INTRODUCTION
The world is currently experiencing rapid rates of
urbanisation, industrialisation and agricultural development.
These activities are contributing signi®cantly
to the pollution of the environment in the form of
accidental chemical discharges, exhaust fumes, wastes,
sewage, by-products of manufacturing processes and
other human activities. Intensive animal production is
believed to be a major contributor to the pollution of
the environment. For example, in pigs, about 65% of
the ingested dietary nitrogen is excreted via urine and
faeces,1 and this general low ef®ciency of N utilisation
makes intensive animal production a main contributor
to N pollution of the environment within the
agricultural system.2
Within the European Economic Community
(EEC), for example, farm animal ef¯uents have been
identi®ed as a major source of nitrate pollution of
water systems. These environmental concerns about
ground water contamination with excess nitrate,
phosphorus as well as other elements resulted in the
issuance of Council Directive 91/670 EEC in 1991 in
an attempt to limit the amount of N in slurry that can
be spread on farm land.3 One of the current
approaches to reducing the risk of N pollution of the
environment is the reduction of N excretion in poultry
in carcass protein percentages. Lower
values were found in both experiments in
the R x AA and V x V crosses than in the IR x
P and P x P crosses.