Is progesterone the key regulatory factor behind ovulation rate in sheep?

Area of Science:

• Reproductive Biology
• Animal Science
• Endocrinology

Background:

• Ovarian follicle waves in ewes follow a predictable pattern, with prolific breeds utilizing follicles from earlier waves for higher ovulation rates.
• Prolific sheep have shorter luteal phases, and progesterone (P4) influences follicle wave emergence and ovulation rate.

Purpose of the Study:

• To investigate if extending the luteal phase duration in prolific sheep impacts follicle wave dynamics and ovulation rate.
• To determine the effect of exogenous progesterone administration on ovulation rate and follicular development.

Main Methods:

• Two experiments were conducted on moderately and highly prolific ewes.
• Exogenous progesterone was administered to extend the luteal phase, while control groups received no treatment.
• Ovarian ultrasonography and blood sampling were performed daily and twice daily, respectively, to monitor follicle development and hormone levels.

Main Results:

• Extending the luteal phase with progesterone did not change the interovulatory interval duration in either breed.
• The number of ovulating follicles from the penultimate wave significantly decreased in treated ewes compared to controls in both experiments.
• Ovulation rate was reduced in moderately prolific ewes treated with progesterone, but not in highly prolific ewes.

Conclusions:

• Administration of progesterone at the end of the luteal phase suppresses the contribution of penultimate wave follicles to ovulation.
• The effect of extending the luteal phase on ovulation rate differs between sheep breeds with varying prolificacy.
• These findings suggest a role for luteal phase length in regulating follicle selection for ovulation in sheep.