Phosphorus requirements of broiler chicks three to six weeks of age as influenced by phytase supplementation

Two studies of identical design were conducted in battery brooders utilizin g male chicks of a commercial strain. The birds were grown to 3 wk on diets with adequate P and from 3 to 6 wk were fed diets ranging from 0.10 to 0.4 5% nonphytate P (nPP) in increments of 0.05%, with or without supplementati on with 800 units of phytase per kilogram of diet. Measurements included BW gain, feed conversion ratio (FCR), mortality, tibia ash, and fecal P conte nt. Nonlinear regression was used to estimate nPP needs for optimizing BW g ain, feed conversion, and tibia ash. In the absence of phytase, nPP levels of 0.33, 0.186, and 0.163% were requi red to optimize tibia ash, BW gain, and FCR, respectively. The estimated le vel for optimum tibia ash is in close agreement with current NRC (1994) rec ommendations. In the presence of 800 units of phytase per kilogram, nPP lev els of 0.24, 0.151, and 0.109% were needed to optimize tibia ash, BW gain, and FCR, respectively. Fecal phosphorus levels were markedly reduced at the lower P levels. Further studies are needed to determine whether maximum ti bia ash values are needed to sustain optimum production of market broilers.