title: Influence of microbial colonization of feed particles on determination of nitrogen degradability by in situ incubation.
abstract: Stable 15N isotope was used to determine the extent of microbial colonization of in situ incubation residues and the influence of colonization on estimates of ruminal degradability of feed N. In an in vitro trial, 15N:total N ratio remained unchanged after the soluble (by water, saline, or buffer solution) fraction of plant N was removed, suggesting that 15N was uniformly distributed in the soluble and insoluble N fractions. In three in situ trials, corn grain, corn husks, and alfalfa hay labeled with 15N as an internal marker were incubated in Dacron bags in the rumen of two lactating Holstein cows for 3, 6, 12, 24, 36, and 48 h. Enrichment of 15N in samples decreased with increased time of incubation for all feeds. At 48 h of incubation, 31, 71, and 65% of the residual N was microbial N for corn grain, corn husks, and alfalfa hay, respectively. Bacterial cell DM, as a percentage of residual DM, increased in a quartic manner, suggesting that microbial colonization depended on substrate availability. For 24, 36, and 48 h incubation, bacterial DM, as a percentage of total residue, averaged 10.0 +/- 1.0% for corn grain, 17.5 +/- 1.4% for corn husks, and 22.0 +/- .2% for alfalfa hay. Apparent degradability of N after 48 h of incubation was approximately 65% for corn grain and alfalfa hay and 19% for corn husks. When corrected for bacterial N, degradabilities of N in all feeds increased and the degradabilities for corn husks became similar to those for corn grain.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Animal Sciences, University of Arizona, Tucson 85721.
authors.forename: R C
authors.initials: RC
authors.lastname: Wanderley
authors.forename: J T
authors.initials: JT
authors.lastname: Huber
authors.forename: Z
authors.initials: Z
authors.lastname: Wu
authors.forename: M
authors.initials: M
authors.lastname: Pessarakli
authors.forename: C
authors.initials: C
authors.lastname: Fontes
mesh_terms: D000821:Animal Feed / Q000382:microbiology*; D000818:Animals; D001419:Bacteria / Q000378:metabolism; D002417:Cattle / Q000378:metabolism*; D004044:Dietary Proteins / Q000008:administration & dosage / Q000378:metabolism; D005260:Female; D005285:Fermentation; D005516:Food Microbiology; D000455:Medicago sativa; D009584:Nitrogen / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism*; D003313:Zea mays
chemical_list: D004044:Dietary Proteins; D009584:Nitrogen