The effects of various dietary phosphorus levels on the circadian patterns of plasma 1,25-dihydroxycholecalciferol, total calcium, ionized calcium, and phosphorus in laying hens.

Six hundred and sixty 75-wk-old Hy-line W36 hens were allocated to one of three dietary levels of total phosphorus, .30, .60, or .90%. Birds were fed the diets for 3 days following which blood samples were collected at six different times, 2, 6, 10, 14, 18, and 22 h postoviposition (POP), and analyzed for 1,25-dihydroxycholecalciferol [1,25-(OH)2 D3], total calcium (TCa), ionized calcium (Ca++), percentage Ca++ to TCa (%Ca++/TCa), and phosphorus (P). Plasma TCa and P significantly (P less than .001 and .025, respectively) peaked at 10 to 14 h POP. The Ca++ and %Ca++/TCa significantly (P less than .001) decreased during eggshell formation and following completion of the shell (22 h POP) levels returned to resting concentrations. Plasma 1,25-(OH)2D3 results confirmed the existence and time of a circadian rhythm in laying hens. Peak concentrations of the metabolite occurred at 10 to 14 h POP, which resulted in a quadratic relationship (P less than .001). Plasma P decreased with decreasing dietary P and plasma 1,25-(OH)2D3 increased (P less than .025). Feeding low dietary P significantly (P less than .001) increased Ca++ and %Ca++/TCa. Results of feeding various levels of dietary P to laying hens indicate that low P stimulates an increase in plasma 1,25-(OH)2D3 as well as Ca++ and %Ca++/TCa, but high P actually suppressed this response.