Effect of age and dietary crude protein content on the apparent ileal calcium digestibility of limestone in broiler chickens

The objective of the present study was to investigate the effects of age on the apparent ileal calcium (Ca) digestibility of limestone for broiler chickens. Six treatment groups of different ages, namely days 1–7, 8–14, 15–21, 22–28, 29–35 and 36–42, were utilised. A maize-based diet (crude protein, 79 g/kg) containing limestone as the sole Ca source and, supplying 9.0 g/kg Ca, was fed to six replicate cages of broilers during each of the six periods. The birds were fed a commercial broiler starter diet until the introduction of the assay diet, except the day 1–7 age group. Ileal digesta were collected on day 7 (twelve birds per cage), 14 (ten birds per cage), 21 (eight birds per cage), 28 (six birds per cage), 35 (six birds per cage) or 42 (six birds per cage). A secondary objective was to examine the influence of dietary protein content on Ca digestibility. An additional diet containing a crude protein content of 153 g/kg was developed and fed to six replicate cages (eight birds per cage) from day 15 to 21 post-hatch. The birds in the age group of 15–21 days and fed the 79 g/kg crude protein diet served as the control treatment. Titanium dioxide (5 g/kg) was included in all diets as an indigestible marker for apparent ileal digestibility measurements. Total tract Ca retention, pH of gizzard digesta and relative weights of gizzard and gizzard digesta were also measured. Apparent ileal Ca digestibility coefficients declined linearly (P < 0.001) with advancing age, from 0.51 at day 7 to 0.27 at day 42. Ca retention coefficients declined quadratically (P < 0.01) with advancing age, from 0.56 at day 7 to 0.30 at day 28 and then plateaued. Age quadratically (P < 0.01) affected the gizzard pH where the pH declined from day 7 to 21 and then increased. Relative weights of gizzard and gizzard digesta were quadratically decreased (P < 0.001) with advancing age. Increasing the dietary protein content had no effect (P > 0.05) on the apparent ileal digestibility and retention of Ca. Increasing dietary protein had no effect (P > 0.05) on the relative weights of gizzard and gizzard digesta. In conclusion, the present results showed that the Ca digestibility of limestone in broilers, fed a maize-based diet with limestone supplying 9 g/kg Ca, declined with advancing age.