Comparison of alternative neutral detergent fiber methods to the AOAC definitive method

Authors: Hall, Mary Beth; MERTENS, DAVID - Mertens Innovation & Research, Llc

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2023
Publication Date: 6/16/2023
Citation: Hall, M., Mertens, D.R. 2023. Comparison of alternative neutral detergent fiber methods to the AOAC definitive method. Journal of Dairy Science. 106(8):5364-5378. https://doi.org/10.3168/jds.2022-22847.
DOI: https://doi.org/10.3168/jds.2022-22847

Interpretive Summary: Neutral detergent fiber (NDF) is the most important measure of fiber in dairy cattle diets. It is used to predict nutrient supply to the animal, determine the value of forages, and has wide application in other fields. Unlike other analytes, NDF is defined by the method used to measure it, and there is a gold standard method for NDF. A variety of alternative methods to measure NDF are in use in the field – do they measure it accurately compared to the gold standard? Using corn silages, corn grains, alfalfa silages, soyhulls, calf starter pellets, and sugar beet pulp as test samples, we found that samples should be ground as recommended in the gold standard procedure, as a finer grind reduces the NDF values. Among the methods tested an alternative method of filtration through a Buchner funnel with a glass fiber filter gave acceptable results, one filter bag procedure did not, and another filter bag procedure appears to be acceptable for some classes of samples. Application of these results will help to ensure that NDF values are the correct ones needed by farmers, forage brokers, nutritionists, researchers, and others who rely on the accuracy of feed analyses.

Technical Abstract: Neutral detergent fiber (NDF) is the most commonly reported metric for fiber in dairy cattle nutrition, with extensive application in other fields. An empirical method, NDF is defined by the procedure used to measure it. The current definitive method for NDF is AOAC Official Method 2002.04 performed with refluxing and then filtration through Gooch crucibles with (AOAC+) or without (AOAC-) a glass fiber filter filtration aid, and with dried samples ground to pass the 1 mm screen of a cutting mill. Other methods in current use include analyzing samples ground to pass the 1 mm screen of an abrasion mill, a modification using filtration through a Buchner funnel with a glass fiber filter (Buch), and use of a system which simultaneously extracts and filters samples through filter bags with 2 different particle size retentions (F57, F58). Our objective was to compare AOAC and alternative methods using samples ground to pass the 1 mm screen of a cutting or abrasion mill. Eleven samples analyzed were 2 alfalfa silages, 2 corn silages, dry ground and high moisture corn grain, mixed grass hay, ryegrass silage, soybean hulls, calf starter, and sugar beet pulp. Samples were run in duplicate in replicate analytical runs performed on different days by professional technicians. Compared to cutting mill ground samples, abrasion mill samples gave or tended to give lower NDF% of dry matter (DM) values for 8 of 11 samples (-0.5 to 2.1 NDF% of DM range, mean = 0.8%), method was significant for all samples, with method by grind interactions for 6 of 11 samples. For ash-free NDF% of DM (NDFOM%) assessed with cutting mill ground samples, method was significant or tended to be for all samples except dry corn grain. Based on a priori determined contrasts, the number of samples for which methods differed or tended to differ from the AOAC+ and AOAC- for NDFOM% were Buch: 4, F57: 7, and F58: 8; AOAC+ vs. AOAC- differed from each other for 2 samples. Significantly different does not necessarily mean substantially different. Using 2 times the standard deviation of the AOAC NDFOM% as the allowable absolute difference between means of the alternative methods and mean of the AOAC methods, the number of samples which exceeded this difference were Buch: 0, F57: 6, and F58: 3; the differences were Buch: 0, F57: 5, and F58: 3 if AOAC+ alone was used for evaluation of the calf starter and ryegrass silage. We conclude that for NDF values to agree with the definitive AOAC method on the samples tested: 1) samples should be ground with a 1 mm screen cutting mill and not an abrasion mill, unless a 2 mm screen is used as allowed in the AOAC procedure; 2) procedure F57 should not be used; 3) F58 may be used for alfalfa, corn silage and grain samples; 4) Buch appears to be acceptable for all samples; and 5) differences noted between AOAC- and AOAC+ suggest use of AOAC+. Further evaluation with an expanded sample set is recommended.