An evaluation of gravimetric and inert marker techniques to measure digestibility in the American lobster

Abstract

Two markers routinely used in digestibility studies, chromic oxide and the generic moiety of total ash, were tested in feeding experiments with adult American lobsters ( Homarus americanus Milne Edwards) to determine if chromic oxide would deter voluntary intake and if ash would be absorbed in the gastrointestinal tract. Chromic oxide incorporated into a test diet at levels up to 1% did not suppress dry matter intake. Observations of pigment excretion patterns in the feces proved there was a differential passage rate of the chromium pigment. It is not valid to use chromic oxide as an indigestible marker to measure apparent digestibility in the American lobster. Total ash as an indigestible tracer was unsatisfactory as it appeared to be absorbed through the gut wall, as measured gravimetrically by the difference in ash intake compared with fecal ash. This resulted in a significant underestimation of apparent digestibility when comparing an ash-ratio measurement with a gravimetric measurement of digestibility. It was concluded that gravimetric measurement of apparent digestibility is the method of choice.