Effect of High Dietary Calcium and Wide Calcium-Phosphorus Ratios in Broiler Diets

Abstract

The effect of varying dietary calcium (Ca) and phosphorus (P) levels and ratios on broiler performance and the incidence of leg abnormalities was studied using 180 Cobb broilers. A 3 × 2 factorial arrangement of treatments involving three Ca (1.0, 2.0, 3.0%) and two P (.6, 1.2%) levels was used. Manganese (Mn) was kept at 70 mg/kg for all diets. Only P main effects were significant for performance parameters, and Ca × P interractions effects were significant for leg abnormality scores and Mn status. High dietary P (1.2%) depressed (P<.01) gains (18.6 vs. 13.3 g/day), increased (P<.01) feed conversion ratio (2.4 vs. 2.8 kg/kg), and caused higher (P<.01) mortality (8.9 vs. 18.8%). At normal P (.6%), high dietary Ca (2.0% and 3%) disrupted Mn metabolism, causing a high incidence and severity of leg problems and associated bone disorders. Increasing dietary P to 1.2% attenuated these disruptive effects of high dietary Ca. This strategy, however, is not a satisfactory management alternative, because high dietary P adversely affected broiler performance and survival. The possibility of managing the condition by increasing dietary Mn was investigated in a second study in which 150 Cobb broilers grouped into five lots of 30 each were fed five diets containing graded levels of Mn (77.5, 140, 200, 255, and 300 mg/kg) for 6 weeks. All diets contained similar Ca (3%) and P (.6%) and adequate levels of other essential nutrients. Control birds (77.5 mg/kg) had the highest (P<.05) leg abnormality score (1.75), which decreased (P<.05) as dietary Mn increased up to and including 200 mg/kg, above which no additional reduction (P>.05) was observed. A similar trend was observed for tibia ash and volume, as well as the Mn status of the birds, but dietary Mn had no effect (P>.05) on performance parameters. Thus, the tendency to induce leg abnormalities of broiler diets containing Ca as high as 3% and, or Ca:P ratios as wide as 5:1, could be alleviated by increasing dietary Mn to 200 mg/kg.