Effect of GnRH and hCG on ovulation and pregnancy in mares.

This research investigates the ways in which Gonadotropin-Releasing Hormone (GnRH) and Human Chorionic Gonadotropin (hCG) affect ovulation and subsequent fertility rates in mares. The aim was to optimize the breeding process by enhancing the knowledge of ovulation patterns and improving controlling measures.

Ovulation in Mares

• The study notes the essentiality of understanding basic mating habits in mares to achieve satisfactory fertility rates. These habits include oestrous behaviour, duration of the oestrous cycle, ovulation, and the timing of insemination.
• Different mares show individual variations in the length of their oestrous period and ovulation which, combined with a short breeding season, can impact fertility rates.
• One significant cause of infertility is anovulation or delays in ovulation in mares that otherwise show normal oestrous behaviour.
• Examination of the ovaries and control of ovulation require methods like ultrasonography and rectal examinations.

Anoestrous and Oestrous Periods

• Anoestrus is a period of reproductive quiescence where no significant follicles are present, as revealed by ultrasonographic examinations.
• During anoestrous periods, the ovaries appear small and hard via rectal palpation with no discernible inner structures.
• At the onset of the breeding season, the ovaries soften, and several 10mm or more follicles can be palpated.
• During the transition to the oestrous period, follicles ranging between 20mm to 50mm start to appear, which are easily observed via ultrasonography.
• In the oestrous period itself, round, black follicles filled with fluid can be observed.
• Just before ovulation, the follicles reach a diameter of 35mm to 45mm, and ovulation rarely occurs in follicles with a diameter less than 30mm. About 90% of ovulated follicles are 35mm or more.

Application of LH, hCG, and GnRH

• Luteinising Hormone (LH), hCG, and GnRH are frequently administered to thoroughbred mares to induce ovulation, synchronize mating with ovulation, and reduce the number of matings.
• The effectiveness of these hormones is typically monitored through methods like ultrasonography.