Ovulation and calcium metabolism in white leghorn hens (Gallus gallus)

Calcium status is a major factor in the regulation of reproductive activity in the hen. Restriction of dietary calcium (Ca) or vitamin D (D) is assumed to cause cessation of ovulation through decreased plasma calcium concentrations. Several studies suggest that there may be a threshold level of ionized calcium (Cai) below which ovulation will not proceed. The objectives of this thesis were to determine how Cai concentration is involved in the process of ovulation by comparing Ca and D-deficient hens, that had ceased laying, with control birds that were laying normally. A secondary objective was to determine the effects of multiple blood sampling (MBS) on the hen's ovulatory cycle. SCWL hens were divided into three groups-control, Ca-deficient and D-deficient groups and fed respective diets. Control birds were serially sampled every two hrs for 24-26 hrs immediately after oviposition until the next oviposition. Deficient birds, that had ceased laying for 10 to 14 days, were sampled at the same time. MBS was achieved with an indwelling vascular access port. Six birds/experimental group were used. Control birds were bled two weeks later from late afternoon until the following day at the same time. Whole blood was analyzed for Cai. Separated plasma was analyzed for total calcium (Cat), inorganic phosphorus (Pi), estrogen (E₂), progesterone (P₄) and l,25(OH)₂D₃ concentrations. Tibiae were ashed for mineral content. In expt. 1, the effect of MBS on the ovulatory pattern of hormones and ions was observed by sampling control birds twice, using two different time courses. Patterns and concentrations of the hormones and ions, regardless of time course, were similar to previous studies. Overall treatment effects were only significant between treatments with regards to total calcium and estradiol concentrations. The large loss of plasma proteins during the bleeding regime resulted in a steady decline in Cat over the 26 hrs, however, it was still within the physiological range of laying birds. . E₂ concentrations were also affected due to interruption of the laying sequence. However, this can be avoided since some birds continued to lay. In expt. 2, the control group had significantly higher mean plasma Cat and Pi concentrations and bone ash than both the deficient groups. Control and D-deficient groups had similar mean Cai concentrations, however, the ovulatory profile of the control group had a significant cyclic pattern over the 24-26 hrs, whereas, both deficient groups did not vary significantly over the 24 hrs. Plasma Pi concentration in the control group, not previously described, was cyclic in nature, related to the egg laying cycle. Plasma l,25(OH)₂D₃ concentrations were significantly higher in the Ca-deficient group than the control group. D-deficient birds had detectable levels of plasma 1,25(OH)₂D₃, but it was significantly lower than the control group. Plasma E₂ and P₄ concentrations were significantly higher in the control group In conclusion, it would appear that an inter-relationship exists among Cai, Pi and 1,25(0H)₂D₃ and the reproductive hormones. A threshold concentration of Cai may be the trigger for ovulation, perceived at the level of the pituitary, hypothalamus or ovary. A threshold of Pi and a window of l,25(OH)₂D₃ concentration may also have permissive roles in ovulation. In addition, MBS, regardless of time course, can be used as a method for determining ovulatory profiles in individual birds without seriously affecting ionic and hormonal concentrations and patterns.Land and Food Systems, Faculty ofGraduat