Ovarian follicles, ovulations and progesterone concentrations in aged versus young mares.

This study looks at the difference in ovulation rates and related factors in older and younger horses. The researchers found that older mares had a higher rate of ovulation failure, a later average day of first ovulation, and higher levels of progesterone, among other findings.

Research Objectives

• The researchers were interested in investigating if there was an ovulation failure rate for mares related to their age. Older horses, in this study between 20 to 33 years of age, were compared to those that are younger, between 3-12 years of age.
• They also wanted to compare whether there was a difference in the number of ovarian follicles, their occurrence of ovulations, and postovulatory concentrations of progesterone between aged mares and their younger counterparts.

Methodology

• The study was conducted using two experiments. In the first, four out of ten aged mares (25 to 33 years old) failed to ovulate between July and September 1989.
• In the second experiment, ovulation failure was observed in two of 25 aged mares (20 to 30 years old), while none of the young mares (3 to 12 years old) failed to ovulate between February and June 1990.

Findings

• The research found that the average day of the first ovulation was later for older mares compared to younger ones, thereby suggesting that age plays a role in the timing of ovulation.
• Furthermore, there were fewer 11 to 20 mm ovarian follicles in aged mares versus young mares, suggesting age could affect the number of certain types of ovarian follicles. However, when looking at the total number of ovarian follicles, there was no significant difference between older and younger mares.
• The number of ovulatory cycles was less for older mares than younger ones. This indicates that age does not just affect the timing of ovulation, but also the number of ovulatory cycles a mare has.
• Lastly, the study looked at plasma progesterone concentrations and found that these concentrations were higher in old mares as compared to young mares on the 10th and 15th days of the first ovulatory cycle. This indicates a potential difference in hormonal regulations between older and younger mares.