Effect of feed 15N incorporation into solid-associated bacteria on the in situ nitrogen degradability of 15N labelled Italian ryegrass

The objective of this study was to examine the effect of microbial contamination of the incubation residue on the in situ nitrogen degradability of Italian ryegrass. Microbial nitrogen in the bag residues was estimated by the feed 15N dilution procedure, and the incorporation of feed 15N into the adherent bacteria was assessed after isolating a bacterial pellet and determining its 15N enrichment. Isotopically labelled Italian ryegrass (fertilized with 15NH415NO3) was harvested either in spring (early cut) or in summer (late cut). Forages were incubated in the rumen of two steers for 6, 24 and 48 h. The proportion of microbial N per unit of total N in the bag residue increased with incubation time, ranging from 184 to 853 mg microbial N/g total N at 6 and 48 h, respectively. 15N enrichment in the bacterial pellet was highest after 6 h of incubation (3.6 mg 15N/g N) and then declined steadily (0.8 mg 15N/g N in 48-h residues). Microbial 15N represented up to 0.422 g/g total 15N in the bag residue, but the incorporation of feed 15N into bacterial N did not account for more than 57 mg/g 15N incubated, this incorporation rate decreasing progressively with incubation time. Correction of apparent N disappearance for microbial N resulted in higher values of N degradability, especially for the late cut grass. A further correction considering the amount of microbial 15N had little effect on the estimation of corrected N degradability values. Therefore, some of the feed 15N is incorporated into bacterial N, but this fraction has a minor effect on the estimation of total microbial N in the bag residue by the feed 15N dilution approach, and hence on the estimation of corrected degradability values.