The fate of dietary phosphorus in the digestive tract of growing pigs and broilers

The reduction of the phosphorus (P) content in the diet without affecting performance is an environmental and economic concern for sustainable pig and poultry production that requires precise knowledge of P requirement and its availability. A prerequisite is thus a good understanding of the fate of ingested P in the digestive tract, according to its origin (phytic, non phytic, mineral), its interaction with dietary calcium (Ca) and exogenous phytase supply. With this aim, a mathematical model was constructed using in vitro or in vivo literature data. The core of the model is based on a compartmental structure which distinguishes three sections in which successively occur: (1) P solubilisation and phytic P hydrolysis, (2) P absorption and (3) P insolubilisation. To better understand the physicochemical and transit characteristics that could be key factors in P utilisation, parameterisation of equations was conducted in both growing pigs and broilers. Sensitivity analysis indicated that P solubilisation/insolubilisation and especially phytic P solubilisation prior to hydrolysis by phytase were sensitive to pH and dietary Ca supply. Transit time also modulated the extent of P hydrolysis. Behaviour analysis indicated that the model was able to predict the lower efficacy of plant phytase compared with microbial phytase in both species. The negative impact of dietary Ca on digestive P utilisation was higher in pigs than in broilers due to physicochemical differences in the gastric section. The model accurately predicted apparent digestibility of P in pigs. External validation could not be performed in broilers due to lack of digestibility data. Before the model can be used as a predictive tool, additional information is needed to validate some parameters, particularly for broilers. The model offers the potential to be combined with a metabolic model that integrates regulation of P and Ca metabolism.