Year of Publication


Document Type





Animal Science

First Advisor

Austin H. Cantor


Studies were conducted to investigate the interactive effects of exogenous enzymes andorganic acids on in vitro and in vivo nutrient digestion and growth performance of broiler chicks.In Study 1, five exogenous enzyme products including ??-glucanase, xylanase, amylase, ??-galactosidase and protease, were assayed in triplicate at their optimum pH levels and at pH levelsof 3.0, 6.0, 6.5, 7.0 and 7.5, which were used to simulate pH levels found in the gizzard, the diet,the crop, and the proximal and distal parts of small intestine, respectively. The pH gradient wasobtained by dissolving the enzymes in different buffers. Results suggested that the pH levelscommonly found in the avian digestive tract were either too high or too low for maximumactivity of the exogenous enzymes, such as ??-galactosidase and protease.In Study 2, broiler chicks were fed corn basal, barley basal or wheat basal diets withdifferent levels or different sources of organic acids. Dietary inclusion of graded levels oforganic acids linearly reduced the pH of the diet and crop content, but not the pH of the digestasampled in the gizzard and small intestine. The inclusion of 2% organic acids (citric acid orfumaric acid) in broiler diets had either no effect or negative effects on chick growthperformance.In Study 3, an in vitro model was used to simulate the chicken's digestive process in thecrop, the gizzard and the small intestine. Soybean meal and raw whole soybeanwere used as substrates. Graded levels of either ??-galactosidase (0 to 13,792 units/kg) or protease(0 to 888 units/kg) and 0 or 2% citric acid were added to the substrates in a factorialarrangement. Reducing sugars, ??-amino nitrogen and trypsin inhibitor content were measured.The data indicated that increasing levels of ??-galactosidase linearly increased the release of thereducing sugars from the soybean meal. Addition of citric acid further increased the activity of ??-galactosidase, resulting in more reducing sugars were released. Increasing the supplementarylevels of protease linearly increased the ??-amino nitrogen release from the soybean meal and rawwhole soybean. Trypsin inhibitor content in the raw whole soybean was not influenced by theapplication of the protease.In Study 4, broilers were fed low energy or normal energy basal diets with ??-galactosidase,amylase and acidification of diet and water. Growth performance, AMEn and digestibility ofDM, CP and NDF were observed. Alpha-galactosidase improved the AMEn of the diets andincreased the weight gain and feed intake of broiler chicks. Citric acid decreased the crop pH andenhanced the activity of ??-galactosidase in the crop. Citric acid decreased the AMEn of the dietsand chick growth performance. These effects were corrected by supplementing ??-galactosidase.The activity of ??-galactosidase was enhanced by simultaneously using organic acid. Thenegative effects on chick growth performance by dietary inclusion of organic acids werecorrected by simultaneously using ??-galactosidase.