Energy supplements can be subdivided into two groups: nonstructural and structural carbohydrate
supplements. When choosing an energy supplement, it is important to consider the effect the supplement will have on the basal diet. The type of carbohydrate has a major effect on the rate and extent of forage digestion (Bowman and Sanson 1996). Caton and Dhuyvetter (1997) noted that sources of highly degraded fiber such as wheat middlings, beet pulp, and corn gluten feed have generally not reduced forage intake as much as nonstructural carbohydrate-based supplements such as cereal grains. However, when feeding structural carbohydrate-based supplements, it is important to consider the possibility that the supplement may substitute for the basal diet. Substitution was defined by Bowman and Sanson (1996) as the change in forage intake in kg dry matter per kg supplement dry matter fed. Feeding nonstructural carbohydrates may lead to substrate substitution effects. Poppi and McLennan (1995, p. 285) stated: “Energy supply to the rumen can be most effective when there is a fast NH3 production and a loss of protein. Sugar beet pulp is capable of capturing NH3, and it is high in digestibility and low in protein. The slower degradation of the fiber may enable better synchrony between energy and NH3 release. However, the high fiber can lead to substitution effects, and use of these feeds as supplements may be more suitable for basal diets that are low in fiber.” Soluble carbohydrates such as starch or sugar may impede cellulose digestion due to factors such as lowered pH, competition between cellulolytic and non-cellulolytic bacteria for essential nutrients other than energy, or use of alternative energy sources by certain of the cellulolytic bacteria (Fahey and Berger, 1988). ∅rskov (1986) found that a small amount of grain (20 to 30% of the diet) produces little or no depression of intake and digestibility of roughages, but a higher level can depress intake to an extent that it is no longer a supplement, but becomes a substitute. In most cases, it is important to insure that the supplement complements the basal diet and does not substitute for it.