True Digestibility of Protein in the Small Intestine and the Hind Gut of Cows Measured with the Mobile Bag Technique using 15 N-Labelled Roughage

Using the mobile bag technique the true small intestinal digestibilities of DM, NDF and N were determined in ten samples of whole plant and straw from pea and barley and ryegrass hay labelled with 15N and harvested at different stages of maturity. The bags were preincubated in a pepsin-HCI solution, unserted into the duodenum of dairy cows and recovered from the terminal ileum or from the faeces. The digestibility in the lower gut was also estimated. The N disappearance was higher in bags recovered from faeces than from the ileum for most of the feeds tested; however differences were significant only in pea and barley straw. The disappearance of Kjeldahl N (NK) underestimated true N digestibility (15N) at both recovery sites. NK digestibility measured using bags recovered from the faeces generally agreed with estimates of 15N digestibility measured using bags recovered from the ileum. It is concluded that the mobile bag technique provides a true measure of intestinal nitrogen digestion.