In 1991, the European Union introduced the Nitrates Directive which aims to prevent the pollution of groundwater and surface water by nitrates arising from agricultural sources. One mandatory measure stipulated by the Directive involves a limit, set at 170 kg/hectare organic N (manure N), on the amount of livestock manure which may be applied to land each year. This limit will have very significant implications for stocking rates on livestock farms. The objective of the present study was to use total diet digestibility data obtained at this Institute to develop prediction equations for manure N output in lactating dairy cows.