Feed nitrogen conversion in lactating dairy cows on pasture as affected by concentrate supplementation

The effect of concentrate supplementation on nitrogen (N) intake and excretion in grazing lactating dairy cows was determined in three herds in Norway. Grazing trials were conducted with each herd in June and August for two consecutive years. The average supplementation was 1.8 (S.D. 2.1) kg DM/day, and the concentrate was based on grain with a N content ranging from 18.7 to 24 g/kg DM. Herbage DM and N intake were reduced with increasing supplementation, but total DM and N intake increased. Milk yield and protein content increased by 1.1 kg milk and 0.28 g protein per kg milk for each kg extra concentrate. Milk N excretion increased with increasing supplementation (6.5 g N/kg DM), and N utilisation improved by 11.7 g N per kg N intake per kg extra concentrate. Excretion of urine N and its share of total excreta N decreased by 4.0 and 9.2 g/kg concentrate, respectively. The reducing effect on urine N excretion of supplementation was, however, numerically low compared with other studies, most likely due to a high crude protein content of the concentrate used.