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Short answer. Acid detergent fibre (ADF) is the gravimetric residue left after a feed or forage sample is boiled in an acidified detergent solution (1 N sulfuric acid with the detergent CTAB). The extraction removes cell solubles, hemicellulose and soluble minerals; what remains is mainly cellulose and lignin, plus heat-damaged protein, some cell-wall protein and ash. The reference method is AOAC Official Method 973.18, built on Goering and Van Soest (1970).
Definition and scope
ADF is a method-defined quantity: the number is whatever residue the stated procedure leaves, reported as a percentage of sample dry matter. The UC Davis Analytical Laboratory describes its Method 640 as based on AOAC Official Method 973.18, “Fiber (Acid Detergent) and Lignin in Animal Feed” (16th edition, 1997), with a scope covering all types of forages. In the lab’s description, ADF is the residue after extraction with hot acidified detergent solution, and that residue contains cellulose, lignin, heat-damaged protein, some cell-wall protein and ash (checked 2026-11-07).
What is the Van Soest method?
The Van Soest system is a family of detergent-fibre procedures developed by P. J. Van Soest and colleagues. The acid detergent procedure is described in Goering and Van Soest (1970), Forage Fiber Analyses, USDA Agriculture Handbook 379, which AOAC 973.18 and current lab manuals cite. Van Soest, Robertson and Lewis (1991, Journal of Dairy Science 74(10):3583-3597, doi 10.3168/jds.s0022-0302(91)78551-2) is the later paper that addresses neutral detergent fibre standardisation and the relationship of total dietary fibre and non-starch polysaccharides to animal nutrition.
How the analysis is run in practice
The figures below are from a named teaching laboratory manual (Dairy Knowledge Portal, manual 8.4, citing Goering and Van Soest 1970) and show the shape of the method; they do not replace your laboratory’s controlled SOP.
- Reagent: 1 N sulfuric acid (49.04 g H2SO4 per 1000 mL), with its normality checked by titration, and 20 g of technical-grade CTAB (cetyltrimethylammonium bromide). Decalin, acetone and optionally hexane are listed as auxiliary reagents.
- Sample: 1 g air-dry material ground to pass 1 mm, with 100 mL of cold acid-detergent solution.
- Extraction: bring to a boil within 5-10 minutes, reduce heat to avoid foaming, and reflux for 60 minutes from the onset of boiling.
- Recovery: filter through a pre-weighed crucible, wash with hot water (3-4 times) and then acetone until no colour comes out, dry at 100 C for 8 hours or overnight, and cool in a desiccator before weighing (see laboratory desiccators).
- Calculation: ADF % of dry matter = (crucible plus fibre minus empty crucible) / dry sample weight x 100. Weighing accuracy matters; see what an analytical balance is.
The UC Davis lab reports a method detection limit of 0.5% ADF and asks for particles of at least 1 mm (No. 18 sieve) with a 20 g submission.
Which acid is used in the detergent?
Sulfuric acid. The acid-detergent solution is 1 N sulfuric acid containing CTAB. Do not confuse it with the 72% sulfuric acid used afterwards for lignin: in the NDSU acid detergent lignin procedure the ADF residue is digested in 72% H2SO4 (specific gravity 1.634 at 20 C) for 4 hours, dried at 100 C and ashed at 470 C (NDSU, citing AOAC 973.18, 18th ed., 2010; checked 2026-11-07).
How ADF connects to other results
ADF is the first step of a chain, which method summaries often leave implicit. The UC Davis method description gives these downstream results:
| Result | How it relates to the ADF residue |
|---|---|
| Acid detergent lignin (ash free) | ADF residue extracted with 72% H2SO4, then ashed |
| Cellulose | ADF minus pre-ash lignin |
| ADIN (acid detergent insoluble nitrogen) | Nitrogen measured on the ADF residue |
| TDN (total digestible nutrients) | Calculated from ADF using an alfalfa formula |
What is the difference between ADF and NDF?
NDF stands for neutral detergent fibre; it is the corresponding residue from the neutral-detergent procedure rather than the acid-detergent one. Both are gravimetric, method-defined residues, so results are only comparable when the same procedure was used. For ADF, the acid step removes hemicellulose, which is why the residue is described as cellulose, lignin and associated material. Standardisation of the neutral detergent procedure is covered in Van Soest, Robertson and Lewis (1991).
Crucible and filter-bag methods
Many labs run ADF in filter bags in an automated fibre analyser instead of crucibles. Published bag-method descriptions use 20 g CTAB per litre of 1 N H2SO4 and grind samples to 1 mm; finer grinds can lose particles through the bag. A blank bag gives a correction factor (a value above 1.0000 indicates particle loss), the acid must be rinsed out fully, and acid should be added to water when preparing the reagent. CTAB irritates mucous membranes, so handle it under your lab’s chemical-hygiene rules and written procedures (how to write a lab SOP).
Pre-analytical checklist
- Grind to the specified particle size (1 mm) and record it.
- Confirm acid normality by titration before use.
- Dry and cool crucibles or bags consistently before weighing.
- Wash until the acetone runs colourless.
- Report on a dry-matter basis and name the method (AOAC 973.18 or your lab’s variant).
Common misunderstandings
- ADF is not a single chemical compound. It is a residue that includes cellulose, lignin, heat-damaged protein and ash.
- ADF is not NDF. They come from different extractions.
- The 72% acid is a different step. It belongs to the lignin determination that follows ADF.
FAQ
What does NDF stand for in animal nutrition?
Neutral detergent fibre, the residue from the neutral-detergent procedure standardised in Van Soest, Robertson and Lewis (1991).
Which acid is used in the detergent?
1 N sulfuric acid with CTAB for ADF; 72% sulfuric acid is used only in the later lignin step.
What is the Van Soest method?
The detergent-fibre system of Van Soest and colleagues; the ADF procedure is set out in Goering and Van Soest (1970) and AOAC 973.18.
What is the difference between ADF and NDF fiber?
ADF is the residue of the acid-detergent extraction, which removes hemicellulose and leaves mainly cellulose and lignin; NDF is the residue of the neutral-detergent extraction. The two are separate, method-defined measurements.
Sources
- UC Davis Analytical Laboratory, Method 640 (Acid Detergent Fiber): anlaborders.ucdavis.edu/analysis/Feed/640, checked 2026-11-07.
- Dairy Knowledge Portal, laboratory manual 8.4: dairyknowledge.in, checked 2026-11-07.
- NDSU, Acid Detergent Lignin beaker method: ndsu.edu, checked 2026-11-07.
- Van Soest PJ, Robertson JB, Lewis BA (1991) J Dairy Sci 74(10):3583-3597, doi 10.3168/jds.s0022-0302(91)78551-2, https://doi.org/10.3168/jds.s0022-0302(91)78551-2, checked 2026-11-07.
- Goering HK, Van Soest PJ (1970) Forage Fiber Analyses, USDA Agriculture Handbook 379; AOAC Official Method 973.18 (as cited in the UC Davis and NDSU documents above, checked 2026-11-07).
Related: standard operating procedure (SOP).
