Oestrous Cycle

Oestrous Cycle

1. Oestrus Phase (in heat)

   • Duration is usually 7 days but depends on daylight so is shorter during the winter.
   • Signs
     • seeks out stallion or pays more attention to them
     • some mares move their ears forward and elevate their tail and squat to urinate to accept the stallion
   • Ovulation
     • Ovulation starts 1- 2 days after the mare goes out of heat. After ovulation the follicle is a CH and is not producing oestrogen, therefore the mare will not show signs of heat after ovulation.

2. Diestrus phase (out of heat)

   • Duration is around 14 - 15 days
   • Called 'cold' or 'out' of the luteal phase because it is dominated by the progesterone produced by the corpus luteum.
   • To anticipate when a mare will next be in heat you count forward 14/15 days after she finishes her heat. this is more reliable then assuming it will be exactly 21 days.
   • Signs
     • The mare will start to reject the stallion, she will do this by pinning her ears back and kicking him.

3. Hormonal events of the oestrous cycle

   • FSH
     • Responsible for the growth of new follicles, granulosa cell multiplication and LH receptor induction
     • peaks during diestrus
     • causes 1-2 waves of follicular growth
     • The first wave might start to grow after 9 days of ovulation. This wave might t be the only dormant follicle.
   • Inhibin
     • Inhibin is a protein produced by the granulosa cells od the dormant follicle
     • prevents further FSH production and therefore inhibits the growth of new follicles
     • The dormant follicle ensures selection of itself over all the other follicles by preventing their growth.
   • LH
     • LH rises at the beginning of the oestrus, and unlike other species, peaks 2 days post ovulation.
     • Has a long half life due to the silica acid residues present.
     • The final follicular rescue is LH dependant and the secretion of LH is modulated by endogenous oxytocin in oestrus
     • Repeated oxytocin administration or sexual stimulation may induce an LH peak and enhance the ovulatory surge
   • Progesterone
     • progesterone is from the transformed granulosa cells and rises shortly after ovulation.
     • The dormant hormone of diestrus
   • Prostaglandin
     • produced by the endometrium when there is no pregancy signal by the early embryo.
     • Enters the systemic circulation via A-V or lymphatic transfer and causes lysis of the mature corpus luteum.
     • The corpus luteum is responsive to prostaglandin about 5 days after ovulation
     • If the endometrium is damaged, there may be no prostaglandin production, so the mare may not return to estrus regularly.
     • Prostaglandin can be released by cervical stimulation, intrauterine infusions, biopsy, culture, or endometritis. This will result in a premature return to estrus.