Oestrogen excretion in mares in relation to various reproductive states.

The research paper investigates the pattern of estrogen excretion in mares under varying reproductive states, discussing how aspects such as seasonal changes and different stages of the reproductive cycle impact hormone level fluctuations.

Research Methodology

• Four mares who had undergone ovariectomy (removal of the ovaries) were chosen for the study. This was done to monitor changes in hormone levels without the influence of the ovaries.
• In two of these mares, the procedure was done when urinary estrogen concentration was high. Their hormone levels fell to a minimum within 48 hours of ovariectomy.
• Oestradiol-17beta (a form of estrogen) was intramuscularly injected in these mares. This resulted in maximum urinary estrogen levels within 3 to 6 hours, returning to the minimum detection level within 24 hours.
• The predominant estrogen in mares’ urine was found to be estrone.

Investigating Seasonal and Reproductive State Changes

• The study was extended to nine maiden mares over a period from September to June.
• The reproductive states including the cycle duration, the durations of diestrus (non-reproductive phase) and estrus (reproductive phase), and the ovulation time were observed.
• The reproductive parameters were found to be similar in both autumn (September to December) and spring (January to June).

Observations on Anoestrus Periods and Estrogen Excretion

• Among the nine mares, periods of anoestrus (a state of sexual inactivity or quiescence) were noted. Both ‘shallow’ and ‘deep’ conditions of anoestrus were observed but the study does not detail what differentiates these terms.
• In all mares displaying cyclicity, the level of urinary estrogen excretion began to rise gradually 6 to 7 days before ovulation.
• There was a rapid and statistically significant (P less than 0-001) drop in estrogen excretion on the day of ovulation and the day after.
• Maximum urinary estrogen levels were twice as high in spring than in autumn cycles. This implies that the season has an effect on hormone levels and potentially fertility in horse mares.