Crude Fiber, ADF or NDF: Choosing the Right Fiber Method (and the Right Filter Bag)

In short: these are three numbers answering three different questions. Crude fiber answers 'what survives boiling acid and alkali' — the legal declaration on many feed labels. ADF (acid detergent fiber) estimates cellulose plus lignin, the least digestible fraction. NDF (neutral detergent fiber) captures the total structural carbohydrate — hemicellulose, cellulose and lignin — and drives ration formulation. On the same sample the result almost always runs crude fiber < ADF < NDF. All three run as gravimetric filter-bag methods on the same instrument family: what changes is the reagent program, not the bag. For the chemistry of the crude fiber step itself, see our crude fiber determination note.

What each fraction actually measures

Crude fiber (Weende)

The sample is boiled in 1.25% sulfuric acid, then in 1.25% sodium hydroxide; the residue is dried, weighed and ashed. What remains is mostly cellulose with some lignin — but much of the lignin and nearly all hemicellulose dissolves along the way, so crude fiber systematically underestimates total fiber. It is an old and imperfect number that survives because feed-labeling law in many countries is written around it.

ADF — acid detergent fiber

An acid detergent solution (CTAB in dilute sulfuric acid) dissolves hemicellulose and protein; the residue is cellulose, lignin and acid-insoluble ash. ADF is the working estimate of the indigestible fraction and feeds directly into energy and digestibility calculations for forage.

NDF — neutral detergent fiber

A neutral detergent solution — with a heat-stable amylase added for starchy samples — leaves the full cell-wall fraction: hemicellulose, cellulose and lignin. NDF is the number nutritionists balance rations against, because it predicts voluntary intake: the more NDF in the diet, the less an animal eats of it.

Side by side

Which method belongs on your bench

Feed mills releasing product against a label need crude fiber — that is the declared figure, whatever its scientific weaknesses. Nutritionists and forage laboratories need NDF first and ADF alongside it, because both feed the ration models. Research and contract laboratories usually end up running all three, which is when batch capacity stops being a convenience and becomes the constraint.

The filter bag's job in all of this

The bag replaces the crucible: reagents must flow freely through the wall while every particle of sample stays inside. The 25 μm pore size used for fiber work — our FB25 bag among them — is the balance point between retention and flow, and it is shared across crude fiber, ADF and NDF programs. That is why one bag type serves all three methods: the geometry does not change, only the chemistry does.

Three practical notes from application support. Grind to pass a 1 mm screen — coarser grinding slows reagent penetration, finer grinding risks particle loss through the bag wall. High-starch samples need the amylase step in NDF work or the result reads high. And high-fat samples should be pre-extracted, because fat films interfere with filtration and with the detergent chemistry.

Choosing the instrument once the method is settled

Method first, instrument second. Once the program mix is known, the choice is batch size and automation depth: the F20 covers routine semi-automated runs, while the F22 automates the full boil–rinse sequence for higher daily throughput. Both run crude fiber, ADF and NDF programs on the same 25 μm bags — and if your laboratory currently runs ANKOM bag-method instruments, our FB25/GB3 compatibility note explains the direct-replacement position, including free trial kits for validation.

Honest limitations

The bag method is a gravimetric technique and inherits gravimetric limits: particle loss on over-fine grinding, matrix effects on very fatty or starchy samples, and the need to validate against your own reference materials before switching any step. One boundary to state clearly: total dietary fiber for nutrition labelling (AOAC 991.43, AOAC 2011.25) is an enzymatic-gravimetric method, not a detergent method — a fiber analyzer built for crude fiber, ADF and NDF is not a substitute for it.