Evaluation of Acid-Insoluble Ash as a Natural Marker in Ruminant Digestibility Studies

Comparisons were made of the total fecal collection method and Acid-Insoluble Ash (AIA) natural marker method for determination of dry matter digestibility coefficients of rations by sheep. Three laboratory analytical procedures Concentrated (Cone.) HCl, 4N HCl and 2N HCl which differed in ashing sequence, ashing temperature and acid strength, were used to determine the AIA content of feed and fecal samples. The dry matter digestibility coefficients estimated by the AIA marker method, by all three analytical procedures, were not significantly different from the coefficients determined by the traditional total fecal collection method. However, the digestibility coefficients estimated using the 4N HCl marker were higher (P<.05) than those estimated using the Cone. HCl and 2N HCl markers. The mean and standard deviation of the recoveries of marker in feces estimated using the Cone. HCl and 2N HCl analytical procedures to determine the marker content were 96.7 ± 6.7% and 95.8 ± 6.1%, respectively, and were lower (P<.05) than the recovery (103.0 ± 7.1%) estimated using the 4N HCl procedure. However, none of these measurements of recovery were themselves significantly different from 100% recovery. There was no evidence of a diurnal pattern of AIA excretion which would influence the estimation of digestibility coefficient. The use of natural markers offers some distinct advantages over the total fecal collection method for digestibility studies. In addition to minimal time and labor involvement, quantitative measurements of feed intake and fecal output are not required and measurements can be made on single feed and fecal samples. AIA, therefore, has possible use in digestibility studies and feed testing laboratories where other methods may not be applicable. Of the three AIA analytical procedures evaluated, the 2N HCl procedure was simpler and more convenient than the Cone. HCl or the 4N HCl procedures.