Acid detergent fibers and neutral detergent fibers are important measurements used in forage food consumed by animals. The two calculations are based on the digestibility of plant material present in an animals' food. Farmers use these two calculations to determine how much food an animal requires and how much energy the animal will receive from that consumed food.

The main difference between acid detergent fiber and neutral detergent fiber is the inclusion of hemicellulose in the calculation of neutral detergent fiber. Both calculations include cellulose and lignin present in plant material. Hemicellulose, which is also a carbohydrate present in plant material, is considered in the calculation of neutral detergent fiber. This small carbohydrate makes the difference on how the two fibers are applied to feed.

The acid neutral fiber is used to calculate the energy that will be derived from feed that can be used by the animal. These calculations are very important in determining how much feed must be given to an animal. For example, a beef cow and a milk cow have vastly different energy requirements. A milk cow requires much more energy from its feed to meet the demands of producing milk.

Neutral detergent fiber is used to calculate how much food that an animal can hold. There is a limit to how much food will fit into an animal at one time. For example, a cow will eat until the first chamber of the stomach, also called the rumen, is full. Once that chamber is full, the cow will no longer eat until the food moves to the gut or is digested. Each type of forage food or fiber will take up different amounts of space and digest differently. The neutral detergent fiber provides information to the quality of the feed.

The two fiber calculations are used in conjunction with one another to determine the amount and energy that will be contained in a feed. Fiber that has low cellulose, lignin and hemicellulose will typically take up less space in the stomach and are able to provide larger amounts of energy to the animal. Fibers high in these materials take up more space and produce less energy for the animal to use.