Nitrogen utilisation by dairy cows fed diets differing in crude protein level with a deficit in ruminal fermentable nitrogen

We studied the efficiency of nitrogen utilisation by dairy cows, using three diets differing in the crude protein (CP) level but with similar deficits (10 g$\cdot$kg$^{-1}$ dry matter, DM) in ruminal fermentable nitrogen. There was no difference in milk yield from the cows offered the three diets (130, 145 and 160 g CP$\cdot$kg$^{-1}$ DM). The milk protein content differed between the two most extreme diets (28.9 vs. 29.9 g$\cdot$kg$^{-1}$, $P < 0.05$), resulting in higher protein yields for the highest CP treatment ($P < 0.01$). The efficiency of nitrogen utilisation, calculated as the proportion of ingested nitrogen recovered in the milk, was significantly higher for the 130 g CP$\cdot$kg$^{-1}$ DM diet than for the other two diets (0.37 vs. 0.33 and 0.32 respectively, $P < 0.01$). The different diets also resulted in different levels of nitrogen excretion into the environment (237, 270 and 330 g N$\cdot$d$^{-1}$, $P < 0.01$). Hepatic deamination of the amino acids may have generated additional energy to enable the animal to make use of the additional nitrogen in the diet, resulting in an increase in plasma urea concentration