5. Conclusions
The study demonstrates the wide variation in sward
DM production and N yield that can exist within a
single silage ®eld most probably as a result of spatial
variability in soil MIT. More importantly, it gives some
indication of the scope for improving N use eciency
and for minimising NH3 and NOx emissions to the
environment through adopting a spatially variant
approach to fertiliser-N management.
The results suggest that N-yield maps might be used
as the basis for varying N application rates across silage
®elds in order to maximise productivity and to minimise
N emissions to air and water. However, it has to be
acknowledged that if farmers were to adopt such an
approach, a more convenient means of mapping N
yields would be needed. Ideally, forage harvesters should
be equipped with sensors that would predict both the
dry weight and the N content of grass ¯owing into
collector trailers and automatically relay the information
to GPS-linked data collection and data mapping
facilities.