Relative bioavailability of phosphorus in low-phytate soybean meal for broiler chicks

A 14-d chick bioassay was conducted to estimate the relative bioavailability (RBV) of P in a low-phytate soybean meal (LPSBM) using slope-ratio techniques. A corn-soybean meal basal diet was formulated to supply total P and Ca at 3.3 and 10.5 g kg-1, respectively. Three reference diets were formulated by the addition of 0, 0.5 or 1.0 g kg-1 total P from monosodium phosphate (MSP). Four test diets were formulated by the addition of 0.5 or 1.0 g kg-1 total P from LPSBM or soybean meal (SBM). The additions of MSP, LPSBM, or SBM were made at the expense of corn starch. A diet consisting of the basal diet plus supplemental methionine, lysine, threonine and tryptophan was also included to confirm that response to diets containing LPSBM or SBM was not due to the higher concentration of amino acids in those diets. Each diet was randomly assigned to six replicate cages of four birds. Birds were fed from 1 to 3 wk of age. Body weight gain (P < 0.01) and feed intake (FI, P < 0.05) increased linearly as supplemental dietary P increased from 0 to 1.0 g kg-1 for chicks fed diets containing MSP or LPSBM. Chicks that received the basal P level had the lowest body weight gain and FI. No differences were observed between the basal diet and the amino-acid-supplemented basal diet in any of the response criteria measured indicating that the response to tests was due to the increasing levels of dietary P. Tibia mineral content (TMC) and tibia ash weight (ASH) increased linearly (P < 0.001) in response to supplemental P from MSP, LPSBM and SBM. The RBV for LPSBM and SBM using ASH as the response criteria were estimated at 52 +/- 10% and 36 +/- 8%, respectively. The RBV for LPSBM and SBM using TMC as the response criteria were estimated at 61 +/- 9% and 39 +/- 7% , respectively. The RBV of LPSBM was 12 to 16 percentage points higher than SBM depending on the response criteria used for estimating RBV. The results of this study clearly indicate that P from LPSBM is more bioavailable than P from SBM.