Predicting Digestibility of Different Diets with Internal Markers: Evaluation of Four Potential Markers

The use of internal markers to predict total digestibility and duodenal flow of nutrients in cattle given six different diets.

The ability of ytterbium acetabe (Yb acetate) to fulfil the requirements of a particulate-phase digesta-flow marker in a dual-phase marker system, and of the indigestible acid-detergent-fibre fraction of the feed (IADF) to act as a digesta flow marker, were examined using six mature wether sheep given a diet of dried grass (1 kg dry matter (DM)/d). CrEDTA was continuously infused (240 mg chromium/d) into the rumen of all sheep and Yb acetate was also continuously infused (100 mg Yb/d) into the rumen of three of the sheep. At this level of infusion the equilibrium concentration of Yb in rumen, duodenal and ileal digesta and in faeces could be reliably measured by atomic absorption spectrometry. Estimates of faecal DM excretion based on either Yb or IADF did not differ (P greater than 0.05) from that determined by total collection, whereas estimates based on Cr were significantly (P less than 0.05) lower. Urinary excretion accounted for 3.1% of the infused Cr but no Yb was detected in urine. Estimates of ileal DM flow, assuming total marker recovery, were similar (P greater than 0.05) with all three markers, whereas the estimate of duodenal DM flow based on IADF was lower (P less than 0.05) than the estimates based on either Cr or Yb. Compared with the infusion of Cr alone, the infusion of Cr and Yb had no effect (P greater than 0.05) on nutrient flows at the duodenum, ileum and in faeces nor on microbial degradative activity volatile fatty acid production and N metabolism in the rumen. Polyester bag and in vitro studies showed that pre-labelling the dried grass with up to 285 mg Yb/g DM did not affect its susceptibility to microbial degradation. The Yb in rumen, duodenal and ileal digesta was predominantly (greater than 90%) associated with the particulate matter but was not uniformly distributed and its concentration increased as particle size decreased. The use of CrEDTA and Yb acetate as a dual-phase marker system proved more reliable in estimating 'true' duodenal flow than the use of the individual markers when the digesta sample was unrepresentative.

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Ytterbium acetate as a particulate-phase digesta-flow marker

We investigated the spatial distribution of radio-marked wildebeest (Connochaetes taurinus) in the Serengeti ecosystem in relation to the distribution of their food resources, comparing patterns in...

Spatial distribution of Serengeti wildebeest in relation to resources

Evaluation of mathematical models to describe neutral detergent residue in terms of its susceptibility to degradation in the rumen.

Digestibility trials were carried out using wether sheep, to determine the organicmatter digestibility of several forages. Acid detergent fibre (ADF) was isolated from samples of feed and faeces, incubated in buffered cellulase solution, and the ash-free residue remaining (IADF) was used as an internal marker to estimate the organicmatter digestibility (OMD) of the feeds. Estimates of the OMD value of the samples were also made using the standard in vitro digestibility technique.IADF had reached a constant value after incubation for 4–5·5 days and predicted OMD more precisely than the in vitro digestibility technique.It is concluded that IADF is a suitable internal marker and predicts OMD well. However, more work is required to test this technique over a wider range of herbage species and digestibilities.

R. H. Johnson

Papers

The use of internal markers to estimate herbage digestibility and intake: 2. Indigestible acid detergent fibre