Nitrogen metabolism in rumen and whole digestive tract of lactating dairy cows fed grass silage

Effects of wilting of grass prior to ensiling on OM intake, ruminal digestibility, efficiency of bacterial protein synthesis, and amount and composition of duodenal N fraction were examined using four lactating Holstein cows. Direct-cut or wilted grass silages were fed for ad libitum intake and supplemented with 5.0 or 5.3 kg of concentrate, respectively. Milk yield was 18.1 and 21.2 kg/d with direct-cut versus wilted silage, and milk protein and fat contents were 2.63 and 4.60% with direct-cut versus 2.75 and 4.27% with wilted silage, respectively. Wilting of the grass (from 17 to 38% DM) increased OM intake but had no effect on ruminal digestibility. Total N flow and amounts of different AA in proximal duodenum were increased, and the difference was principally of bacterial origin. The efficiency of bacterial protein synthesis in rumen was related to dilution rate of liquid phase and to the time devoted to ruminating. These results indicate that the passage of bacterial protein to the lower digestive tract is a key factor for improving the assimilation of the soluble N fraction of grass silage and that this phenomenon is related to chewing activity.