EFFECTS ON DIETARY PHOSPHORUS OF TREATING COTTONSEED MEAL WITH CRYSTALLINE FERROUS SULFATE FOR THE PREVENTION OF BROWN YOLK DISCOLORATION

1. Two experiments were carried out to examine whether the adverse eff ects on laying hen performance of treating cottonseed meal (CSM) with crystalline ferrous sulphate heptahydrate (FSH) to prevent brown yolk discolouration, was a consequence of iron reducing the availability of dietary phosphorus. Two batches of CSM, with different free gossypol and cyclopropenoid fatty acid contents, were treated with FSH at a 4:1 weight ratio of iron to free gossypol, and incorporated into diets at 300 g/kg with or without supplementation with sodium dihydrogen phosp hate (SHP), and fed to hens. 2. FSH treatment of CSM reduced food inta ke and egg production. Although performance was improved by supplement ing the FSH-treated CSM diets with SHP, it was still, poorer than that achieved by hens fed a non-CSM control diet. 3. Brown yolk discoloura tion was prevented by treatment of CSM with FSH. Additional phosphorus did not produce any significant effects on brown colour development i n yolks, indicating that the bioavailable gossypol content of the diet s had not been altered; however, chemical analyses showed an apparent increase in the free gossypol content of the diets with the inclusion of SHP, suggesting that the assay gives misleading results.