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 (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 δ13C (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 (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 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 δ13C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.

Stable isotope‐labelled feed nutrients to assess nutrient‐specific feed passage kinetics in ruminants

“…Because these isotopes are present in low quantities in feed, the isotopic ratio needs to be increased in favour of the stable ( 13 C) or radioactive ( 14 C) carbon isotope relative to the main carbon isotope ( 12 C) to distinguish the marker from the bulk material. Various procedures were described for ruminant nutrition studies and include either switching from a naturally low‐C‐enriched diet based on C 3 plant species (e.g. ryegrass, lucerne, wheat) to a naturally higher‐ 13 C‐enriched diet based on C 4 plant species (e.g.…”

“…the C/C ratio) may be constant between the moment of marker administration and the time of collection in the undigested faecal matter. Recent studies have shown that microbial fermentation did not affect the C/C ratio in apparent undigested material in vitro and in vivo (Ferreira LMM, unpublished) and that the 13 C/ 12 C ratio remained constant over time. Hence digestion of the labelled nutrient did not alter the isotopic ratio and its FPR in those studies.…”

“…However, when damaged microbial material (mainly originating from the rumen) is included, simulations indicate that 12–24% of faecal carbon is microbially derived, and this higher value is of significance in FPR estimation. Determination of δ C in an indigestible fraction will circumvent this problem, as no or a negligible amount of microbial matter is analytically recovered.…”

“…The production of intrinsically labelled plant material for passage studies can be constrained by its costs and difficulties in producing uniformly labelled material of a comparable nutritional composition to that of the bulk diet. Costs of production of material are negligible when the natural difference in C abundance between diets is used but can be considerable for plant material isotopically enriched above natural abundance, depending on the desired enrichment level and pulse dose size. Growing plants can be labelled in vivo in flexible isotope assimilation chambers on field, but continuous labelling is difficult.…”

“…However, data based on such markers are scarce. Isotopically labelled feed has been applied in early ruminant metabolic studies but was only recently applied in ruminant digestion studies or specifically to quantify passage kinetics . Here we provide an overview of recent digesta passage research and discuss opportunities and limitations of using the isotopic signature ( δ ) of dietary components as a potential novel marker to quantify FPR in ruminants.…”