Effect of calcium and phosphorus specifications and limestone solubility on mineral utilisation and performance on modern broilers.

Abstract

Modern broilers exhibit extremely rapid weight gain that needs to be supported by concomitant increases in the size and structural integrity of the skeleton. Calcium (Ca) and phosphorus (P) are vital minerals for metabolism, bone formation, and regeneration and in order to support the required rate of skeletal mineralization, an understanding of Ca and P requirements that matches the continuously changing genetic potential of modern broiler genetics is required. Poultry diets are formulated to total Ca (tCa) and either available P (AvP) or retained P (oP) using fixed estimates of the availability, or retention of dietary P. However, recent research has shown that characteristics of limestone included in diets such as the geology, purity, particle size and solubility speed can profoundly alter phytate P utilisation, Ca and P digestibility and phytase efficacy. These effects of limestone on Ca and P digestibility can therefore also potentially alter dietary recommendations for these nutrients. The study objectives were to assess the effects of three Ca and P recommendations (REC) on broiler performance, bone mineralisation and production costs, and mineral excretion and evaluate the effect of limestone solubility in broiler response to the Ca and P recommendations. The Ca and P REC were from a Dutch Nutrition Group (NL), University of Maryland (UMD), and Commercial (COM) following breed company recommendations. Further, the solubility profile of two commercially available limestones was identified, and these were utilised as either fast soluble (FS-LS) or slow soluble (SS-LS). A commercial type of corn-soy diet was fed ad libitum to 3600 as-hatched Cobb-500-day-old chicks for 32d, which were randomly allocated to six treatments with twelve replicates and 50 birds/pen in 72 pens. The study had a 2x3 factorial design with treatments arranged in a completely randomized-blocking design. All diets of all phases were supplemented with 2000 FTU/kg feed of a 6-phytase. Performance parameters were recorded at placement and further at 7, 10, 14, 21, 28, and 32 days of broiler age. Five birds/pen were sampled for tibia ash at 10 and 32d. Litter sampling for mineral excretion, and foot pad (FPD) and hock burn (HB) was done at 32d. Each pen was regarded as an experimental unit. The data were analysed using JMP (V.15) (SAS Inst. Inc., Cary, NC, 2016) in a two-way ANOVA in a mixed model. Differences between means were analysed using a protected Tukey’s test, and significant differences were reported at P < 0.05 level of significance. The FPD and HB data were analysed using a Rao-Scott Chi-square method. The REC had significant effect on BW performance throughout the duration of the study. The lower Ca and P contained in the NL-REC depressed 7d body weight (BW) by 1.6% and 2.5% compared to the COM and UMD REC and by 2.6% and 2.8% compared to the UMD and COM REC at 32d, respectively (P < 0.001). Feeding Ca and P (UMD-REC) improved the FCR by 0.02 and 0.03 compared to the COM and NL REC at 7 and 32d, respectively (P < 0.001). Low dietary Ca and P (NL-REC) were detrimental to bone mineralisation at 10 and 32d, irrespective of LS solubility (P < 0.05). Dietary treatments influenced the incidence of FPD and HB. Increased dietary Ca and P (COM) promoted high P excretion in litter over 32d irrespective of LS solubility (P < 0.001). A cost saving of ZAR0.17 and ZAR0.21 cents can be made in feed cost/kg broiler harvested at 32d on the UMD REC compared to the COM and NL REC, respectively (P < 0.05). These results suggest that Ca and P REC, and LS used in commercial broiler diets can impact live performance, bone mineralisation and production costs.