The digestibility of three calcium phosphates for pigs as measured by difference and by slope-ratio assay

Taking into account a series of factors possibly responsible for the wide range in P-digestibility of Ca-phosphates for pigs as found in the literature, an experiment was performed in order to measure, as accurately as possible, the apparent digestibility of three Ca-phosphates: di-hydrated dicalcium-phosphate; anhydrous dicalciumphosphate and monocalciumphosphate. A digestibility experiment involving seven treatments with twelve 35-40 kg castrates per treatment, was carried out. Total faeces collection was applied over 10 days. The control diet was a very low-P, semi-synthetic diet (0.37 g P/kg) supplemented with 1 or 2 g of P from each of the three phosphates investigated. In order to guarantee the absolute intake of the supplemented feed phosphates, the phosphates were weighed separately per meal and, after double checking, hand mixed with the basal diet in the feeding trough of the metabolic cages. The results of the P-digestibility measurements were virtually independent of the method of calculation: difference method versus regression analysis (slope-ratio-technique). It may be concluded that the three Ca-phosphates differ significantly in apparent digestibility: CaHPO4 . 2H(2)O, 72.9%, (95% confidence limits: 68.6-77.2); CaHPO4, 63.0%, (95% confidence limits: 58.9-67.2); Ca(H2PO4)(2), 91.6%, (95% confidence limits: 87.5-95.8). The digestibilities of CaHPO4 . 2H(2)O and CaHPO, are close to the Dutch standards. The digestibility of Ca(H2PO4)(2) however, is much higher (91-92% versus 83%), but in close agreement with some recent German findings. In relative terms, these results show that CaHPO4 is 31% less digestible and CaHPO4 . 2H(2)O. is 20% less digestible than Ca(H2PO4)(2).