Evaluation of techniques for improving phosphorus utilisation in meat poultry

Phosphorus plays an important role in bone development and mineralisation, but approximately two-thirds of the phosphorus contained within the plant-based ingredients of poultry feed occurs in the form of phytate which is poorly available to poultry. Consequently, the low availability of phytate phosphorus necessitates the inclusion of supplementary dietary phytase enzyme or inorganic phosphorus (a finite global resource). However, inorganic phosphorus is often added at levels that exceed requirement to ensure dietary sufficiency, as there are concerns that the current phosphorus requirement guidelines (NRC, 1994) may not accurately reflect the current physiological needs of the modern broiler strain. To this end, the accurate evaluation of the effect of dietary phosphorus supply on the skeleton of modern broilers is required. The overarching aim of this project was twofold: to characterise the bone and whole body phosphorus content of commercial broilers, and to examine possible approaches for improving phosphorus utilisation in broilers. Before accurate evaluation of broiler phosphorus requirements or response to interventions could be assessed, it was necessary to establish the optimum method for determining bone ash content. Four investigations, conducted using two bird trials were initially carried out to examine the effect of four common divergences in the bone ash methodology: the effect of fat extraction, the inclusion of cartilage caps, the minimum fat extraction time required and the effect of autoclaving prior to fat extraction. Sensitivity in elucidating differences in treatment means improved for tibia ash % when fat was extracted, and when cartilage caps were removed from the bones prior to ash determination. However, increase in ash % have been reported when cartilage caps are affected by disease, and it may be advantageous to include the cartilage caps for better comparison of data, particularly when the health status of a bird flock is unknown. A minimum fat extraction time of 6h using the Soxhlet procedure iv was adequate in extracting fat from bones while autoclaving the bones prior to fat extraction did not have any significant effect on bone ash or ash %. There is a current lack of bone ash and phosphorus content reference dataset for commercial broilers. A normal range of age and sex-related normal bone morphology and mineralisation values of healthy commercial broilers were sequentially sampled across 6 commercial farms from 24 flocks of birds and data presented. The whole body phosphorus content determined showed that, despite the improvements in growth rate and muscle mass observed in modern broiler strains, when broilers are fed nutritionally adequate diets whole body phosphorus content retained (5g/kg) has remained constant. The partitioning of calcium in relation to phosphorus in the whole body of broilers has also remained the same. A 4-day transient reduction in available dietary phosphorus during early life not only confirmed previous findings that nutritional imprinting improves phosphorus utilisation in later life, but also leads to improved bone strength. The nutritionally imprinted group of birds had significantly stronger bones (P < 0.012) than the control group of birds. Finally, the potential use of a high phytase wheat (HIGHPHY) variety with high purple acid phosphatases content was examined in this project by feeding broilers with standard wheat diets, which were replaced with graded levels of HIGHPHY (33%, 67% and 100%). A 100% substitution of standard wheat with HIGHPHY resulted in the best ileal digestibility coefficients for P and Ca (34.6% and 22.9 % greater than control respectively). This project demonstrates that HIGHPHY has a promising potential for improving phosphorus digestibility in animal feed, and provides fundamental initial data into the use of acid phosphatase phytase in broilers through plant breeding to improve the phytase activity of grains. To conclude, a minimum Soxhlet fat extraction time of 6h with cartilage caps included is proposed to be adopted by researchers in order to improve accuracy when comparing bone ash data from unrelated studies. The dataset of bone measurements reported in this thesis provides a significant contribution to the knowledge gap of the current range of bone data applicable to commercial broiler production. Nutritional imprinting for better phosphorus utilisation and the use of high phytase wheat varieties are useful techniques that can be employed to improve phosphorus utilisation in broilers.