Variations in Methods for Quantification of Crude Ash in Animal Feeds

Crude ash is categorized as an empirical method playing an important role in the nutritional interpretation of animal feeds, allowing indirect estimation of total organic matter (OM).

Our objective was to evaluate variations in laboratory procedures for crude ash quantification regarding physical parameters (i.e., time, temperature) and ashing aids and their influences on crude ash, repeatability, and discrimination power among feeds.

The "control" method was based on a simple ignition time of 3 h at 550°C. The variations are briefly described: increasing ashing time to 6 h; increasing temperature to 600°C; and using two 3 h ignition cycles at 550°C with ashing aids inclusion between them: fresh air supply, fresh air supply plus distilled water, and fresh air supply plus hydrogen peroxide. A color evaluation was also performed using a colorimetric technique. Twenty-four study materials from eight different feed types were evaluated.

The crude ash results differed among the method variations, but a consistent decrease in the estimates was observed when liquid aids were applied, which also improved repeatability. Ash residues did not present a consistent color pattern among methods, but the residues were darker when the control method was applied.

The method of obtaining ash residues in animal feeds based on 550°C × 3 h does not have enough robustness and may overestimate crude ash in some feeds. Adjustments in either ignition time or temperature might improve crude ash test results, but the best test results are obtained using liquid ashing aids between two ignition cycles.

The recommended method is based on the use of 550°C and two 3 h ignition cycles with water added to the ash residue between cycles.