1. Rapeseed (Brussicu nupus), barley grain, ryegrass (Loliurnperenne) and barley straw were labelled with15N as an internal marker. The dilution of15N 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 μm and 40 μm) with different sample sizes (1 g and 5 g respectively) was also studied.2. For all feeds, the average disappearance of15N was faster than that of total N, the difference between N and15N disappearance being marked with barley, ryegrass and barley straw. The disappearance of microbially corrected dry matter (DM; correction calculated from the15N values) was, accordingly, always faster than the uncorrected DM disappearance. Except for the bag pore/sample size effect for N disappearance, significant (P < 0.01–0.001) feed, pore/sample size and incubation-time effects were always found for the disappearance values.3. Errors (%) resulting from the microbial contamination (calculated from the15N values) in N-loss measurement with rapeseed, barley, ryegrass and barley straw, at 5, 12 and 24 h in 20 μm 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 μm 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, –1284.6.4. The largest residual proportions of RMN15N, RMNDAP-Nand DAP-N (% of total N) were found in barley straw, followed by barley, ryegrass and rapeseed, in that order. RMN15N(g/kg residual DM) followed the descending order: barley, ryegrass, straw, rapeseed. RMNDAP-N, (g/kg residual DM) and DAP (mg/kg residual DM) followed the descending order: barley, ryegrass, rapeseed and barley straw. Feed, pore/sample size and incubation-time effects were always significant (P < 0.001).5. With 40 μm bags RMN15Nvalues of barley, ryegrass and barley straw (expressed as % of total N or g/kg residual DM) were substantially higher than those of RMNDAP-NWith 20 μm bags the RMN15Nand RMNDAP-Nvalues were generally quite close for these feeds. With rapeseed residues, RMN15Nwas clearly lower than RMNDAP-Nwith 20 μm bags, but only small differences were found with 40 μm bags.6. A more diverse microbial colonization of feed samples was indicated in the 40 μm bags compared with the 20 μm bags. It was also noticed, consequently, that in most cases larger error resulted from the microbial contamination in 40 μm bags.
