Cell walls become thicker as the plant matures. This thickening of plant cell walls increases the fiber content of the total plant, and decreases the cell content portion of the plant. It is this relationship, the amount of cell content vs. the amount of cell wall content present, that is measured by a forage test. Cell contents are measured as protein and energy. The cell wall is measured as fiber, which can be further divided into acid and neutral detergent fiber.
Ruminants adjust their intake in response to their energy requirements.
When ruminate diets contain large portions of indigestible fiber, the quantity of feed consumed may be limited by rumen capacity. In some instances rumen capacity is reached before an animal has consumed enough forage to meet energy demands. This phenomenon can be seen when a high fiber hay is fed to cows with high energy demands such as cows nursing calves. While the cow's energy requirement increases her potential intake, her intake becomes limited due to the low digestibility and rate of passage of the hay. The rumen becomes full of hay before enough hay has been consumed to meet energy demands.