Stable Isotope Labeled n-Alkanes to Assess Digesta Passage Kinetics through the Digestive Tract of Ruminants

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 (C(27)–C(36)) and passage kinetics were determined for the most abundant C(29), C(31) and C(33) 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.