Animal Feed Science and Technology 141 (2008) 92–103 Available online at www.sciencedirect.com Live performance and environmental impact of broiler chickens fed diets varying in amino acids and phytase  W.A. Dozier III a,∗ , M.T. Kidd b , A. Corzo b , P.R. Owens c , S.L. Branton a a United States Department of Agriculture, Agriculture Research Service, Poultry Research Unit Mississippi State, MS 39762-5367, USA b Department of Poultry Science, Mississippi State University Mississippi State, MS 39762, USA c United States Department of Agriculture, Agricultural Research Service, Forage and Waste Management Unit, P.O. Box 5367, Mississippi State, MS 39762-5367, USA Received 11 July 2006; received in revised form 20 April 2007; accepted 27 April 2007 Abstract This research examined the effects of decreasing dietary amino acid density (AAD) and potential interactions of AAD and phytase on growth and nitrogen (N) and phosphorus (P) excretion of broiler chickens. In experiment 1, diets were formulated to high (H) or moderate (M) AAD during prestarter (1–7 days), starter (8–19 days), and grower (20–35 days) resulting in the following dietary treatments: HHH, HHM, HMM, and MMM. The HHH feeding regimen improved (P≤0.05) body weight gain (BWG) and feed conversion (FCR) from 1 to 35 days of age, but increased (P≤0.05) N content of the excreta by 180 g/kg. In Experiment 2, three diets were fed from 36 to 49 days varying in AAD (H, M, and low, L) with (500 units/kg) or without supplemental phytase. Broilers fed a diet formulated to L AAD had high FCR. Increasing AAD increased (P≤0.05) N excretion by 0.56 and 0.80 g per bird Abbreviations: AAD, amino acid density; N, nitrogen; H, high; M, moderate; L, low; BWG, body weight gain; FCR, feed conversion; CP, crude protein; AA, amino acid; Exp, experiment; P, phosphorus; TSAA, total sulfur amino acids; FI, feed intake; DM, dry matter  Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. ∗ Corresponding author. Tel.: +1 662 320 7505; fax: +1 662 320 7589. E-mail address: bdozier@msa-msstate.ars.usda.gov (W.A. Dozier III). 0377-8401/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.anifeedsci.2007.04.012