The use of indigestible dry matter (iDM) and indigestible neutral detergent fiber (iNDF) as internal markers to estimate digestibility in lambs was evaluated. Data and samples were obtained from six independent digestion trials with lambs housed in metabolism cages given ad libitum different forage and concentrate feedstuffs. Indigestible residues were determined after 144 hours of in situ incubation of feed and faeces samples. Recovery rate varied from 64.8 to 108.5% for iDM, and from 49.5 to 67.9% for iNDF. As the marker ratio between marker concentration in the faeces and the feed was not corrected for fecal recovery, the large part of the estimated apparent organic matter digestibility (AOMD) of the experimental means were lower than the in vivo that obtained. As the marker ratio was corrected for fecal recovery of the marker, the AOMD means estimated using both markers were similar to the in vivo data in all experiments. As individual estimates, corrected for fecal recovery of the marker, were related to in vivo observations through regression analysis, the slope of the regression for iDM was not different from 1 whereas for iNDF, the slope was lower than 1. Individual AOMD values estimates using both markers, however, were poorly related to the in vivo observations (r² varied from 0.60 to 0.63). Once corrected for fecal recovery, residual DM after 144 hours of in situ incubation is suitable for use as an internal marker to estimate mean diet digestibility consumed for a group of animals. However, individual estimates were not accurate to detect small differences in digestibility imposed by different treatments within an experiment.
digestibility; fecal recovery; indigestible dry matter; indigestible neutral detergent fiber; intake
