Effect of age on the apparent ileal calcium digestibility of dicalcium phosphate and monocalcium phosphate in broiler chickens

The objective of the present study was to investigate the effects of age on the apparent ileal calcium (Ca) digestibility of dicalcium phosphate (DCP) and monocalcium phosphate (MCP) for broiler chickens. Six treatment groups of different ages, namely days 4-7, 11-14, 18-21, 25-28, 32-35 and 39-42 post-hatch, were utilised. Maize-based starter and finisher (228 and 201 g/kg crude protein) assay diets containing either DCP or MCP as the sole Ca source, supplying 9.5 and 7.0 g/kg Ca, respectively, were fed to six replicate cages of broilers during each of the six periods. Except for those age groups fed assay diets, birds were offered a commercial broiler starter diet until 14 days of age, and a commercial broiler finisher diet thereafter. Assay diets were offered for the 3 days prior to sampling in each age group. Ileal digesta was collected on days 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). Titanium dioxide (5 g/kg) was included in all diets as an indigestible marker for apparent ileal digestibility measurements. In vitro solubility of DCP and MCP ranged from 89-94% and 91-98%, respectively. During the starter period, the coefficient of apparent ileal Ca digestibility (CAID) was greater ( P = 0.04) at day 7 (0.69) compared to day 14 (0.59). Additionally, CAID of DCP (0.65) and MCP (0.63) were similar ( P > 0.05) during the starter period. During finisher periods, CAID decreased ( P < 0.001) with advancing broiler age (from 0.63 at day 21 to 0.40 at day 42). Additionally, CAID was greater ( P = 0.016) for DCP (0.54) compared to MCP (0.49). During starter period, digestible Ca intake (DCI) was greater for DCP compared to MCP on day 14, but not on day 7 (age × ingredient, P < 0.05). During finisher period, DCI increased until day 35 and decreased thereafter. Additionally, DCI was greater for DCP compared to MCP. During the starter period, gizzard pH decreased in birds fed DCP from 1.93 at day 7 to 1.58 at day 14, while pH remained unchanged in birds fed MCP (1.77) at both ages, resulting in an age × ingredient interaction ( P < 0.035). During finisher periods, gizzard pH of birds was higher ( P = 0.029) for DCP, and higher ( P < 0.001) at days 28 and 42 compared to other ages. Relative gizzard weight was higher ( P < 0.001) in birds fed MCP, and it decreased with advancing broiler age from day 28 to day 42. In conclusion, average digestibility of Ca in DCP and MCP was 0.65 and 0.63, respectively, during the starter period (day 7-14) and 0.54 and 0.49, respectively, during the finisher period (day 21-42). Broiler age influenced Ca digestibility, where digestibility decreased as age increased, during both the starter and finisher periods for MCP, and during the finisher period for DCP.