Gonadotrophin secretion during the periovulatory period in Galway and Finnish Landrace ewes and Finnish Landrace ewes selected for high ovulation rate.

Rates of ovulation differed significantly (P less than 0.01) among ewes of the different genetic lines. However, of the reproductive characteristics studied, only progesterone concentration at the height of luteal function, duration of oestrus, and interval from onset of oestrus to peak of the preovulatory gonadotrophin surge showed significant positive association with rate of ovulation. The pattern of secretion of LH during the periovulatory period did not differ in the Galway and Finnish Landrace breeds. The total amount of LH secreted during the preovulatory surge did not differ amongst lines. Similarly, no difference in the plasma concentration of LH at the height of the preovulatory surge was noted among Galway and reference Finnish Landrace lines. However, the concentration of LH at the height of the surge was significantly (P less than 0.05) reduced in the selected Finnish Landrace line. Plasma concentrations of FSH during the preovulatory period were significantly (P less than 0.05) elevated in the breed (Galway) with the lowest prolifcacy. When contrasted with either of the Finnish Landrace lines, the magnitudes of the preovulatory surge of FSH and the secondary surge of FSH were significantly greater (P less than 0.05) in Galway ewes. These results suggest that genetic difference in rate of ovulation among sheep breeds is not tightly coupled to quantitative differences in plasma concentration of gonadotrophic hormones during the periovulatory period.