Measurement of true ileal phosphorus digestibility in feed ingredients for poultry : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Poultry Science at Institute of Veterinary, Animal and Biomedical Sciences (IVABS), Massey University, Palmerston North, New Zealand
Global interest in improving the utilisation of phosphorus (P) by poultry has recently increased due to concerns over environmental pollution through excess P excretion, depletion of non-renewable inorganic phosphate deposits, and increasing price of inorganic phosphate supplements. Use of a sound criterion, preferably based on P digestibility, to assess P availability is needed to enable greater efficiency of utilisation of dietary P. No established methodology is currently available to measure the true digestible P contents in common feed ingredients for poultry.
The first experiment of this thesis (Chapter 3) investigated the effects of dietary calcium (Ca) concentrations (6, 9 and 12 g/kg) on the digestibility of P, Ca, nitrogen, fat and starch in different intestinal segments and on the apparent metabolisable energy of diets in young broiler chickens. The results showed that the digestion of P and Ca was completed by upper ileum and jejunum, respectively. The site of digestion of P and nitrogen was found to shift depending on the dietary Ca concentrations. The digestibility coefficients of P in low, normal and high Ca diets at the lower ileum were determined to be 0.417, 0.379 and 0.325, respectively. The overall data showed that increasing dietary Ca concentrations negatively influenced the digestion of P, nitrogen and fat, but had no effect on those of Ca, starch and apparent metabolisable energy.
The second experiment (Chapter 4) was conducted to determine endogenous losses of P and Ca in broiler chickens. The data showed that the ileal endogenous P losses in birds differed depending on the methodology employed. The ileal endogenous flow of P in birds fed P-free, gelatin-based and casein-based diets were 25, 104 and 438 mg/kg dry matter intake (DMI), respectively. Ileal endogenous flow of Ca in birds fed casein-based diet was estimated to be 321 mg/kg DMI.
The next three experiments (Chapters 5, 6 and 7) investigated the potential usefulness of regression method to evaluate true ileal P digestibility of seven feed ingredients. True ileal P digestibility coefficients of maize, canola meal, wheat, sorghum, soybean meal and maize-distiller‟s dried grains with solubles for broiler chickens were determined to be 0.676, 0.469, 0.464, 0.331, 0.798 and 0.727, respectively. For plant-based ingredients, the determined true digestible P values were consistently higher than corresponding non-phytate P values (Maize, 1.72 vs. 0.75; canola meal, 4.55 vs. 2.82; wheat, 1.49 vs. 1.11; sorghum, 0.78 vs. 0.55; soybean meal, 5.16 vs. 2.15; maize-distiller‟s dried grains with solubles, 5.94 vs. 4.36 g/kg, as fed
basis, respectively). Phytate P in maize (54.25%), soybean meal (69.7%) and maize- distiller‟s dried grains with solubles (41.5%) were well digested by broilers compared to canola meal (25.2%), wheat (18.1%) and sorghum (13.0%). True ileal P digestibility coefficients of three meat and bone meal (MBM) samples ranged from 0.420 to 0.693. Total and true digestible P contents of three MBM samples (MBM-1, MBM-2 and MBM-3) were determined to be 37.5 and 26.0; 60.2 and 36.6; and 59.8 and 25.1 g/kg, as fed basis, respectively, suggesting that P in MBM is not highly digestible. The overall data suggested that the use of regression approach to estimate true ileal P digestibility in feed ingredients has number of limitations. Overestimation as a result of using Ca- and P-deficient diets and the negative endogenous P losses observed for some ingredients (canola meal, sorghum and MBM-3) were main concerns. Negative ileal endogenous P losses were also shown to be associated with low true ileal P digestibility in these ingredients.
In the final experiment (Chapter 8), two regression-based methodologies were compared for the measurement of true ileal P digestibility in maize and soybean meal. The results showed that the methodology influenced P digestibility in maize and soybean meal. The use of assay diets containing a narrow Ca:total P ratio yielded higher P digestibility for both ingredients.
In this thesis research, the regression method was used to determine true ileal P digestibility of ingredients, but this approach suffers from several drawbacks. The data reported in this thesis also demonstrated that high dietary Ca concentrations were detrimental to the digestibility of nutrients, particularly of P, nitrogen and fat.