Often called woody-fiber, lignin is the cement that binds other fiber fractions together for plant strength. Lignin increases with plant maturity and is the primary factor limiting digestibility. Lignin decreases plant digestibility both by increasing indigestible volume as plants mature and by making a protective coating around more digestible fiber components. Cellulose and hemicellulose are complex, structural carbohydrates and are digested slowly compared to more soluble sugars, starches, proteins and fats. Hemicellulose is more digestible than cellulose, but not all hemicellulose or all cellulose digest at the same rate or to the same degree.
Crude fiber is the residue remaining after successive boiling in dilute acid and alkali.
Crude fiber measures the cellulose and part of the lignin. However, it is an incomplete measure of the less digestible part of feeds as it fails to measure hemicellulose and most of the lignin. Neutral detergent fiber is the best indicator of rumen fill or intake potential. Therefore, ADF is a good indictor of digestibility. The potassium permanganate procedure is preferred because of its accuracy and higher repeatability.