title: Stable isotope labeled n-alkanes to assess digesta passage kinetics through the digestive tract of ruminants.
abstract: We describe the use of carbon stable isotope ((13)C) labeled n-alkanes as a potential internal tracer to assess passage kinetics of ingested nutrients in ruminants. Plant cuticular n-alkanes originating from intrinsically (13)C labeled ryegrass plants were pulse dosed intraruminally in four rumen-cannulated lactating dairy cows receiving four contrasting ryegrass silage treatments that differed in nitrogen fertilization level (45 or 90 kg nitrogen ha(-1)) and maturity (early or late). Passage kinetics through the gastrointestinal tract were derived from the δ(13)C (i.e. the ratio (13)C:(12)C) in apparently undigested fecal material. Isotopic enrichment was observed in a wide range of long-chain n-alkanes (C27-C36) and passage kinetics were determined for the most abundant C29, C31 and C33 n-alkanes, for which a sufficiently high response signal was detected by combustion isotope ratio mass spectrometry. Basal diet treatment and carbon chain length of n-alkanes did not affect fractional passage rates from the rumen (K 1) among individual n-alkanes (3.71-3.95%/h). Peak concentration time and transit time showed a quantitatively small, significant (p≤0.002) increase with carbon chain length. K 1 estimates were comparable to those of the (13)C labeled digestible dry matter fraction (3.38%/h; r = 0.61 to 0.71; p≤0.012). A literature review has shown that n-alkanes are not fermented by microorganisms in the rumen and affirms no preferential depletion of (13)C versus (12)C. Our results suggest that (13)C labeled n-alkanes can be used as nutrient passage tracers and support the reliability of the δ(13)C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.
authors.affiliation: Animal Nutrition Group, Wageningen University, Wageningen, The Netherlands.
authors.forename: Daniel
authors.initials: D
authors.lastname: Warner
authors.forename: Luis M M
authors.initials: LM
authors.lastname: Ferreira
authors.forename: Michel J H
authors.initials: MJ
authors.lastname: Breuer
authors.forename: Jan
authors.initials: J
authors.lastname: Dijkstra
authors.forename: Wilbert F
authors.initials: WF
authors.lastname: Pellikaan
mesh_terms: D000473:Alkanes / Q000378:metabolism*; D000818:Animals; D002247:Carbon Isotopes / Q000378:metabolism; D002417:Cattle; D005260:Female; D041981:Gastrointestinal Tract / Q000378:metabolism; D007700:Kinetics
chemical_list: D000473:Alkanes; D002247:Carbon Isotopes