Relative bioavailability of phosphorus in feed and agricultural phosphates for poultry

Nine hundred fifty male Hubbard chicks were used in a 21-d study (10 birds per battery cage) to determine relative bioavailability of P (RBP) in four feed-grade phosphates (FP) [two Brazilian dicalcium and two U.S.-made phosphates (di-monocalcium and defluorinated)] and four Brazilian agricultural grade phosphates (AP) [single (AP-1), and triple (AP-2) superphosphates, monoammonium (AP-3), and thermomagnesium (AP-4) phosphates]. The reference standard was a purified-grade calcium phosphate dibasic (SP). Phosphates were added to the corn-soybean control diet (22% protein; 0.40% P + 0.08% P from SP), providing 0.08 and 0.16% additional P. Calcium level was 1.0% for all diets. Slope ratio was used to determine RBP, with BW, bone ash (BA), or bone strength (BS) regressed on P added within each P source. A relative biological value (RBV) was estimated using BW, BA, and feed efficiency. Performance was depressed (P < 0.01) by AP as compared with FP; BW was decreased by 11%, and feed intake (FI) was decreased by 14%. Mortality increased (P < 0.05) by 154% (7 vs 2.8%). Phosphate source AP-4, which had the lowest content of P and a high content of F, Fe, Ba, Ti, and Th, was toxic based on a 44% decrease (P < 0.01) in BW, 46% decrease in FI, 19% decrease in BA (32.4 vs 40.0%), 55% decrease in BS (7.1 vs 15.8 kg), and mortality increase (P < 0.05) from 0.7 to 26% compared with the average of AP-1, -2 and -3. The RBP could not be estimated for AP-4; and average availabilities for FP and AP, respectively, were 100.6 and 107.6% (BW), 88.3 and 93.2% (BA), 84.2 and 96.3% (BS), and 100.0 and 99.9% (RBP). The AP varied in RBP, with particularly high values calculated for AP-3. Performance and bone parameters in this study were not strongly affected by high levels of potentially toxic mineral elements in certain AP; this result may be explained by the low levels of phosphate addition and the short duration of the feeding period (21 d). However, considering their relatively high levels of F, Fe, Mg, S, Ba, Ti, and Th, agricultural-grade phosphate may represent considerable risk of toxicity for use in animal diets.