Which statement best describes NDF and ADF and their relevance to intake and digestibility?

• A. NDF measures crude protein; ADF measures ash.
• B. NDF measures moisture content; ADF measures soluble sugars.
• C. NDF measures soluble sugars; ADF measures energy.
• D. NDF measures total cell wall components, and is inversely related to intake; ADF measures lignin and cellulose and relates to digestibility.

Answer: (D)

NDF and ADF are fiber measurements used to predict how much a ruminant will eat and how well it will digest forage. NDF (neutral detergent fiber) quantifies the plant’s total cell-wall material—hemicellulose, cellulose and lignin. Because these walls contribute to rumen fill, higher NDF means the animal feels full sooner, so voluntary intake tends to drop as NDF increases. ADF (acid detergent fiber) measures the portion of the cell wall that is less digestible—mainly lignin plus cellulose. Lignin is largely indigestible and interferes with the digestion of nearby polysaccharides, so feeds with higher ADF generally have lower digestibility and energy availability. The statement that correctly links these concepts is the one describing NDF as total cell-wall components inversely related to intake, and ADF as comprising lignin and cellulose with a relationship to digestibility.