Protein digestion and intestinal amino acids in dairy cows fed fresh Lolium perenne with different nitrogen contents

An experiment was carried out to study digestion and intestinal AA in three ruminally and duodenally cannulated, lactating cows fed freshly cut grass (Lolium perenne) fertilized with 500 and 275 kg of N/ha per yr, respectively. High N grass was fed in June and October, and low N grass was offered in July and September. Composite samples of the grass fed in each period also were tested for in situ degradation of OM, CP, and NDF. When low N grass was fed, the digestibilities of OM and CP were lower than when high N grass was fed. On low N grass, the duodenal N flow expressed per unit of N intake was higher, although the flow of AA N on low N grass was reduced in September, mainly because of reduced microbial protein synthesis from slower OM degradation of low N grass. Duodenal N flow per unit of N intake was related negatively to the N:OM ratio of the diet. The rate of N fertilization had no effect on ruminal OM and NDF turnover rates. Turnover and passage rates in this experiment were not different from reported data on cows on winter rations with similar DMI.