Nitrogen partitioning into faeces, urine and milk according to the feeding strategy of dairy cows

Increasing nitrogen (N) efficiency thereby reducing N losses towards air or water and increasing N excretion in milk is a major challenge for European dairy production. Since N losses in faeces and in urine do not have the same fate at the farm level, a precise prediction of N partitioning into milk, faeces and urine at the cow level is required for plausible modelling of farm emissions and management recommendations. In that context, a database gathering more than 1700 complete individual N balances (N intake, in milk, faeces and urine) of lactating dairy cows, of which at least two thirds have a known diet composition, was constructed within the REDNEX European project (FP7, KBB-2007-1). Data originate from trials conducted over the last 40 years in France, United Kingdom and the Netherlands. A wide range of feeding practices is covered with grass hay, grass silage, fresh grass and maize silage based diets, supplemented with 0 to 70% concentrate, and ranging from 120 to 220 g CP/kg DM (5th and 95th percentiles). A wide variation is also observed in performance, with milk yields varying from 12 to 39 kg/day, BW from 480 to 720 kg, and DM intakes from 12 to 24 kg/day. N outputs vary from 80 to 300 g/day in urine, 100 to 230 g/day in faeces and 65 to 190 g/day in milk. The urinary part of N excretion, which varies from 35 to 70%, is strongly reduced with a decrease in CP content of the diet as expected. It is also reduced with an increase in N use efficiency for milk production, defined as the ratio of N in milk to N intake, which varies from 16 to 37%. Further meta-analysis on this large database will precisely characterise N partitioning according to the diet and cow characteristics.