REEVALUATION OF CALCIUM AND PHOSPHORUS REQUIREMENTS OF LAYING HENS FOR OPTIMUM PERFORMANCE AND EGGSHELL QUALITY

A long-term experiment was conducted to re-evaluate the Ca and available P (AP) requirements of laying hens for optimum performance and eggshell quality. The experiment lasted from 20 to 64 wk of age and was divided into three phases: Phase 1, 20 to 36 wk of age; Phase 2, 36 to 54 wk of age; and Phase 3,52 to 64 wk of age. The treatments (T) consisted of diets with 3.5 to 5.5% Ca in increments of .5% and a constant level of AP (.4%) (T1 to T5); a step-up Ca phase feeding regimen of 3.5, 4.5, and 5.5% with a constant level of AP (.4%) (T6); a step--down AP phase feeding regimen of .4, .3, and .2% with a constant level of Ca (3.5%) (T7); a concurrent step-up Ca phase feeding and step-down AP phase feeding regimen with or without substitution of 50% oyster shell for pulverized limestone (T8 and T9); and a regimen similar to T9 with a step-up cholecalciferol phase feeding of 2,200,4,400, and 8,800 ICU/kg (T10). Increasing the dietary level of Ca did not have any beneficial effects on shell quality, indicating that the National Research Council (1984) estimated requirement of 3.75 g Ca per hen per day is adequate for optimum shell formation. A step-up Ca phase feeding regimen, a step-down A.P phase feeding regimen, or their combination, were without effect on shell quality. Also, increasing the dietary level of cholecalciferol with the hen's age did not have any beneficial effects on shell quality. The presence of oyster shell had a beneficial effect on shell quality during Phases 2 and 3 even when diets contained generous levels of Ca. Percentage retention of Ca was reduced and absolute retention of Ca remained constant with an increase in the dietary level of Ca in each phase of the experiment. Plasma and bone Ca levels were not influenced by dietary treatments in each phase of the experiment. Bone ash was not different among treatments in Phases 1 and 2 of the experiment. In Phase 3, bone ash of hens fed a step-down AP regimen of .4 to.3 to.2% with a constant level of Ca (3.5%) was significantly lower than hens fed the other dietary treatments. Egg production was not influenced by dietary treatments.