Effects of dietary Ca concentration on the performance of broiler chicks fed various sources of dietary P with and without phytase supplementation

Two experiments were conducted to determine the effects of high dietary calcium (Ca) on broiler chicks fed low phosphorus (P) diets from either nonphytate (nPP) or phytate P (PP) with or without phytase. The response criteria that were analyzed was bird performance, tibia ash, Ca and P digestibility and duodenal P transporter mRNA abundance. Newly hatched broiler chicks were housed in raised-wire starter batteries (65.8 in2) for 16 d. Each experiment had 12 treatments were replicated 6 times using 8 chicks per cage that resulted in a total of 576 Cobb 500 chicks. Experiment 1 had a 3x2x2 factorial arrangement included 3 P treatments (0.225%nPP, 0.225 + 0.1%P from nPP and 0.225 + 0.1%PP), 2 concentrations of Ca (0.9 vs.1.7%Ca) and either (0 or 1,000 FTU of phytase). Experiment 2 also had a 3x2x2 factorial arrangement but set closer to commercial standards (0.325% nPP, 0.325 + 0.1% P from nPP and 0.325 + 0.1% P from PP), 2 concentrations of Ca (0.9 vs.1.4% Ca) with and without phytase (0 or 1,000 FTU of phytase). Body weight gain (BWG), feed intake, feed conversion and mortality were calculated over the 16d period. On 16d, all remaining chicks were euthanized, and the right tibia was analyzed for tibia ash weight (TAW) in mg/tibia and expressed as a percentage (TAP). All data were analyzed as a 3x2x2 factorial using ANOVA in JMP 14 (P ≤ 0.05). In Experiment 1, the phytase interactions were driving FI, which increased BWG (P ≤ 0.05). However, the phytase interactions did not influence bone mineralization (P ≤ 0.05). So, the diet is potentially more appetizing with phytase supplementation. Calcium x phosphorus interaction was driving bone mineralization, but not influencing BWG or FI (P ≤ 0.05). Additionally, bone mineralization was impacted when additional P was put into high Ca diets (P ≤ 0.05). Phytase upregulated P transporter mRNA abundance in normal Ca diets, but the results were muted in high Ca diets (P ≤ 0.05). Experiment 2 used more practical P and Ca conditions to validate the responses from Experiment 1 with the understanding that the potential responses would be blunted. The addition of phytase improved tibia ash when Ca and P were unbalanced (P ≤ 0.05). High concentrations of Ca lowered BWG and FI, but again the addition of phytase to the diet had a more beneficial effect (P ≤ 0.05). Diets with reduced P had an improved P and Ca digestibility and upregulation of P transporter mRNA abundance (P ≤ 0.05). Also, the addition of phytase improved both P and Ca digestibility and downregulated P transporter mRNA abundance (P ≤ 0.05). Overall, both experiments exhibited that source of P did not influence performance but did influence tibia ash. High calcium had a negative effect on performance and digestibility. However, phytase was able to ameliorate negative impacts of high calcium diets. The multiple interactions of Ca, P and phytase indicate a complex relationship among the three factors that require balancing when looking for optimal conditions commercially, especially to maximize performance and skeletal health while balancing cost and environmental considerations.