PHOSPHORUS REQUIREMENTS OF LAYING HENS FED ON WHEAT-BASED DIETS

1. Three experiments were carried out to determine the phosphorus (P) requirements of laying hens aged 34 to 58 weeks (experiment 1), 59 to 70 weeks (experiment 2) and 22 to 50 weeks (experiment 3) given diets containing wheat, sorghum and soyabean meals as the main ingredients. Dietary total P (P-t) varied between 3.2 and 7.3 g/kg (experiment 1), 3.2 and 4.6 g/kg (experiment 2) and 3.0 and 6.6 g/kg (experiment 3). H ens were housed at either 18 degrees or 30 degrees C (experiments 1 an d 2) and uncontrolled temperature (experiment 3), and in experiment 2 diets were fed without or with a phytase supplement of 500 units/g. 2. Dietary P-t had no significant effect on production measures in any e xperiment. Increases in dietary P-t adversely influenced egg shell qua lity although uterine calcium (Ca), ATPase and carbonic anhydrase acti vities were unaffected. 3. A 3-d-feeding trial in experiment 1 gave ma ximum P-t retentions of 228 mg/d at 18 degrees C and 204 mg/d at 30 de grees C. These were obtained with diets containing, respectively, 4.6 and 6.0 g P-t/kg. 4. Plasma inorganic P (P-i) increased consistently w ith increases in dietary P-t at all temperatures but plasma total Ca, and tibia Ca and P, were unaffected. 5. The inclusion of the phytase s upplement in diets containing 3.2 and 4.6 g P-t/kg had an adverse effe ct on egg production at both temperatures in experiment 2. 6. A dietar y P-t concentration of 3.2 g/kg, providing a calculated 1.2 g availabl e P (P-av)/kg, with a dietary phytase activity of less than 200 units/ kg, satisfied the P requirements of the hens used in these studies. Ho wever, the data from experiment 3 suggest that the P-t requirement of some flocks fed on wheat-based diets may be lower than 3.2 g/kg.