Efficiency of feed nitrogen conversion in dairy cattle herds

To evaluate the effect of feasible strategies to mitigate dairy herd's environmental impact in a homogeneous area, the actual level of N excretion and conversion efficiency was determined in fifteen farms located in the North-West of Italy. Main factors affecting N efficiency (feeds consumption and composition, live weight, productions, milk urea and reproductive indexes) were recorded for cows (C), heifers (H, 12-24 months) and young heifers (YH, 6-12 months). N requirements, retention, excretion and efficiency were calculated. N excretion was respectively 131.39, 60.88 and 47.49 kg/year/head for C, H and YH and 14.07 kg/year per litre of milk in C. Feed N conversion efficiency was respectively 28.40, 8.90 and 6.64 in C, H and YH. In the studied farms N efficiency could be increased acting on the most important factors affecting N utilisation, so reducing the environmental impact of dairy cattle.