No abstract

excreted by ruminants is of endogenous origin (metabolic losses) and some corresponds to dietary nitrogen (undigested or unused nitrogen). These losses of dietary nitrogen could be reduced by increasing the efficiency of dietary nitrogen use. However, INTRODUCTIONImprovements in the management of effluents from animal production systems have been recommended to limit environmental damage. Some of the nitrogen Abstract -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 . 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 . kg -1 DM). The milk protein content differed between the two most extreme diets (28.9 vs. 29.9 g . 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 . 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 . 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.nitrogen balance / dairy cow / utilisation / milk / urine Reprod. Nutr. Dev. 42 (2002) 545-557 545

Wheat and potato are rich in starch but their starches differ in their rate of ruminal degradation. Kinetics of in sacco disappearance and profiles of ruminal fermentation were studied for these two concentrates in total mixed rations based on grass silage or corn silage. Wheat starch was more rapidly (34%/h) degraded by rumen microorganisms than potato starch (5%/h). The differences in starch degradation in sacco were found again in the VFA concentrations, mainly in grass silage-based diets. Overall ruminal pH, total VFA concentration, and proportions of acetate, propionate, and butyrate are more variable for wheat during the kinetic (amplitude and quickness) than for potato in grass silage-based diets. In these diets, risks of acidosis were more elevated with wheat than with potato but the VFA concentrations were also higher. These differences of fermentation profile were so reduced in corn silage-based diets that, in this case, wheat can be substituted by potato without any effect on digestion and no risk of acidosis.