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Evaluating Inorganic Feed Phosphate Type and Further Potential of Phytase Supplementation using a Commercial Broiler Model Angela Elsie Lamp Optimizing feed formulation of poultry diets may be achieved by proper utilization of nutrients and feed additives. This will provide a less expensive and nutrient efficient diet to the bird. An experiment was conducted to evaluate early broiler performance, tibia mineralization, and mineral digestibility of broilers fed diets that differed in inorganic feed phosphates (IFP) but that were formulated to be either similar in dietary non-phytate phosphorus (nPP) and calcium to phosphorus ratio (Ca:P), total mineral content, or calculated prececal digestible phosphorus (CPDP). Dicalcium phosphate (DCP), monodicalcium phosphate (MDP), monocalcium phosphate (MCP), and defluorinated phosphate were used in formulations with similar nPP and Ca:P. In addition to these four treatments, a mixture of MCP, monosodium phosphate, and DCP was used to assimilate total mineral content of MDP (DMM). Moreover, a mixture of MCP and silicon dioxide was used to produce a similar CPDP as the DCP diet (MCP+SiO2). The MCP+SiO2 diet had the lowest IFP inclusion. The study utilized a randomized complete block design with ten replications of ten male broilers fed each of six dietary treatments for 21 days. Linear contrasts were utilized to compare treatments of interest. Birds fed MCP demonstrated increased live weight gain, tibia ash (mg/chick), and mineral digestibility compared to DCP when diets were formulated to similar nPP and Ca:P (P < 0.05). When diets were formulated to similar total mineral content, MDP fed birds had an increase in tibia ash (%) and mineral digestibility compared to birds fed DMM (P < 0.05). Birds fed MCP+SiO2 had increased mineral digestibility compared to birds fed DCP when diets were formulated to similar CPDP (P < 0.05). Dietary inclusion and efficacy data suggest that diets formulated using CPDP may optimize feed P incorporation and utilization. A second experiment was conducted to evaluate correlations between in vitro phytase activity post pelleting and broiler performance, tibia mineralization, and plasma myo-inositol of broilers fed diets containing different phytase enzymes conditioned at three different temperatures (70, 80, and 90°C). Dietary treatments included a positive control (PC; 0.9% calcium (Ca) and 0.4% nPP) and negative control (NC; 0.7% Ca and 0.2% nPP) and four additional diets containing commercially available phytase products (A, B, C, and D) added to the NC based on analyzed activity and manufacturer’s recommendations. Treatments were arranged in a 6 (diet formulation) x 3 (conditioning temperature) factorial in a randomized complete block design with ten replications of ten male broilers fed each of 18 dietary treatments for 21 days. Correlations were determined with categorical and replicated data. A conditioning temperature x diet formulation interaction occurred for d14 and 21 FCR and d 21 tibia ash (P < 0.05). Day 14 and 21 FCR increased and d21 tibia ash decreased as conditioning temperature was increased from 70 to 90 ̊C for all phytase treatments except Phytase B that did not demonstrate performance or tibia ash detriment. Correlations were most apparent with replicated data. In vitro Phytase A activity correlated with d21 FCR (P = 0.04, r = -0.38), Phytase B did not demonstrate correlation (P > 0.05), Phytase C correlated with tibia ash mg/chick (P = 0.01, r = 0.46), and Phytase D showed a moderate correlation with tibia ash mg/chick (P = 0.16, r = 0.26) and plasma myo-inositol concentration (P = 0.15, r = 0.27). This study did not demonstrate a common in vivo performance metric that was correlated with in vitro phytase activity post manufacture among commercial phytase products that decreased in activity due to increased conditioning temperature. Both in vivo and in vitro measurements should be assessed to properly evaluate the efficacy of an enzyme. The third and final study conducted was conducted to determine the ME release of a commercial phytase fed to broilers at multiple doses in basal diets that varied in ME. Dietary treatments included three negative controls (NC; 0.75% calcium and 0.30% non-phytate phosphorus) formulated at three ME levels (2701, 2868, and 3000 kcal/kg). A commercial phytase product was added to each NC diet at 1500, 4500, 13500, and 30000 FTU/kg, creating a total of 15 dietary treatments. Treatments were arranged in a 5 (phytase dose) x 3 (ME level) factorial in a randomized complete block design with nine replications of ten male broilers fed each of 15 dietary treatments for 21 days. Alpha was designated as 0.05 for significance and 0.10 for trending data. For all measurement periods (d7, 14, and 21), FCR decreased as ME increased (P < 0.0001). On d21, ending bird weight and live weight gain (LWG) increased with the addition of phytase and as ME increased (P < 0.05). Furthermore, the addition of phytase significantly improved d14 and 21 FCR (P < 0.05). Trends were observed for ME level x phytase dose interactions for d41 and 21 FCR. These trends demonstrate that phytase is most beneficial with the low energy diets. Eight treatments were selected for AMEn analysis based on d14 and 21 FCR data; AMEn ranged from 2773 to 3457 kcal/kg. Diets formulated at 2701 kcal/kg dosed at 1500, 13500, and 30000 FTU/kg released 114, -179, and -2.76 kcal/kg, respectively. Whereas, diets formulated at 2868 kcal/kg dosed at 1500 and 13500 FTU/kg released 281 and 423 kcal/kg, respectively. Contrary to past studies, clear results were not observed for the energy releasing efficacy of graded levels of phytase. Performance improvements with phytase supplementation were likely more associated with P availability than AMEn availability.