Date of Award

12-2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Michael O. Smith

Committee Members

Kelly R. Robbins, Charles H. Goan, Alan G. Mathew, Naima Moustaid-Moussa

Abstract

Defatted rice bran is a by-product from the solvent-extraction of rice bran. It contains high amounts of phytate and non-starch polysaccharides, which are considered to be two major antinutritional factors that limit the use of rice bran in poultry diets. Two experiments were conducted to evaluate broiler performance and mineral utilization of defatted rice bran diets supplemented with commercial enzyme phytase alone or a combination of phytase and xylanase under two different environmental temperatures. In the first experiment, six replicate groups of day-old chicks were assigned to five dietary treatments comprised of a control corn-soybean meal diet with 25% corn starch and four diets containing two levels of defatted rice bran (10 and 25%) with or without phytase (500 units/kg diet) supplementation. In experiment 2, five replicate groups of chicks were assigned to dietary treatments similar to first experiment, but using both phytase (500 units/kg diet) and xylanase (1000 units/kg diet) instead of phytase alone plus an additional basal corn-soybean meal based diet. Available phosphorus in diets containing phytase was calculated to be 0.1% reduction from the requirement for broilers. On day 21, some birds were slaughtered and tibias harvested for mineral analysis. The remaining birds were transferred to either a thermoneutral temperature chamber (23.9oC) or a heat stress chamber (23.9-35oC, diurnal). Digesta retention time was determined on day 35. Birds were slaughtered at seven weeks of age, tibias harvested, and intestines examined.

In both experiments, using up to 25% defatted rice bran in diets was not detrimental to broilers. The beneficial effect of high fat level can overcome some problems associated with high phytate phosphorus and non-starch polysaccharide in defatted rice bran. As a result, birds fed 25% defatted rice bran diet had the highest body weight gain. Supplementing with enzymes improved feed conversion ratio of 3-week-old chicks fed 10% defatted rice bran diet. No significant differences were found in weight percentage of most parts of the carcass, as well as in morphology of the duodenum and the jejunum examined by villi height and crypt depth measurements. Abdominal fat was increased in birds fed 25% defatted rice bran diets. Tibia phosphorus, zinc and manganese content of chicks fed 10% defatted rice bran diet were improved by supplemental phytase alone, while tibia calcium, phosphorus, and manganese content of chicks were improved when both enzymes were added to 25% defatted rice bran diet. However, only the improvement of tibia phosphorus of 7-week-old birds was detected when phytase was added to 25% defatted rice bran diet or when both enzymes were added to 10% defatted rice bran diet. Although percent phosphorus in the excreta was decreased in both experiments in birds fed 25% defatted rice bran diets supplemented with enzyme, phosphorus retention of birds did not improve by using phytase alone. Since the price of rice bran was cheaper than other major ingredients, use of 25% defatted rice bran diet lowered feed cost per weight gain of broilers. From the growth performance and feed cost per weight gain of birds, supplemental enzyme did not alleviate the adverse effects of heat stress.

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