Modelling phosphorus intake, digestion, retention and excretion in growing and finishing pigs: model description

Abstract

Low phosphorus (P) digestibility combined with intensive pig production can increase P diffuse pollution and environmental load. The aim of this paper was to develop a deterministic, dynamic model able to represent P digestion, retention and ultimately excretion in growing and finishing pigs of different genotypes, offered access to diets of different composition. The model represented the limited ability of pig endogenous phytase activity to dephosphorylate phytate as a linear function of dietary calcium (Ca). Phytate dephosphorylation in the stomach by exogenous microbial phytase enzymes was expressed by a first order kinetics relationship. The absorption of non-phytate P from the lumen of the small intestine into the blood stream was set at 0.8 and the dephosphorylated phytate from the large intestine was assumed to be indigestible. The net efficiency of using digested P was set at 0.94 and assumed to be independent of BW, and constant across genotype and sex. P requirements for both maintenance and growth were made simple functions of body protein mass, and hence functions of animal genotype. Undigested P was assumed to be excreted in the feaces in both soluble and insoluble forms. If digestible P exceeded the requirements for P then the excess digestible P was excreted through the urinary flow; thus the model represented both forms of P excretion (soluble and insoluble) into the environment. Using a UK industry standard diet, model behaviour was investigated for its predictions of P digestibility, retention and excretion under different levels of inclusion of microbial phytase and dietary Ca, and different non-phytate P : phytate ratios in the diet, thus covering a broad space of potential diet compositions. Model predictions were consistent with our understanding of P digestion, metabolism and excretion. Uncertainties associated with the underlying assumptions of the model were identified. Their consequences on model predictions, as well as the model evaluation are assessed in a companion paper.Keywords: modelling, phosphorus, phytase enzymes, phytate, pig ImplicationsCurrently there is some disagreement about the phosphorus (P) requirements of pigs of different genotypes and how digestible P contents of pig diets are calculated, especially for diets that include different amounts of phytate and nonphytate P. Achieving a balance between meeting digestible P requirements for optimum growth and health, and avoiding excess P intake would lead to a reduction in diffuse P levels in manure and effluents, and environmental impact from pig systems. A simulation model that predicts P intake, digestion, retention and excretion is the first, necessary step towards achieving this aim. IntroductionPhosphorus (P) is an important mineral for both the metabolism and skeletal development of the growing pig (NRC, 2012).In pig diets, P is the third most expensive nutrient required, after carbohydrates (energy) and protein. The high cost of P is due to the low digestibility of plant dietary P, which results in the need to sup...

Quantifying the consequences of nutritional strategies aimed at decreasing phosphorus excretion from pig populations: a modeling approach

“…Single animal model description The dynamic, deterministic pig growth model of Wellock et al (2003), as adopted by Symeou et al (2014a) was used to predict the fate of dietary P in groups of pigs. Briefly, the model represented the limited ability of pig endogenous phytase activity to dephosphorylate phytate as a linear function of dietary calcium (Ca).…”

“…P requirements for both maintenance and growth were made simple functions of body protein mass, and hence functions of animal phenotype undigested P was assumed to be excreted in the feces in both soluble and insoluble forms. For justification of the values of the model parameters and mathematical relationships, the reader is referred to Symeou et al (2014a). The model was extensively evaluated by Symeou et al (2014b) and was found to predict satisfactorily the quantitative pig responses, in terms of P digested, retained and excreted, to variation in P supply, Ca and exogenous phytase supplementation.…”

“…The potential rate of protein retention was determined by pig phenotype and current protein weight only. The maximum (potential) protein retention was then used to determine the potential gains of the other chemical components, including P (Emmans and Kyriazakis, 1997;Wellock et al, 2003;Symeou et al, 2014a). Potential ADG was the sum of the potential gains of protein, lipid, ash (including P) and water.…”

“…If the feed was deficient in P then the actual, as opposed to potential rates of P retention were calculated. Symeou et al (2014a and2014b) predicted the P digestion, retention and ultimately excretion in growing and finishing pigs of different genotypes, offered access to feeds of different P content. The total P excreted comprised of fecal and urine P. The feces contained both insoluble and soluble P, while urinary P was only soluble (Jendza and Adeola, 2009;Selle et al, 2011).…”

“…The total P excreted comprised of fecal and urine P. The feces contained both insoluble and soluble P, while urinary P was only soluble (Jendza and Adeola, 2009;Selle et al, 2011). For a complete description of the model including inputs and outputs, see Symeou et al (2014a and2014b).…”