Measurement of true ileal calcium digestibility of feed ingredients for broiler chickens : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Institute of Veterinary, Animal and Biomedical Science (IVABS), Massey University, Palmerston North, New Zealand
The recent interest towards the use of digestible phosphorus (P) in poultry feed
formulations necessitates the measurement of true calcium (Ca) digestibility of feed
ingredients because of the close relationship between these two minerals for their
absorption and post absorptive utilisation. When this thesis research was initiated, no
published data were available on Ca digestibility of feed ingredients for broiler
chickens. The major objective of the studies reported in this thesis was to determine the
true Ca digestibility of feed ingredients for broiler chickens. In total, nine studies were
The first study (Chapter 4) was conducted to determine the effect of
methodology on ileal endogenous Ca losses. Three methods, namely feeding a Ca- and
P-free diet, maize gluten meal based diet and egg albumen based diet, were used. Ileal
endogenous Ca losses differed among different methodologies. The highest ileal
endogenous losses of 125 mg/kg dry matter intake (DMI) were recorded on the Ca- and
P-free diet, followed by 77 and 43 mg/kg DMI on maize gluten meal and egg albumen
In the second and third studies (Chapters 5 and 6), regression and direct
methods, respectively, were used to determine the true Ca digestibility of meat and bone
meal (MBM). The true Ca digestibility coefficient of MBM samples were ranged from
0.41 to 0.60. No difference was observed between true Ca digestibility coefficients of
MBM determined by regression and direct methods. Since the direct method is less
laborious and cost effective compared to regression method, this method was used in
subsequent studies (Chapters 7 to 10) to determine the true Ca digestibility of a range of
In fourth and fifth studies (Chapters 7 and 8), the influence of dietary P,
particle size and Ca to non-phytate P ratio was investigated on the true Ca digestibility
of limestone for broiler chickens. The true Ca digestibility of three limestone samples
varied from 0.56 to 0.62. Supplementation with recommended dietary P (4.5 g/kg)
increased the true Ca digestibility of limestone when compared to diets without P. An
increase in particle size from <0.5 to 1-2mm improved the true ileal Ca digestibility of
limestone. Widening the Ca to non-phytate P ratio reduced the true Ca digestibility of limestone for broiler chickens.
The sixth study (Chapter 9) was conducted to determine the effect of Ca source and particle size on the true Ca digestibility and total tract retention. Limestone and oyster shell were used as Ca sources. No difference was observed between the true Ca digestibility of limestone and oyster shell. An increase in particle size from <0.5 to 1-2 mm increased both the Ca digestibility and retention of both Ca sources, and increased the Ca concentration of gizzard contents.
The study reported in Chapter 10 was conducted to determine the true Ca digestibility of dicalcium phosphate (DCP), monocalcium phosphate (MCP), canola meal, poultry by-product meal and fish meal, and to compare the effect of dietary adaptation length on true Ca digestibility of DCP and MCP. The true Ca digestibility coefficients of these feed ingredients were lower than MBM, limestone and oyster shell, and ranged from 0.24 to 0.33. It was speculated that the length of adaption to the assay diets may be responsible for the lower than expected estimates. The effect of dietary adaptation length (24, 48 or 72 hrs) was subsequently examined, but had no effect on true Ca digestibility of DCP and MCP.
In the final study (Chapter 11), the true Ca digestibility of DCP was determined using different methodologies (regression, difference and direct methods). The true Ca digestibility coefficients of DCP were 0.34 and 0.21 with direct and different methods, respectively. A very low digestibility coefficient of 0.13 was determined by the regression method.
In conclusion, the true Ca digestibility coefficient of major Ca sources (limestone, oyster shell and MBM) is not high and varied from 0.40 to 0.70. Particle size of limestone and oyster shell influenced Ca digestibility, with coarser particles having higher digestibility. The direct method appears to be suitable for the determination of true Ca digestibility of limestone, oyster shell and MBM, but may not be appropriate for other Ca sources with intrinsic imbalance of Ca and P.