Describe the relationship of detergent fiber components NDF and ADF to digestibility in forages.

• A. NDF relates to lignin content; ADF relates to cellulose content, and higher levels always increase energy.
• B. NDF reflects cell wall content affecting intake; ADF reflects lignin and cellulose affecting digestibility; higher content reduces digestibility and intake.
• C. NDF and ADF measure only the protein fraction.
• D. NDF is a measure of energy density.

Answer: (B)

The main idea is how fiber components in forage influence how much a ruminant will eat and how well it digests the feed. Neutral Detergent Fiber measures the total cell-wall material (bulk), so higher NDF means more bulk in the feed and less voluntary intake because the rumen fills up faster. Acid Detergent Fiber captures the more indigestible portion of the cell wall, mainly lignin plus cellulose. Higher ADF indicates less digestible forage because lignin is largely indigestible to rumen microbes, and more ADF means fewer truly digestible carbohydrates are available. Together, higher fiber reduces both digestible energy and intake, lowering the energy the animal can get from the forage. So NDF relates to intake through cell-wall bulk, and ADF relates to digestibility through its lignin and cellulose content, with higher levels diminishing both intake and digestibility.