δ13C as a marker to study digesta passage kinetics in ruminants: a combined in vivo and in vitro study

The aim of the current study was to explore the use of the tracer C-13 as an internal marker to assess feed fraction-specific digesta passage kinetics through the digestive tract of dairy cows. Knowledge on feed-specific fractional passage rates is essential to improve estimations on the extent of rumen degradation and microbial protein efficiency; however, this information is largely lacking. An in vivo and in vitro experiment was conducted with grass silages (Lolium perenne L.) that were enriched with C-13 by growing the grass under elevated (CO2)-C-13 conditions. In a crossover design, two dairy cows received pulse doses of two C-13-enriched grass silages and chromium-mordanted neutral detergent fibre (Cr-NDF) into the rumen. The two C-13-enriched grass silages used differed in digestibility and were grown under identical field conditions as the bulk silages fed to the animals. Faecal excretion patterns of C-13-enriched dry matter (C-13-DM), neutral detergent fibre (C-13-NDF) and Cr-NDF were established, and a nonlinear multicompartmental model was used to determine their rumen passage kinetics. In addition, the C-13-enriched silages were incubated in rumen liquid in an in vitro batch culture system at different time intervals to determine the effect of fermentation on C-13-enrichment in the residue. The in vitro study showed that the C-13 : C-12 ratios in DM and NDF residues remained stable from 24 h of incubation onwards. In addition, in vitro fractional degradation rates for C-12 in the DM and NDF did not differ from those of C-13, indicating that fermentative degradation does not affect the C-13 : C-12 ratio in the DM nor in the NDF fraction of the residue. Model fits to the faecal excretion curves showed a significant difference in fractional rumen passage rates between Cr-NDF, C-13-DM and C-13-NDF (P <= 0.025). Silage type had no clear effect on rumen passage kinetics (P >= 0.081). Moreover, it showed that peak enrichments for C-13-DM and C-13-NDF in faeces were reached at 30.7 and 41.7 h post dosing, respectively. This is well after the time (24 h) when the C-13 : C-12 ratios of the in vitro unfermented residues have reached stable enrichment level. Fractional rate constants for particle passage from the rumen are estimated from the descending slope of faecal excretion curves. The present study shows that the decline in C-13 : C-12 ratio after peak enrichment is not affected by fermentative degradation and therefore can be used to assess feed component-specific fractional passage rates.