1 Dry Matter Intake

Dry matter intake (DMI) is fundamentally important in nutrition because it establishes the amount of nutrients available to an animal for health and production. Actual or accurately estimated DMI is important for the formulation of diets to prevent underfeeding or overfeeding of nutrients and to promote efficient nutrient use. Underfeeding of nutrients restricts production and can affect the health of an animal; overfeeding of nutrients increases feed costs, can result in excessive excretion of nutrients into the environment, and at excessively high amounts may be toxic or cause adverse health effects. Many factors affect voluntary DMI. Individual theories based on physical fill of the reticulorumen (Allen, 1996; Mertens, 1994), metabolic-feedback factors (Illius and Jessop, 1996; Mertens, 1994), or oxygen consumption (Ketelaars and Tolkamp, 1996) have been proposed to determine and predict voluntary DMI. Each theory might be applicable under some conditions, but it is most likely the additive effect of several stimuli that regulate DMI (Forbes, 1996). Feeds low in digestibility are thought to place constraints on DMI because of their slow clearance from the rumen and passage through the digestive tract. The reticulorumen and possibly the abomasum have stretch and touch receptors in their walls that negatively impact DMI as the weight and volume of digesta accumulate (Allen, 1996). The neutral detergent fiber (NDF) fraction, because of generally low rates of digestion, is considered the primary dietary constituent associated with the fill effect. The conceptual framework for the metabolic-feedback theory contends that an animal has a maximal productive capacity and maximal rate at which nutrients can be used to meet productive requirements (Illius and Jessop, 1996). When absorption of nutrients, principally protein and energy, exceeds requirements or when the ratio of nutrients absorbed is incorrect, negative metabolic-feedback impacts DMI. An alternative to the metabolic theory is the theory Ketelaars and Tolkamp (1996) proposed based on oxygen consumption. This theory suggests that animals consume net