Evaluation of Extra-Phosphoric Effects of Phytase on Growth Performance, Carcass Characteristics, Phytate Degradation, Plasma Inositol Concentrations, Gene Expression of Hypothalamic Appetite Hormones, and Catecholamine Concentrations in Broiler Production
Type of DegreePhD Dissertation
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Phytase is an enzyme responsible for the hydrolysis of phytate at the carbon-phosphate linkage. Commonly, phytase is added in poultry diets at 500 to 1,000 phytase units (FTU)/kg to liberate phosphorus from phytate. However, when adding phytase for phosphorus liberation, previous research viewed the degradation of IP6 as the end goal of phytase supplementation. Unfortunately, lower phytate esters may also have a similar capacity to chelate nutrients, which causes these nutrients to be unavailable to broilers. Increasing phytase dose may be an alternative to rapidly hydrolyze phytate with lower number of phosphate groups. The benefits of adding phytase in a high dose (beyond 1,500 FTU/kg) to broiler diets have been realized through the improvement in growth performance beyond phosphorus liberation. While these benefits have mainly been attributed to the enhancement of nutrient utilization, other contributors such as liberation of inositol, degradation of lower phytate esters, and feed intake stimulation may also warrant investigation as participants of extra-phosphoric effects of phytase. Five experiments were conducted to evaluate the effects of phytase beyond phosphorus liberation in broilers. Experiment 1 was conducted to determine the influence of phytase supplementation on plasma inositol concentrations for its use as a biomarker for phytase efficacy. Experiments 2 and 3 were designed to evaluate the extent of phytate degradation and inositol liberation through high phytase doses (0, 500, 1,500, 4,500, 13,500, and 40,500 FTU/kg). Experiments 4 and 5 aimed to determine if the benefits of extra-phosphoric effects of phytase specifically originate from the increased nutrient intake. In Experiment 1, despite observing phytate degradation and inositol liberation in the gastrointestinal tract of broilers, plasma inositol concentration of broilers was not altered (P > 0.05) by phytase supplementation. Phytase inclusion up to 40,500 FTU/kg in Experiments 2 and 3 removed 98% of all phosphate groups of phytate and increased inositol liberation, which resulted in enhanced (P < 0.05) growth performance and carcass characteristics of broilers. In Experiment 4, the resulting higher amino acid digestibility and AMEn due to phytase addition was used to calculate nutrient uplifts in Experiment 5. In Experiment 5, additive effects of phytase and nutrient uplifts were observed (P < 0.05) on BW gain, FCR, and carcass characteristics. The effects of phytase supplementation on increasing feed intake of broilers approached significance (P = 0.06) compared with broilers fed diets without phytase inclusion. Interestingly, hypothalamic dopamine concentration was greater (P < 0.05) in broilers provided diets with phytase addition compared with those receiving diets without phytase. Nevertheless, hypothalamic gene expression of appetite hormones was not influenced by phytase and/or nutrient uplifts. Results of this research provide evidence on the extra-phosphoric effects of phytase on phytate degradation, inositol liberation, and feed intake. However, the mechanism of phytase supplementation on feed intake stimulation remains inconclusive, which warrants future research.