Acid insoluble ash (AIA) has been used as a marker for nutrient digestibility studies in various animals for a long time. In aquaculture, it has been used as an internal or external marker for apparent nutrient digestibility. However, there have been conflicting results among studies regarding reliability of AIA as the marker in aquaculture. One major reason identified is that the method for AIA measurement is rudimentary and lacks sensitivity and reliability. To address this matter, a few years ago, we developed a significantly improved method for measuring AIA. Therefore, it is necessary to compare AIA with other known markers for apparent nutrient digestibility studies with fish, using the new method.
The present study with rainbow trout involved spiking a reference diet mixture with Celite (a pure commercial AIA) at 0, 0.3 and 0.6%, respectively, and yttrium oxide at 0.1%, mixing each of the three spiked references with fish meal, soybean meal, and DDGS, respectively, at 7:3 ratio, and making 12 diets (including the original three reference ones) by extrusion. Digestibility trials involved 36 tanks, three tanks per diet and 25 fish per tank. Each diet was randomly assigned to a tank of fish and fed to apparent satiation twice daily for 7 days prior to fecal collection.
There were drastic differences between the two markers in apparent digestibility coefficients (ADC) determined for several nutritional attributes in diets fed to rainbow trout, containing each of the three protein ingredients. For example, using yttrium oxide as a marker, average ADC of proteins in diets containing fish meal, soymeal or DDGS were 89.2, 87.8, 85.6%, respectively. In contrast, when AIA was used as a marker, mean ADC of proteins in the same diets containing the fish meal, soymeal or DDGS changed to 76.4, 41.2, 17.3%, respectively. The variation in ADC of a specific attribute among three tanks was also drastically larger for AIA than yttrium oxide. Careful examination of data revealed a huge difference in changes from diets to feces between yttrium and AIA composition within total ash. The percentages of yttrium in total ash in feces increased to 25.8-84.6% of those in diets, with an average of 53.1%. In contrast, the percentages of AIA in total ash in feces decreased to 27.6-67.7% of those in diets, with an average of 53.8%. Consequently, AIA content in feces was significantly reduced for most diets, indicating substantial loss as it went through the biological process within trout guts and under an aqueous environment. It is concluded that comparing to yttrium oxide, AIA was a much less reliable marker for digestibility study with trout.