The apparent ileal phosphorus digestibility is not always correlated with the dietary phosphorus concentrations in the raw material, according to studies from Massey University in New Zealand.
The Massey University research team studied this estimating the true ileal digestibility of phosphorus (P) in wheat, sorghum, soybean meal, and corn distiller's dried grains with solubles (DDGS) in broiler chickens. Four semipurified diets were formulated from each ingredient (wheat and sorghum: 236.5, 473, 709.5, and 946 g/kg; soybean meal and corn DDGS: 135, 270, 405, and 540 g/kg) to contain graded concentrations of nonphytate P. The experiment was conducted as a randomized complete block design with 4 weight blocks of 16 cages each (5 birds per cage). A total of 320 21-d-old broilers (Ross 308) were assigned to the 16 test diets with 4 replicates per diet. Apparent ileal digestibility coefficients of P were determined by the indicator method and the linear regression method was used to determine the true P digestibility coefficients.
Effects depends on raw material
The results showed that the apparent ileal P digestibility coefficients of wheat-based diets were not influenced (P > 0.05) by increasing dietary P concentrations, whereas those of diets based on sorghum, soybean meal, and corn DDGS differed (P < 0.05) at different P concentrations. Apparent ileal P digestibility in broilers fed diets with soybean meal and corn DDGS linearly (P < 0.001) increased with increasing P concentrations. True ileal P digestibility coefficients of wheat, sorghum, soybean meal, and corn DDGS were determined to be 0.464, 0.331, 0.798, and 0.727, respectively. Ileal endogenous P losses in birds fed diets with wheat, soybean meal, and corn DDGS were estimated to be 0.080, 0.609, and 0.418 g/kg DMI, respectively. In birds fed sorghum-based diets, endogenous P losses were estimated to be negative (-0.087 g/kg DMI). True digestible P contents of wheat, sorghum, soybean meal, and corn DDGS were determined to be 1.49, 0.78, 5.16, and 5.94 g/kg, respectively. The corresponding nonphytate P contents in wheat, sorghum, soybean meal, and corn DDGS were 1.11, 0.55, 2.15, and 4.36 g/kg, respectively.
These differences between digestible P and nonphytate P contents may be suggestive, at least in part, of overestimation of P digestibility under the calcium-deficient conditions used in the regression method.
This study was published in the December issue of the Journal of Animal Sciences.