In the UK unless a mare is put under lights they usually start to season around March. Obviously the TB industry want foals born as close to Jan 1st as possible so it is regularly practice to manipulate the TB mares cycle. My TB mare was born Feb 12th 1990 and I have had her 15 years. Since I had her she has always had her first season/ heat on or very close to the 1st January. This is totally natural as I have never tried to manipulate the heat or until a few years ago breed from her.

This year I took her to an ET clinic mid March and they said it would probally be the next heat they would try as she would of only just started cycling this year. I explained that she was now on her third or fourth heat and in full flow by early Spring (March).

My mare took only once on her second try early in April (and I have a 60 day embryo )the next 3 goes with diffrent stallions produced no embryos. So my question is
could she have been more fertile in March/April as this was already mid way through her natural cycling?
Does fertility decrease over the course of the breeding season?
Or is the first season as fertile as the last season and the ones in the middle?

I am really just intrested in the answer but if I was mad (and rich) enough to try again next year with a 20 year of horse should I have start in February and stop in May?