166 Associative Effects and Management – Combinations of Processed Grains

INTRODUCTION When feeding grains to feedlot cattle, the goal is to maximize starch digestion while managing acidosis in order to provide optimal economic utilization of the finishing diet. Starch, the main component of grains, is digested primarily in the rumen with some digestion occurring in the small intestine and cecum. The small intestine is the preferred location of starch digestion to occur because digestion in the small intestine is more efficient (20 to 25%; Waldo, 1973) than digestion by the rumen microbes. However the capacity of the small intestine to digest starch may by limited (Karr et al., 1966; White et al., 1971). Starch digestion and acidosis are closely intertwined. Acidosis usually occurs in feedlots with cattle fed high-energy diets. Therefore, the relationship between rate of starch digestion, acidosis, and intake is important in determining whether the level of performance attained equals the level predicted for the cattle and feedstuffs used. Acidosis is one of the most important nutritional disorders in feedlots. In general, acidosis is considered one disorder, but it needs to be separated into acute and subacute types. During acute acidosis, the animal’s life may be threatened or at least some physiological function, usually absorption, may be impaired (Britton and Stock, 1987). In subacute acidosis, the major response observed is a reduction in feed intake with a concomitant reduction in performance (Fulton et al., 1979). Processed grains may differ in site of digestion, rumen versus post-rumen. Processed grains also may differ in their rate and extent of starch digestion in the rumen. In addition, the amount of starch fed (% grain in the diet) has an important effect on acidosis, and consequently, on cattle performance. Thus, it is important to consider rate, extent, site, and amount of starch digestion when determining an appropriate combination of processed grains to be fed.