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

Abstract

The aim of the current study was to explore the use of the tracer 13C 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 13C by growing the grass under elevated 13CO2 conditions. In a crossover design, two dairy cows received pulse doses of two 13C-enriched grass silages and chromium-mordanted neutral detergent fibre (Cr-NDF) into the rumen. The two 13C-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 13C-enriched dry matter (13C-DM), neutral detergent fibre (13C-NDF) and Cr-NDF were established, and a nonlinear multicompartmental model was used to determine their rumen passage kinetics. In addition, the 13C-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 13C-enrichment in the residue. The in vitro study showed that the 13C : 12C ratios in DM and NDF residues remained stable from 24 h of incubation onwards. In addition, in vitro fractional degradation rates for 12C in the DM and NDF did not differ from those of 13C, indicating that fermentative degradation does not affect the 13C : 12C 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, 13C-DM and 13C-NDF (P ⩽ 0.025). Silage type had no clear effect on rumen passage kinetics (P ⩾ 0.081). Moreover, it showed that peak enrichments for 13C-DM and 13C-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 13C : 12C 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 13C : 12C ratio after peak enrichment is not affected by fermentative degradation and therefore can be used to assess feed component-specific fractional passage rates.

Passage kinetics of concentrates in dairy cows measured with carbon stable isotopes

“…Marker excretion curves for individual animals and markers ( Figure 1) were characterised by a sharply ascending curve phase, a distinct peak at 20 h after marker dosage and a slowly descending curve phase. Marker excretion patterns were comparable to studies in dairy cattle in which a 13 C pulse dose was mixed with previously fully evacuated rumen content (Pellikaan et al, 2013). The different routes of marker administration need to be considered when comparing markers.…”

“…All fractions, including Cr and Co concentrations, were analysed as described in detail by Pellikaan et al (2013). The NDS fraction was obtained by calculating the difference between the DM and NDF.…”

“…Passage kinetics of 13 C isotopes were determined from the specific carbon isotope ratio ( 13 C : 12 C ratio) at several time intervals after marker dosage. Pellikaan et al (2013) showed that the 13 C : 12 C ratio did not change with fermentative degradation in the rumen and, therefore, can be used to assess feedspecific and feed component-specific passage kinetics.…”

“…Predominantly, studies on passage dynamics have employed external digesta markers although their use has been criticised for their representativeness (Tamminga et al, 1989;Owens and Hanson, 1992;Pellikaan et al, 2013). In contrast, internal markers would be preferred as they undergo the same biological processes to which the feed fractions under investigation are exposed to.…”

“…Differences in natural abundance of the stable isotope 13 C relative to the major carbon isotope 12 C, as occurs between carbon-fixating C 3 and C 4 plant species (Boutton et al, 1988), can be used to determine fractional passage rates (Südekum et al, 1995;Sponheimer et al, 2003). Recent studies further emphasised the advantages of stable isotopes in estimating feed-specific fractional passage rates (Huhtanen and Hristov, 2001;Pellikaan et al, 2013). In the latter study, 13 C-labelled grass silage yielded lower fractional rumen passage rates compared to the external marker chromium-mordanted fibre (Cr-NDF), likely because the latter marker typically associates to the small indigestible rumen particle pool (Bosch and Bruining, 1995).…”