TRUE DIGESTIBILITY OF PROTEIN IN THE SMALL-INTESTINE AND THE HIND GUTOF COWS MEASURED WITH THE MOBILE BAG TECHNIQUE USING N-15-LABELED ROUGHAGE

Using the mobile bag technique the true small intestinal digestibiliti es of DM, NDF and N were determined in ten samples of whole plant and straw from pea and barley and ryegrass hay labelled with N-15 and harv ested at different stages of maturity. The bags were preincubated in a pepsin-HCl solution, inserted into the duodenum of dairy cows and rec overed from the terminal ileum or from the faeces. The digestibility i n 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 t ested; however differences were significant only in pea and barley str aw. The disappearance of Kjeldahl N (N(K)) Underestimated true N diges tibility (N-15) at both recovery sites. N(K) digestibility measured us ing bags recovered from the faeces generally agreed with estimates of N-15 digestibility measured using bags recovered from the ileum. It is concluded that the mobile bag technique provides a true measure of in testinal nitrogen digestion.