1. Introduction
Ineciencie in N usage for grass-based agricultural
production have resulted in signi®cant emissions of
NH3, N2O, NO
ÿ
3 and NOx (NO and NO2) to air and
water (Buijsman et al., 1987; Jarvis, 1997). Part of the
problem may be attributed to the use of ÔuniformÕ applications
of fertilisers across whole ®elds even though
parts of the ®elds may already be well supplied with
mineral or organic-N and, therefore, are incapable of
utilising additional N inputs for grass production. Unfortunately,
owing to the lack of suitable automated
yield-monitoring equipment for non-combinable crops,