Rumen Degradability and Post-ruminal Digestion of Nitrogen and Amino acids by Cows Grazing Temperate Pasture

This experiment was conducted to evaluate nutrient digestion in the rumen and flow to the duodenum of steers grazed on orchardgrass (Dactylis glomerata L., OG) or meadowfescue (Festuca pratensis Huds., MF.) pastures located on northern part of Japan without supplement. Fifty-two digestion trials were carried out grazing Holstein steers fitted with cannulas in the rumen, duodenum and distal ileum in the three years. The pastures were divided into several paddocks, and the steers grazed each paddock for a day and allowed double of dry requirements the grazing cattle. The pre-grazing herbage mass did not significantly differ between pastures but the crude protein (N×6.25) content of herbage ranged from 19.3% to 27.5% on OG pasture and from 20.0% to 32.8% on MF pasture. Total N and AA-N intake did not differ but the degradability of dietary nitrogen in the rumen (RDN) was higher in MF than in OG pasture. The ratio of RDN to OM truly digested in the rumen was negatively related to the apparent N absorption in the rumen, and it was suggested that the amount of apparent N absorption in the rumen would be positive when the ratio was above 25g/kg. Non-ammonia nitrogen (NAN) and AA-N flows to the duodenum of steers were lower in MF than in OG pasture. Duodenal AA flows were more linked to duodenal NAN flows and accounted for approximately 60% of duodenal NAN flows. The proportion of methionine and lysine slightly increased in the duodenal flows compared with the consumed AA, but a comparison between the essential AA composition of milk and the lean tissue indicated that duodenal digesta was most limiting in methionine, lysine, arginine, and histidine, and that deficiencies of arginine and histidine for milk production were relatively small.