Determination of Calcium and Phosphorus Digestibility in a Short-term Bioassay with Broilers

Calcium digestibility can vary dramatically depending on the Ca source, limestone solubility, source of phytate and addition of phytase (Li et al., 2021). However, there is no agreement in the literature about the most appropriate method for determining Ca digestibility (Li et al., 2017). In the current study, the apparent ileal Ca and P digestibility was determined in diets varying in Ca content, with and without phytase supplementation, based on the short-term protocol described by Kim et al. (2019).

Day old, Ross 308, male chicks were fed a wheat-sorghum-soybean meal broiler diet. The diet contained 0.65% Ca and 0.25% ileal digestible P with supplemental phytase at 500 FTU (AXTRA® PHY TPT 10,000) and a mixture of carbohydrases (AXTRA® XB 201 TPT). On day 20, the birds were weighed and allocated by stratified randomisation into cages, with 8 birds per cage and 8 replicates per treatment. There were 4 dietary treatments. The control birds were fed the mash basal diet (Diet 1) containing low Ca maize and soybean meal diet (Ca = 0.15 %), with an added indigestible marker, celite, at 20 g/kg. Diet 2 consisted of the basal diet, plus phytase at 1000 FTU/kg. Diet 3 consisted of the basal diet to which was added limestone (PureCal 12-40) to increase the dietary Ca concentration to 0.65%. Diet 4 was prepared by supplementing Diet 3 with phytase (1000 FTU/kg). After 36 hrs of feeding, the contents of the distal half of the ileum were collected, pooled per replicate, freeze dried and ground. Feed and digesta were analysed in duplicate for Ca, P. Ileal digestibility coefficients were calculated and are shown in the Table.

The data were analysed using the General Linear Model procedures (Minitab version 17.0). The significance level is P < 0.05. The results show the impact of both dietary concentrations of Ca and phytase on Ca and P digestibility. The negative impact of Ca concentrations can be largely overcome by dietary supplementation with phytase. These relationships have been described in the literature (Li et al., 2017) and in the present study, they demonstrate that this bioassay is appropriate for determining Ca and P digestibility. This short-term feeding procedure avoids some of the difficulties of determining digestibility, namely the physiological adaptation to P deficient or Ca and P imbalanced diets when longer feeding periods are used, making it suitable for Ca and P digestibility determination.

ACKNOWLEDGMENTS: Poultry Hub of Australia and Feedworks provided funding.