Abstract
The effects of dietary levels of available phosphorus (AP) on calcium (Ca) tolerance of growing chickens were investigated in 3 experiments.
In the first experiment, increasing dietary Ca to 21.2 g/kg with AP at 5 g/kg significantly reduced tibia P, plasma P and increased plasma total Ca concentration without any significant effect on performance. With dietary Ca at 25.7 g/kg and AP at 5 g/kg, growth and plasma P were reduced and plasma total Ca was increased, whilst Ca at 30 g/kg diet reduced growth, plasma P, tibia P and increased feed conversion ratio (FCR) and plasma total Ca. Increasing dietary AP to 10 g/kg diet with Ca level at 10.6 g/kg reduced growth and increased FCR without affecting any of the other parameters.
Experiment 2 was a 3-dimensional composite design with a central basal diet having Ca and AP concentrations of 12.7 and 4.6 g/kg. The first dimension was 4 dietary concentrations of Ca from 15 to 30 g/kg; the second dimension was 4 dietary concentrations of AP from 6.3 to 12.4 g/kg; and the third dimension was 4 dietary Ca:AP combinations spanning the above ranges but with the Ca:AP ratio maintained at approximately 2.5 (i.e. 15.8 and 6.3; 20.5 and 8.2; 25.5 and 10.2; 30 and 12.1 g/kg for Ca and AP, respectively).
A dietary Ca concentration of 25.4 g/kg increased FCR, whilst 30 g Ca/kg reduced growth and increased FCR. Dietary AP of 12.4 g/kg reduced growth and increased FCR. Growth and FCR were less affected when dietary Ca:AP ratio was kept constant at approximately 2.5 than when the ratio was altered by increasing either mineral alone. This relationship was used to develop a linear regression relationship between chicken growth and dietary Ca:AP ratio.
In experiment 3, the effects of excessive dietary levels of Ca (12.7, 25.5 and 33.1 g/kg) and AP (9 and 13 g/kg) were investigated in sexed chickens in a factorial design. Ca at 33.1 g/kg significantly reduced growth, tibia P and plasma P, whilst FCR, plasma total Ca and excreta moisture were significantly increased. Dietary Ca at 25.5 g/kg reduced tibia P and increased excreta Ca. High dietary AP significantly reduced growth and tibia Ca and increased tibia ash, excreta moisture and excreta Ca. Significant interactions between dietary levels of Ca and AP were found for growth, tibia ash, excreta moisture and excreta Ca, and for tibia P and plasma total Ca. Significant interactions between sex and either dietary Ca or AP for tibia ash were found. A Ca X sex interaction was found for tibia P, excreta moisture and excreta P.
It was concluded that high dietary levels of Ca (up to 21.2 g/kg) can be tolerated by growing chickens without any significant effect on performance, providing the corresponding level of AP is also high.