Assessment and Improvement of the Efficiency of Nitrogen Use in Clover Based and Fertilizer Based Dairy Systems. 2. Beyond Benchmarking to Internal System Components

Abstract

ABSTRACT In order to understand better how the losses of N from farm systems may be controlled and reduced, it is necessary not only to consider farm gate budgets, which provide a useful benchmark for the overall potential environmental impact of a system. Internal flows of nitrogen were quantified for two dairy systems; GC relying on N fixation by white clover and GN using purchased mineral N fertilizer, both before and after modifications to improve the nitrogen efficiency (N in milk/N in inputs) of the whole systems. The greatest difference in N balance between GC and GN systems was found in the soil/crop component. This component showed a greater imbalance between inputs and outputs for GN (58%) than for GC (originally 82%, improved to 88%), indicating more potential for losses from the GN system to the environment. The animal was shown to be the least efficient component of all systems, with an initial efficiency of 20%, improving to 23% (GC) or 24% (GN) following modifications. The efficiency of nitrogen use for milk production is greater when animals are housed than during grazing, since diets can be more carefully controlled. The limitations of assessing the environmental impact of systems using parameters based partly on estimates must be recognized. Tolerance limits must be established if such values are used in advice or legislation.