Accounting for the effects of interactions between Phosphorous, Calcium and exogenous phytase on pig dietary requirements and performance

The project addresses BBSRC Strategic Priority: Agriculture and Food Security relating to the Animal Sector, and in particular the priority of Biotechnologies as applied to animal feed. It aims to investigate the interactions in the digestive tract of the pig between exogenous phytase and Phosphorous (P) and Calcium (Ca) contents in pig feed in order to lead to a better feed utilisation. This will be done in the context of an existing simulation model that supports a Decision Support Tool (DST) to reduce P excretion and improve growth in growing and finishing pigs in the UK.

Both Ca and P are well-known to have important skeletal and physiological functions. At the same time, P is the third most expensive component in pig diets due to the low digestibility of plant dietary P and the consequent need to supplement with expensive inorganic P supplements. From an environmental point of view, P supplies are limited and non-renewable and low feed conversion efficiency leading to the high excretion of water-soluble P in manure causes water pollution in the form of eutrophication. The Industry, e.g. AHDB Pork, recognises the importance of reducing P excretion whilst ensuring that pig performance and health are maintained, and has been involved in research to achieve these objectives.

The majority of plant based P and hence most P in pig feed is bound to the phytate molecule. This form of P is poorly digested by non-ruminant animals as they do not generate sufficient levels of endogenous phytase required for phytate dephosphorylation. One, increasingly popular way of improving bioavailability of plant based P is through supplementation of exogenous phytase. Exogenous phytase hydrolyses the phytate molecule to release inorganic P, which can then be absorbed by an animal. It has been estimated that exogenous phytase supplementation can lead to 30-60% reduction of P excretion, which would permit lower levels of supplement of P in pig diets.
However, in order to fully understand the problem at hand, digestion, absorption and excretion of P must be studied simultaneously with Ca, as these two major macrominerals are interdependent and interact in their utilisation by the pig in its digestive tract. Historically, Ca has not generated as much academic interest as P and Ca is considered to be the most underestimated factor contributing to phytate-P bioavailability. This is mostly due to the fact that inorganic Ca supplementation is cheap as well as widely accessible, and excessive Ca excretion does not explicitly cause any environmental concerns. There have been very few studies that have attempted to separate the effects of the Ca: P ratio on the basis of digestibility, which takes into account endogenous losses of nutrients, rather than total concentrations in feed ingredients. Over the past 10 years, the Industry has formulated diets on the basis of total Ca: digestible P ratio (2:1). Formulating diets based on digestible Ca: digestible P ratio is essential to minimise P for a more effective minimisation of P excretion.

Mathematical modelling can be seen as a useful tool to study P retention and excretion, incorporate factors such as Ca levels and phytase supplementation affecting these processes and in general, undertake many in-silico experiments which would not be possible in-vivo. Development of a new mathematical model accounting for the interactions between Ca and P should inform practices for overall reduction of the environmental impact caused by P and help to maximise nutrient utilisation benefiting swine production industry. The new model and DST will aim to accurately predict digestibility of Ca and its impact on animal excretion and growth. Simulation of the fate of digestible Ca in the model will lead to a more accurate prediction of reduction in P excretion and utilisation of P for growth. This objective has not been achieved so far by any prior model.