Growth rate of ovulatory follicles during the first ovulatory oestrus (after seasonal anoestrus) and subsequent oestrous period in Irish Draught mares.

The research examined the growth rates of ovulatory follicles in Irish Draught mares during their first ovulatory estrus period after seasonal anoestrus and subsequent periods. The study found no significant difference in growth rate or pre-ovulatory diameter in the first ovulation compared with following ovulations, challenging previous beliefs that the first ovulation of the year is less fertile due to slower follicle growth.

Research Overview

• The research investigated the growth rates of ovulatory follicles during the first ovulatory estrus in Irish Draught mares after seasonal anoestrus and in subsequent estrus periods.
• The common belief is that during the spring transition, i.e. the initial period following seasonal rest (anoestrus), the developing follicle grows more slowly, lasts longer and grows to a larger diameter prior to ovulation than at later estrus periods.
• It was hypothesized this slower growth rate could result in the production of a subfertile oocyte, possibly explaining decreased fertility in the year’s first ovulation as compared to later ovulations.

Methodology

• The same group of Irish Draught mares were examined in detail over three winter/spring periods to collect data on follicle growth rates and pre-ovulatory diameters.
• The mean growth rates and maximum pre-ovulatory diameters were calculated for both the first post-anoestrus ovulation and subsequent ovulations.

Findings

• Contrary to the initial hypothesis, no significant differences were found in growth rate or pre-ovulatory diameter between the first and subsequent ovulations.
• For both first and subsequent ovulations, the mean growth rates over the ten days prior to ovulation and mean maximum pre-ovulatory diameters were very similar.
• It was suggested that practitioner observations of slower follicle development during the spring transition might be due to less frequent examinations and thus a failure to note that follicles may grow and then regress during this period.
• The implication is that the largest follicle at an examination is not necessarily the one that was observed few days before.