Estimation of microbial nitrogen in nylon-bag residues by feed¹⁵N dilution

Abstract

1. Rapeseed ( Brussicu nupus ), barley grain, ryegrass ( Loliurnperenne ) and barley straw were labelled with 15 N as an internal marker. The dilution of 15 N was used to estimate microbial N (RMN15 N ) 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 (RMN DAP-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 of 15 N was faster than that of total N, the difference between N and 15 N disappearance being marked with barley, ryegrass and barley straw. The disappearance of microbially corrected dry matter (DM; correction calculated from the 15 N 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 the 15 N 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 RMN15 N , RMN DAP-N and DAP-N (% of total N) were found in barley straw, followed by barley, ryegrass and rapeseed, in that order. RMN15 N (g/kg residual DM) followed the descending order: barley, ryegrass, straw, rapeseed. RMN DAP-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 RMN15 N values of barley, ryegrass and barley straw (expressed as % of total N or g/kg residual DM) were substantially higher than those of RMN DAP-N With 20 μm bags the RMN15 N and RMN DAP-N values were generally quite close for these feeds. With rapeseed residues, RMN15 N was clearly lower than RMN DAP-N with 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.