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

Abstract

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 15 N dilution procedure, and the incorporation of feed 15 N into the adherent bacteria was assessed after isolating a bacterial pellet and determining its 15 N enrichment. Isotopically labelled Italian ryegrass (fertilized with 15 NH 4 15 NO 3 ) 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 48h, respectively.15 N enrichment in the bacterial pellet was highest after 6 h of incubation (3.6 mg 15 N/g N) and then declined steadily (0.8mg 15 N/gN in 48-h residues). Microbial 15 N represented up to 0.422 g/g total 15 N in the bag residue, but the incorporation of feed 15 N into bacterial N did not account for more than 57 mg/g 15 N 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 15 N had little effect on the estimation of corrected N degradability values. Therefore, some of the feed 15 N 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 15 N dilution approach, and hence on the estimation of corrected degradability values. Abbreviations:ADF, acid detergent fibre; D c , N disappearance corrected for microbial contamination; D c* , N disappearance corrected for the microbial contamination and for the amount of 15 N of microbial origin; EC, early cut grass; LC, late cut grass; NDF, neutral detergent fibre; N i , nitrogen incubated in the bag; N r , nitrogen remaining in the bag residue after incubation; N m , microbial nitrogen in the bag; N SAB, nitrogen in solid-associated bacteria; SAB, solid-associated bacteria

Comparison of three 15N methods to correct for microbial contamination when assessing in situ protein degradability of fresh forages1

“…Varvikko and Lindberg (1985) suggested that this can happen when attached microorganisms use the dietary 15 N to become labeled, resulting in underestimation of real microbial contamination. However, Kamoun et al (2007), using 15 N-labeled RG, reported that incorporation of feed 15 N into microbial cells is possible but has a negligible effect on estimation of microbial contamination of the residues and on feed N degradability. Indeed, at early incubation times, this incorporation did not account for more than 5.7% of the 15 N incubated in the bag and decreased progressively with incubation time.…”

“…A correction for this contamination should be considered when assessing N degradability values. The use of 15 N seems to be an accurate and reliable method of identifying microbial N (Sadik et al, 1990;Kamoun et al, 2007). Isotopic 15 N can be used to label either the feed proteins (Varvikko and Lindberg, 1985;Kamoun et al, 1993;Wanderley et al, 1999) or the rumen microbial N directly (Dixon and Chanchai, 2000;Reynal et al, 2005;Colombini and Broderick, 2008).…”

Effects of the ruminal comminution rate and microbial contamination of particles on accuracy of in situ estimates of ruminal degradability and intestinal digestibility of feedstuffs

“…(2008) also showed low microbial contamination for SFM. This contamination is always high in forages, especially in grasses (Bernard et al., 1988; Rodríguez and González, 2006; Kamoun et al., 2007). In contrast, disappearance of SAB from bags during intestinal incubation seems to be high and similar in concentrates and forages (97.1 and 94.2% in SFM, and 97.6 and 98.3% in RGH disappear considering either only k p or this rate together with k c respectively).…”