Uric acid, urea, and ammonia concentrations in serum and uric acid concentration in excreta as indicators of amino acid utilization in diets for broilers

Five experiments were conducted to determine if serum uric acid, serum urea N (SUN), serum ammonia, and the uric acid content of the excreta (UAE) could be used to determine the efficacy of amino acid (AA) utilization in diets for broilers. All experiments were conducted with Ross x Ross 308 or 708 broilers from 0 to 14 or 0 to 18 d posthatching in brooder batteries. Treatments had 6 or 7 replications with at least 6 broilers per replicate pen. All diets were corn and soybean meal-based and formulated to contain 1.0% Ca and 0.45% nonphytate P and to meet or exceed the requirements of all nutrient requirements except total Lys, Met, and Thr (experiment 1) or Met (experiments 2 to 5). Experiment 1 consisted of 2 dietary treatments. Diet 1 was formulated to be deficient in Lys, Thr, and Met and diet 2 was formulated to be adequate in all nutrients. Broilers fed the AA-adequate diet had increased (P < 0.01 to 0.03) ADG, ADFI, and G: F compared with broilers fed the AA-deficient diet. Serum uric acid, SUN, serum ammonia, and UAE were not affected (P = 0.34 to 0.70) by dietary treatment. In experiments 2 to 5, diets contained 1.35% total Lys, 2 levels of Met (0.50 or 0.76 TSAA: Lys), and without or with Gly supplementation up to 2.32% Gly + Ser. Broilers fed diets containing supplemental Met in experiments 2 to 5 had increased (P = 0.01 to 0.03) ADG, ADFI, and G: F. Gain: feed was increased (P = 0.01 to 0.07) in broilers fed supplemental Gly. Serum uric acid and SUN were decreased (P < 0.01) after a 2-h fast in broilers fed supplemental Met and Gly. Serum uric acid and SUN also were decreased at other times after fasting, but the 2-h fast gave the most consistent response. Uric acid content of the excreta was decreased (P < 0.01) in broilers fed supplemental Met. Serum ammonia was decreased (P < 0.01 to 0.02) in experiments 2, 3, and 4 at varying times postfeeding but was not affected by diet in experiment 5. The results of this research indicate that serum uric acid, SUN, and UAE concentrations can be used as an indicator of AA utilization in broilers fed AA-adequate and AA-deficient diets.