Operational nitrogen fertilizer management in dairy farming systems: identification of criteria and derivation of fertilizer application rates

Fertilizer‐nitrogen (N) management is a decisive factor in grass‐based, intensive dairy farming, as it strongly influences economic and environmental performance but little attention has been paid to providing guidance on N‐fertilizer management at an operational level to meet these criteria of performance. Essential criteria in operational N‐fertilizer management were identified as target dry matter (DM) yield of herbage, growth period per cut, herbage N concentration, N use efficiency (NUE), amounts of unrecovered N and marginal N response. Statistical relationships between fertilizer‐N application rates per cut and these criteria were derived from field experiments. These relationships were then used to explore the effects of the criteria on optimum fertilizer‐N applications. Optimum fertilizer‐N rates depended strongly on target levels for NUE, amounts of unrecovered N, growth period and DM yield of herbage. Calculations showed that target DM yield of herbage and growth period per cut are essential in estimating the effect of applied N on marginal N response, NUE and amounts of unrecovered N. The derived relationships can be used to explore the effects of changes in target levels of the criteria on optimum fertilizer‐N applications. The study showed that operational fertilizer‐N management set constraints to the decisions made at strategic and tactical management levels and vice versa.