The ADF portion of fiber is most closely associated with digestibility because higher ADF indicates lower digestibility.

• A. ADF indicates protein content.
• B. ADF is unrelated to digestibility.
• C. ADF includes all fiber.
• D. ADF reflects lignin and cellulose and higher content reduces digestibility.

Answer: (D)

ADF (Acid Detergent Fiber) comes from the portion of fiber that includes cellulose and lignin. The method leaves behind hemicellulose, so ADF rises as more indigestible lignin and related components accumulate. Lignin is largely indigestible and also hinders access to other cell-wall carbohydrates, so feeds with higher ADF provide less digestible energy. That’s why ADF is closely tied to digestibility: higher ADF generally means lower digestibility. The other statements miss the fiber components ADF measures, or mischaracterize its relationship to digestibility, or incorrectly link ADF to protein content.