Influence of dietary cholecalciferol, calcium, and phosphorus on urinary calcium in commercial Leghorn hens.

An experiment was conducted to determine the influence of various dietary levels of cholecalciferol, Ca, and P on urinary Ca and pH, and on plasma concentrations of inorganic P (Pi) and total Ca (TCa) in commercial Leghorn hens. All hens were fed a layer diet containing 500 ICU of cholecalciferol/kg for 30 days and then allocated equally to treatment diets. Twelve treatment diets were in a 3 x 2 x 2 factorial arrangement that comprised three levels of cholecalciferol (0, 2,200, and 4,400 ICU/kg), two Ca levels (3.75 and 5.75%) and two P levels (.3% and .7%). After 9 days of feeding, blood (12 hens per time per treatment) and urine (6 hens per time per treatment) were sampled at 8 and 16 h after oviposition. Urinary Ca and plasma TCa concentrations increased (P less than .05) when dietary cholecalciferol level was increased from 0 to 2,200 ICU/kg, but did not change when cholecalciferol was increased from 2,200 to 4,400 ICU/kg. High levels of dietary Ca elevated (P less than or equal to .05) the urinary Ca concentration of hens fed the low-P treatments. Low levels of dietary P reduced plasma P and increased urinary Ca and pH at all levels of cholecalciferol and Ca; however, the magnitude of increases in urinary Ca were not uniform across all levels of cholecalciferol and Ca. Increases in urinary Ca resulting from low dietary P were relatively small when the diet lacked cholecalciferol, but was increased three- to fourfold when the diet contained adequate or excess levels of Ca and excess but not toxic levels of cholecalciferol.