[Effect of level and nature of degradable nitrogen on rumen digestion with a maize based silage diet in lactating cows].

Four diets differing in level and source of nitrogen were given in a 4 x 4 latin square design using four dairy cows canulated in the rumen and duodenum. A low N diet (T-, 17.4 g N/kg DM) with only 17 g of degradable N per kg of fermentable OM (FOM) (covering 73% of microbial requirements) was supplemented with up to 21.6 g/kg DM and 22 g/kg FOM (covering 95% of microbial requirements) of increasing levels of preformed amino acids. Three mixtures of urea and casein [100:0 (U); 50:50 (UC) and 0:100 (C)] were used in order to supply 30, 15 and 0% of the total N of the diet as urea-N. The four diets (66% maize silage) were isoenergetic and were offered 95% of ad libitum twice daily. OM and NDF digestibilities in the total digestive tract and in the rumen did not differ between T- and U diets. These digestibilities increased linearly with the proportion of casein-N in the diet but the response remained low. The levels and the source of degradable N did not affect the non ammonia nitrogen (NNA) flow into the duodenum, the microbial N flow and the efficiency of microbial synthesis. NNA flow exceeded N intake for all diets but the net recycling into the rumen was markedly higher for the LN diet (+110 vs +31 g/day). This is related to a lower excretion of urea-N in urine (22 vs 80 g/day). These results suggest that in cows fed at 95% of ad libitum, the shortage of degradable N hardly affects the digestion of the diet and that the supply of preformed amino acids is not an important limiting factor for bacterial growth when maïze silage based diets are fed. Therefore, the supplementation with such preformed carbon chains is not of practical interest.