Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Animal Science (Animal, Dairy, and Poultry Sciences)

First Advisor

L. Lee Southern


A series of experiments was conducted to determine the effect of increasing phytase coefficient of variation (CV) in chick diets, and to determine the effect of phytase addition on nutrient availability in diets for swine and poultry. The first experiment (EXP) simulated varying mix uniformity of phytase by alternately providing two diets with two different concentrations of microbial phytase (FTU). Increasing phytase CV had little effect on growth performance, whereas bone ash and breaking strength and Ca and P retention and excretion decreased only at the most extreme CV. These data suggest that a phytase CV of between 34 and 69% is adequate to support optimum chick performance. Seven EXP were conducted using slope ratio assays to determine the nutrient values that might be attributed to phytase addition to corn-soybean meal diets for chicks. For chicks, nutrient values of 26% for lysine, 25% for total sulfur amino acids, and 46,000 kcal of metabolizable energy per kilogram of phytase could be used for phytase providing 600 FTU with an inclusion rate of 0.1%. In an experiment conducted with pigs to determine the effect on growth performance of phytase addition to diets deficient in amino acids, improvements in feed efficiency and lean gain and increased fat indicate possible increases in energy and amino acid availability with phytase addition. This result was supported by an EXP where reducing the concentration of dietary Ca and P, phytase addition, and the combination of the two treatments in corn-soybean meal diets for pigs increased apparent ileal digestibility of amino acids, DM, energy, and N. Similar digestibility EXP were conducted in chicks, broilers, and laying hens. While the effect of phytase on the individual amino acids varied, there was an overall tendency for improved amino acid digestibility with phytase addition in diets for chickens. Thus, phytase increased nutrient availability in diets for poultry and swine.