45Calcium dynamics of hens laying thick- or thin-shelled eggs

An experiment was carried out to investigate rate of shell secretion in two groups of hens selected for differences in shell deformation. Forty 50-wk-old Single Comb White Leghorn hens from two groups shown to produce eggs differing in shell thickness were fed a 3.5% Ca diet. Hens in their third to fifth ovulatory cycle of a sequence were intubated with 15 μCi of 45Ca 0.5 h post-oviposition. At 6, 12, 15 and 24 h, post-intubation blood was collected from five hens, which were subsequently euthanized, and bone samples were taken.Hens producing thick shells had higher (P < 0.05) plasma proteins, although there was no difference (P > 0.05) in plasma Ca or plasma protein-bound phosphorus content. While plasma 45Ca specific activity decreased (P < 0.05) with post-intubation time, there were no differences (P > 0.05) in plasma 45Ca specific activity between the two groups of hens. There were no differences (P > 0.05) in Ca reserves of bone ends or medullary bone alone or combined for hens secreting either thick- or thin-shelled eggs, nor were there any changes (P > 0.05) in these parameters with post-intubation time. Total 45Ca and 45Ca specific activity of both bone ends and medullary bone decreased as the ovulatory cycle proceeded. Regression analysis showed that the rate of depletion of 45Ca from the bone compartment, as indicated by the slope coefficient, was greatest for hens selected to produce thicker shells (Y = 13.8 − 3.11X1, R2 = 0.351, RSD = ± 4.917 for thick-shelled hens; Y = 6.7 − 0.95X1, R2 = 0.360, RSD = 1.1479 for thin shelled hens, where Y is the percentage 45Ca-intubated bone; and X1 is the time in hours). Key words: Calcium, shell formation, bone