On the specificity of different methods for neutral detergent fiber and related problems

We compared alternative methods for analyzing Neutral Detergent Fiber with the reference AOAC 2002.04 method for specificity. We reported the chemical composition of major nutrients for the matrixes of 19 feeds and one non-feed (sawdust). Two independent matrixes were from animal origin: meat and bone meal, and milk powder. All fiber residues were ash and blank-corrected to yield values comparable to the reference aNDFom obtained with the reflux in crucibles (Fibertec, method 1) or in Berzelius beakers without spouts (method 2). The alternative fiber methods were as follow: reflux in Berzelius beakers with spouts (method 3), the ANKOM pressurized filter bag system (method 4), a non-pressurized filter bags (nonwoven tissue) immersion system (TECNAL, method 5), and micro-NDF digestion in crimp-sealed penicillin flasks deposited in an autoclave (method 6). The major nutrients were fractional mass proportions described as Beta-distributed variables. Assuming aNDFom as normally distributed because of negative fiber values recorded, an information-theoretic approach identified the relevance of the random effects of analyst (three analysts of the same laboratory) and their interactions with matrix, method, and matrix-method interaction as fixed effects. We also challenged the traditional homoscedastic assumption. The GLIMMIX procedure of SAS was used to fit the models. In sequence, we chose the model without all random effects except for the analyst-method-matrix interaction defined as the subject and a complete heteroscedastic structure with one variance estimate for each method-matrix group. We removed milk powder from the dataset to fit the same model by assuming that aNDFom/1000 was Beta-distributed, and Pearson residuals demonstrated a comparable fit to the normal assumption. Within the limits of the experimental error, the ADFom was entirely formed by Lignin (sa) for meat and bone meal and for powdered milk. We observed a significant method-matrix interaction for aNDFom. Therefore, effect slices for methods within matrix and vice-versa revealed several significant contrasts between reference and alternative methods for aNDFom. Methods 1 and 2 were not different. With powdered milk removed from the analysis, the fit under Beta-distributed aNDFom resulted in an increased number of significant contrasts; however, contrasts between methods 1 and 2 remained non-significant, regardless of the matrix analyzed. Significant contrasts demonstrated the lack of specificity of the alternative methods used to measure insoluble fiber compared with the reference methods. In terms of variability, within-laboratory variance for aNDFom corresponded to the method-matrix-analyst component, and the variation introduced by different analysts in the same laboratory was negligible.