Response of growing pekin ducks to supplementation of monobasic calcium phosphate to low-phosphorus diets

The effects of a supplementation of P from monobasic calcium phosphate (MCP; Ca(H2PO4)(2)) to low-P basal diets were studied in growing Pekin ducks. Body weight gain and feed conversion were studied in two separate periods between Days 1 to 21 (Experiment 1) and between Days 21 to 49 (Experiments 2 and 3). Retention of P was measured by comparative slaughter technique in Experiment 1. Additionally, two balance trials with quantitative determination of intake and excretion of P were conducted between Days 12 to 17 and between Days 30 to 35. MCP was supplemented in 7 or 6 graded levels at the expense of sand. In cases when ANOVA showed a significant effect of MCP supplementation, the response of ducks was described by nonlinear functions. No significant effect of supplemental MCP on growth, feed intake or feed/gain ratio was detected in the period between Days 21 and 49 with a basal P level of 3.0 g/kg. Between Days 1 and 21, ducks needed 5.1 g P/kg diet to achieve 95% of y(max) in BW gain. The y(max) for P concentration in gained BW, determined from balance trials, was 5.6 and 5.1 g/kg between Days 12 to 17 and Days 30 to 35, respectively. Ninety-five percent of y(max) in P retention was achieved with a dietary P concentration of 6.2 and 4.3 g/kg between Days 12 to 17 and Days 30 to 35. The cumulative efficiency of utilization (retention/intake X 100) of dietary P from the basal diet was 49% (Days 12 to 17) and 43% (Days 30 to 35), and approached maximum with increasing supplementation of MCP of 55 and 53%; respectively, before it decreased again with further increase in MCP supplementation. The marginal efficiency of supplemental P. (,Deltay/Deltax) showed a maximum of 86% (Days 12 to 17) and 92% (Days 30 to 35), and this maximum was achieved where only 75 and 72% of y(max) in P retentions were achieved. It is concluded that ducks require a lower P concentration in the diet with increase in age, but that the efficiency of utilization of P from inorganic salts is not clearly affected by age. Conclusions regarding the P requirement largely depend on the response criterion chosen. Based on P retention data, a dietary level of available P is recommended to be 3.4 (Days 1 to 21) and 2.3 g/kg (Days 21 to 49), although growth was unaffected by P even at lower concentrations of available P. Future comparative studies on the availability of P from ingredients should be conducted at a dietary P concentration that allows for identifying the maximum in utilization.