I have a couple of mares that each are doing the same thing: A follicle reached 44 and 42 mm. I have been ultrasounding every other day, and the follicles are starting to shrink. They are down to 39 and 37mm. They have no other follicles (of mentionable size). Are they going to ovulate off these follicles? Why are they regressing?
Mares develop follicles in response to the release of Follicle Stimulating Hormone ("FSH"). FSH can be multi-phasic in release, that is there can be several waves of release of FSH during a single combined diestrus/estrus phase period (i.e."cycle" - typically approximately 21 days). If there is insufficient Luteinizing Hormone ("LH") circulating, then despite there being a larger follicle present, ovulation will not occur. This lack of LH can be due to one of several factors - seasonality (during early to mid-transitional phase, LH levels are not high enough to achieve ovulation), the fact that it is a diestrus follicle, or hormonal issues in the mare (less likely).
Diestrus follicles, or follicles not exposed to adequate amounts of LH to stimulate final maturation and ovulation will regress. It is perfectly normal for multiple follicles to behave in such a manner during both transitional phase and diestrus.
It is important to note that the presence of a large apparently pre-ovulatory follicle alone (per ultrasound) is not an adequate indication of the correct time to breed. The mare should be in estrus, otherwise one runs the risk of breeding on a diestrus follicle (some mares can even ovulate a diestrus follicle, which is unique among mammals), which can result in uterine infection as the uterine immune response is suppressed during that time period.
I believe you will find that the OP is referring to follicles that are not ovulating, but are regressing.
You are correct in that follicles tend to get smaller slightly before ovulation - you will find that and some other pointers in our article on predicting ovulation (follow that link to go to it).
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