Effects of dietary total phosphorus concentration and casein supplementation on the determination of true phosphorus digestibility for broiler chickens

A study was conducted to compare the regression-driven estimates of true precaecal digestibility (TPD) of phosphorus (P) in soybean meal (SBM) for broiler chickens fed diets with or without casein supplementation at moderate or low total P concentration. A total of 768 male Ross 308 broilers were allocated to 12 diets in a 2 x 2 x 3 factorial arrangement of two total P concentration (moderate-P or low-P), two diet types (with or without casein) and three SBM levels (low, medium or high). There were 8 chicks per cage and 8 replicate cages per treatment group in a randomised complete block design. The birds were fed experimental diets form d 14 to 21 post-hatching. Chromic dioxide was used as an indigestible marker. The results showed that dietary casein supplementation improved body weight (BW) gain, feed intake and feed efficiency of broilers (p < .01). Broilers fed the moderate-P diets had greater dietary total P intake, precaecal flow and total tract output of P, precaecal digested P, and total tract retained P compared with birds fed the low-P diets (p < .01). Dietary casein supplementation increased precaecal digested P, total tract retained P, apparent precaecal digestibility (APD) and total tract retention (TTR) of P (p < .01). Broilers fed the moderate-P diets had a linear decrease in the APD and TTR of P with increasing SBM levels (p < .01). Precaecal digested P and total tract retained P increased with graded inclusion of SBM levels (linear, p < .01). Regression of precaecal digested P against dietary P intake showed that chicks fed the moderate-P diets had lower estimates of TPD of P in SBM than that for chicks fed the low-P diets (p < .05), while dietary casein supplementation had no effects on the determined values of TPD of P. In conclusion, the regression-driven true phosphorus digestibility in assay feed ingredients for broiler chickens was affected by dietary total P concentration.