused on agricultural production systems as non-point sources of pollution affecting the quality of streams, e stuaries and ground water resources. The objective of the current stud y was to develop a model of nitrogen management on the dairy farm, and to perform sensitivity analyses in order to determine the relative im portance of manipulating herd nutrition, manure management and crop se lection in reducing nitrogen (N) losses from the farm. The importance of the method of N input to the farm (purchased feed, legume fixation, inorganic fertilizer, imported manure) was investigated, and the pote ntial to reduce N losses from dairy farms was evaluated. Nitrogen bala nce equations were derived, and related efficiency coefficients were s et to reference values representing common management practices. Total farm N efficiency (animal product N per N input), and N losses per pr oduct N were determined for different situations by solving the set of simultaneous equations. Improvements in animal diet and management th at increase the conversion of feed N to animal product by 50% would in crease total farm N efficiency by 48% and reduce N losses per product by 36 to 40%. In contrast, reducing losses from manure collection, sto rage and application to improve the percentage of manure N that become s available in soil by 100% would only improve total farm N efficiency by 13% and reduce total N losses by 14%. Selecting crops and manageme nt that can use soil nutrients 50% more efficiently would improve tota l farm efficiency by up to 59% and reduce N losses by up to 41% depend ing on the predominant nitrogen sources to the farm. Legume production would reduce N losses per product compared with non-legumes. There wa s more than a five fold difference in N losses per animal product N be tween the most extreme scenarios suggesting considerable opportunity t o reduce N losses from dairy farms. (C) 1997 Academic Press Limited.