Stable isotope labeled n-alkanes to assess digesta passage kinetics through the digestive tract of ruminants

We describe the use of carbon stable isotope (13C) labeled n-alkanes as a potential internal tracer to assess passage kinetics of ingested nutrients in ruminants. Plant cuticular n-alkanes originating from intrinsically 13C 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 d13C (i.e. the ratio 13C:12C) 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 (K1) 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. K1 estimates were comparable to those of the 13C 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 13C versus 12C. Our results suggest that 13C labeled n-alkanes can be used as nutrient passage tracers and support the reliability of the d13C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.