Ask the Vet – When do mares first go into heat?
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Ask the Vet – When do mares first go into heat?

February 29, 2020

DAN: “About what age does a
mare begin going into heat?” So actually, we answered
a question last month from Equestrian Grace about
the work-to-rest ratio. DR LYDIA GRAY: Oh. OK. DAN: So you are
doing a great job with your question
submitting and voting. DR LYDIA GRAY: Yeah. I wonder what’s
going on in her barn. DAN: We’re starting to
paint a picture here. Yeah. DR LYDIA GRAY: Yeah, yeah, yeah. So the short answer is just
that female horses, the fillies, become mares. So they reach sexual
maturity at anywhere from 9, 10 months,
all the way– it can be some as late as 18 months. But I would say between
12 and 15 is most common. DAN: When they’ll
start going into heat? DR LYDIA GRAY: And
that means– yeah. And that means in
their yearling year– now it has to do with–
which seems early, right? DAN: A little bit. DR LYDIA GRAY: Yeah. But it has to do
with what month they were born because
when they come in heat and when they’re in
heat during the year, it’s all related to
the amount of light. So as daylight increases,
mares begin to cycle. And then as daylight shortens,
like this time of year, then they begin to
go out of cycle. So they’re not in estrus
during the winter. So if your filly was born super
early, like January, February, she’s not going to cycle
until all the mares cycle in the next spring. DAN: The following spring. DR LYDIA GRAY: So she
might be a 15-monther. DAN: Oh, that makes sense. DR LYDIA GRAY: But if
your filly was born, I don’t know, June,
then her yearling year– she’ll be nine months, and she
might be getting the signals that she should go into heat. DAN: Because in April, when the
weather starts to get nicer– DR LYDIA GRAY: Yeah, but
she’ll only be technically 9, 10 months old. So that’s why there’s a range. But it’s very
individual, and you have to watch for the signs. And so the signs of a
mare going into heat are they can squat a little
bit and urinate little bits. They wink their vulvar lips. They will stand to be mounted. They’re anxious. They can scream and carry on. So when she begins to
notice those things– I think the important thing is,
because in the same things– same age about happens
with the colts, anywhere from 12 to 14 months. They’re probably
not sexually mature, and that means
capable of reproducing is what that term means,
by like 15 months, because there’s a little
bit more involved there. But basically, the
short answer is– so if you’re worried, if
you have fillies and colts, once you’re into their yearling
year, like January or February, separate them if you want
to avoid those unwanted– DAN: That’s when
they go the boys’– DR LYDIA GRAY:
–surprise pregnancy. DAN: –paddock and
the girls’ paddock. DR LYDIA GRAY: Yeah. Yeah. So it can it could be super
early, 9 to 10 for girls and– DAN: Wow. DR LYDIA GRAY: –12
to 14 for boys. Yeah. DAN: Wow. Well, hopefully that helps you. And I can’t wait to hear
what your question is for next month.

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