THE RESPONSE OF BROILER CHICKS TO PHASE FEEDING FOR P, CA AND PHYTASE

This trial with 2800 broiler chicks (0-6 weeks of age) was performed t o study the influence of different phase feeding schedules for both Ca and P and the bio-effectiveness of microbial phytase on zootechnical performances and bone mineralisation. The zootechnical results, feed i ntake and weight gain, were clearly affected by dietary Ca- and P-conc entration, Ca/P-balance, and phytase supplementation. Feed efficiency was only slightly affected by dietary treatments. Bone mineralisation results were also clearly affected by dietary Ca- and P-concentration, Ca/P-balance and phytase supplementation. Ln comparison with the mono -phase feeding schedule, dietary Ca- and P-levels can be further dimin ished during the growing period (4-6 weeks of age) of a two phase feed ing schedule. As a consequence, P-excretion can be obviously decreased . A wider Ca/P-ratio resulted in an increased bone mineralisation and reduced incidence of tibial dyschondroplasia. Reductions in body weigh t could be nearly completely compensated by phytase supplementations, but the reductions in bone mineralisation were only for the most part compensated by phytase supplementation. P-excretion can be reduced gre atly after supplementing low P-diets with phytase. Based on linear reg ression analysis, the average P-equivalency value of microbial phytase mas 584 (the average of 460 and 709; the latter in turn being the ave rage of 1020, 408 and 699) U/g P (from DCP . 2 H2O), thereby greatly d epending on dietary P- and Ca-concentration. Based on response surface modelling the optimal Ca/P-ratio was about 2.30 for weight gain and a bout 2.20 for bone mineralisation; the corresponding requirements of C a and P were about 0.91 and 0.40 g/day, respectively.