Wheat starch digestion rate affects broiler performance

Two experiments were conducted to determine the differences in starch digestion rate (KDS) among wheats from different cultivars and origins and to verify if chickens would benefit from a certain digestion rate of starch. In the first experiment, 192 chickens (21 d) were assigned to 4 diets containing 55% of each wheat sample (3 cultivars, one of them from 2 origins). Starch and protein digestion were calculated from the remaining starch and protein in 4 segments of the small intestine and in excreta, using chromic oxide as a marker. Mean retention time was measured in each segment, which enabled calculations of digestion rates. In the second experiment, 2,600 chickens were assigned to 5 isoenergetic and isonitrogenous diets (with KDS from 1.80 to 2.56 h(-1)) and growth performance was determined (1 to 34 d). In 3 treatments, dietary starch was provided each by the wheat cultivars (same origin), whereas in the other 2 treatments, 25 and 50% of the wheat starch with the highest KDS was substituted by pea starch. Clostridium perfringens and Lactobacillus in the cecal chyme and glucose in the blood (glycemic index) were measured in broilers at d 19 and 25, respectively. Starch was gradually digested along the small intestine, mainly in the jejunum (48.5 and 80.4% at proximal and distal jejunum) where the largest differences among wheat samples were found. Starch digestion rate varied with origin (from 1.96 to 2.56 h(-1)) and cultivar (from 2.17 to 2.56 h(-1)). Crude protein digestion rate (average 2.21 h(-1)) was not affected by either cultivar or origin. Broiler growth and feed conversion ratio improved in a quadratic way with KDS. The maximum broiler performance was observed with KDS around 2.2 h(-1). Blood glucose response (glycemic index) was not affected by KDS; therefore, it cannot be used to predict broiler performance. In conclusion, the rate of starch digestion varies among wheats, depending on both genetic and environmental conditions of the grain, and affects broiler performance.