Estimation of microbial nitrogen in nylon-bag residues by feed 15N dilution.

Rapeseed (Brassica napus), barley grain, ryegrass (Lolium perenne) and barley straw were labelled with 15N as an internal marker. The dilution of 15N was used to estimate microbial N (RMN15N) in the feed residues in nylon bags incubated in the rumen for 5, 12 and 24 h. For comparative purposes, diaminopimelic acid (DAP) content of the residues was also determined and rumen microbial N (RMNDAP-N) in the feed residues calculated using DAP as a bacterial marker. The influence of two bag pore sizes (20 microns and 40 microns) with different sample sizes (1 g and 5 g respectively) was also studied. For all feeds, the average disappearance of 15N was faster than that of total N, the difference between N and 15N disappearance being marked with barley, ryegrass and barley straw. The disappearance of microbially corrected dry matter (DM; correction calculated from the 15N values) was, accordingly, always faster than the uncorrected DM disappearance. Except for the bag pore/sample size effect for N disappearance, significant (P less than 0.01-0.001) feed, pore/sample size and incubation-time effects were always found for the disappearance values. Errors (%) resulting from the microbial contamination (calculated from the 15N values) in N-loss measurement with rapeseed, barley, ryegrass and barley straw, at 5, 12 and 24 h in 20 microns bags were respectively: -1.8, -3.9, -0.9; -3.8, -22.4, -3.8; -7.2, -4.1, -2.9; -164.5, -146.3, -204.6. In 40 microns bags the corresponding errors were respectively: -4.4, -1.2, -0.7; -26.1, -10.5, -3.9; -13.2, -6.4, -5.5; -221.2, -310.1, -1284.6.(ABSTRACT TRUNCATED AT 250 WORDS)