Intestinal digestibility of rumen undegraded N of concentrates measured by the mobile nylon bag technique

Abstract

Two experiments were carried out on sheep to measure the effects of some factors of the mobile nylon bag (MNB) technique on the intestinal digestibility (DgI) of rumen undegraded N of meat and bone meal (MBM), soybean meal (SBM) and wheat bran (WB). The pore size of the nylon cloth (10 and 42.5 μm) influenced ( P < 0.05) DgI of SBM and MBM N irrespective of the site of recovery of the bags (ileum and faeces) and the incubation time of concentrate feeds in the rumen (0, 2, 4, 8, 16 and 24 h). The faecal recovery of the bags increased ( P < 0.05) DgI of SBM N in comparison with ileal recovery. The opposite was found with WB incubated 4, 8, 16 and 24 h in the rumen. DgI of MBM N was not influenced by site of bag recovery ( P > 0.05). Increasing incubation times in the rumen decreased ( P < 0.05) DgI of undegraded feed N. The total tract digestibility (rumen plus intestines) was independent of incubation time in the rumen for MBM and SBM, excepted for SBM incubated 4 or 8 h in the rumen when bags were collected in the ileum. Previous ruminal incubation of WB for 4, 8, 16 and 24 h increased ( P < 0.05) the amount of N digested in the total tract in comparison with unincubated WB and WB incubated for only 2 h. In the second experiment, residues obtained from the incubation of the feeds in the rumen (0, 2, 4, 8, 16, 24 and 48 h) were mixed in different proportions in order to obtain the true fraction of the undegraded feed in the rumen. This method was based on the DM degradability of the concentrate feeds and the outflow rate constant of feeds from the rumen, supposed equal to 6% h −1. Bags containing feeds mixed according to this method were incubated in a solution of pepsin-HCl for 0, 1, 2 and 3 h to simulate abomasal digestion and the DgI of N was measured afterwards with the MNB technique. Increasing action of pepsin led to higher digestibilities of N at this stage ( P < 0.05) but decreased the digestibility of N in the intestines excepted for MBM incubated in pepsin-HCl for 3 h and in any case for SBM. However, the omission of the incubation in pepsin had little influence on the amount of N postruminally digested.