Due Date Calculations

[OveroHunter]

Okay, I am fairly new to the breeding world. I met my SO about 4 years ago and he and his family have been breeding APHA and AQHA horses for 30+ years. I was always a rider, never a breeder. Well, they always calculated due dates as 11 months and 10 days after the LAST breeding. According to his old timey parents, a mare conceives on the last day of heat…

Well to me, it makes since to calculate from the FIRST breeding, because just like little, stupid teenagers, you can always get pregnant the first time. Plus based on a 340 day gestation, the actual formula would be 11 months plus 5 days:

365/12 = 30.4 days per month, 340/30.4 = 11.18 months, 0.18*30.4 = 5.47… Therefore roughly 11 months and 5 days (in case anyone wanted the math behind it… This is what I do for a living)

How do you calculate due dates? I know I haven’t been in the breeding business long, but this just makes sense to me!

[Laurierace]

I go 342 days from last cover date which is roughly 11 months one week. To do the actual calculation I use this website: http://www.sawyercreek.com/page8.htm
Then I sit back and prepare to lose countless hours of sleep because mares foal whenever they feel like it!

[Graymaresrule]

I use Excel

[Rhyadawn]

[QUOTE=Laurierace;5653162]
I go 342 days from last cover date which is roughly 11 months one week. To do the actual calculation I use this website: http://www.sawyercreek.com/page8.htm
Then I sit back and prepare to lose countless hours of sleep because mares foal whenever they feel like it![/QUOTE]

Mares don’t have actual due dates It is just a study of average (for proof read all the threads on foaling dates and what a due date is. There are LOTS)

For myself I count 340 days from last breeding, but then go 10 to 15 days in either direction for an estimate if the mare is maiden. If the mare has foaled before (under my watch) then I might have a better idea of a tighter timeline, but…

My coach has a list of due dates based on cover dates, but because she uses 345 days and I prefer 340 I have to pack count 5 days.

I think equine reproduction has a foaling calculator on their website as well…

[Hillside_H_Ranch]

I use 340 days from the day of ovulation. Of course, if you aren’t using ultrasound then day of ovulation probably isn’t known. However, b/c of the huge variability in gestational length you are in the ball-park pretty much no matter which way you calculate it !

[Home_Again_Farm]

@Hillside H Ranch;5653191

I use 340 days from the day of ovulation. Of course, if you aren’t using ultrasound then day of ovulation probably isn’t known. However, b/c of the huge variability in gestational length you are in the ball-park pretty much no matter which way you calculate it !

This.:yes:

[rideagoldenpony]

Since mares tend to ovulate towards the end of a cycle, I calculate 340 days from the date of last breeding. However, I also try not to breed to the very end of a cycle.

Most of my mares (particularly the ones carrying foals by Mardi Gras), tend to go “early” – ie 1-3 weeks ahead of 340 days.

[amastrike]

I went from the day bred…

Date A + X Days = Date B
Number of days between two dates

[rodawn]

[QUOTE=Hillside H Ranch;5653191]
I use 340 days from the day of ovulation. Of course, if you aren’t using ultrasound then day of ovulation probably isn’t known. However, b/c of the huge variability in gestational length you are in the ball-park pretty much no matter which way you calculate it ![/QUOTE]

THIS!

Once a mare ovulates, her heat pretty much flatlines and she loses interest in the stallion and starts rejecting him. So it is safe, if they’re doing live cover, to assume the last breeding date is the one that his swimmers catch the little eggy. If they’re AI’ing, then the last insemination date (which hopefully occurred just hours before she ovulated) is from when the dates are counted.

So if Mr. Studly last covered her or, alternatively, she was last AI’d on June 5, then day 340 is May 11. But she could deliver anywhere from April 22 (320) to roughly June 10 (370), a few have gone even longer, I’ve heard, that’s rare tho.

I use a Mare Care/Foal Care dates wheel supplied by Intervet. I slide the wheel to last breeding date, it marks the Day 340 for me, also marks reminder dates for ultrasound checks, deworming, vaccinating, even tells me the safe time frame to open a Caslicks. On the flip side is the foal care - slide the wheel to date of birth and it tells you the deworming schedule, vaccinations, etc., right up to 1 year of age. FANTASTIC tool. Love it love it love it - beats having to sit down and calculate each mare and each foal on my calendar.

Mares don’t have a due date, as someone else noted. They have a general date range. You have to rely on the mare’s physical signs and behavior to tell you what’s up.

Most mares, however, are pretty consistent within a few days of where they average-length their gestations. So, if you have a mare who has birthed at around day 335, chances are she’ll probably birth again very close to that. Not guaranteed, but roughly an average.

The general popular saying is: The foal chooses the date of maturity and the mare picks the date and time.

[Equine_Reproduction]

[QUOTE=OveroHunter;5653118]
Okay, I am fairly new to the breeding world. I met my SO about 4 years ago and he and his family have been breeding APHA and AQHA horses for 30+ years. I was always a rider, never a breeder. Well, they always calculated due dates as 11 months and 10 days after the LAST breeding. According to his old timey parents, a mare conceives on the last day of heat…

Well to me, it makes since to calculate from the FIRST breeding, because just like little, stupid teenagers, you can always get pregnant the first time. Plus based on a 340 day gestation, the actual formula would be 11 months plus 5 days:

365/12 = 30.4 days per month, 340/30.4 = 11.18 months, 0.18*30.4 = 5.47… Therefore roughly 11 months and 5 days (in case anyone wanted the math behind it… This is what I do for a living)

How do you calculate due dates? I know I haven’t been in the breeding business long, but this just makes sense to me![/QUOTE]

As others have noted, there really isn’t any such thing as a due date. 320 to 370 days gestation is considered well within the norm with 340 being an average of averages. Indeed, if you read the article “Is My Mare Overdue?” - http://www.equine-reproduction.com/articles/overdue.shtml there is a poll on that article for mare owners to enter their foal’s gestation length - with over 3,000 foalings reported, MORE mares go over 370 days (18.53%) than foal in the 339-344 day period (15.51%).

Typically mares ovulate 24 hours before the end of their estrus cycle, so if you are not ultrasounding, you would typically estimate from the last breeding date as the egg hasn’t been released from follicle prior to that time. It’s not about when she is bred, but about when she actually ovulates.

To figure 340 days, we take the day she ovulated - so say June 5th…add five days making it June 10th and subtract a month, so her 340 day would be May 10th…give or take a day ;).

Hope that helps!

[coloredcowhorse]

If I’m hand breeding I breed my mares on day 3 and day 5 of their cycle (assuming that I was teasing and actually have the first day of their cycle pinned down)…rarely on day 7 if still in a standing heat (maybe 3-4 times in over 30 years). Once the mare ovulates she usually will be out of heat (showing no signs or only minimal ones and not interested) within about 24-30 hours…ovulation occurs in most mares about 24 hours prior to the end of their obvious heat cycle. If I tease her on day 6 and she’s still in standing heat I skip that day and go to day 7…figuring that there are live swimmers still present from the day 5 breeding and fewer covers results in less fluid issues/irritation. If I cover on day 3 and on day 4 she’s still interested but on day 5 she’s not I figure either a short cycle or (more likely) I missed the first day she was in and I leave it at the single cover on day 3. I count back 25 days from the last day covered…this gets me to 340 days from the last day bred. IE…breed the last time on 4/30…count back 25 days to get to 340 and estimated due date the following spring will be 4/5. Then I figure in 2 weeks on either side of this if a mare I don’t have experience with…maybe a little less if it is a mare I’ve bred before (preferably several times) and there’s a pattern. If I pasture breed I keep an eye on things and usually am aware of when a mare is flirting, standing, been covered and when she’s telling the stud to “bug off, dude, we’re done here!” and still have a pretty good idea of when she’s due. I rarely have mares that are covered a second cycle and almost never have one fail to conceive in two cycles. Seems to work.

[Kyzteke]

@Hillside H Ranch;5653191

I use 340 days from the day of ovulation. Of course, if you aren’t using ultrasound then day of ovulation probably isn’t known. However, b/c of the huge variability in gestational length you are in the ball-park pretty much no matter which way you calculate it !

Also pretty much this. I go a year forward then count backwards 25 days.

One year I had 4 foals due, which is alot for me. So I carefully spaced out the breedings – one was due in April, one May, one June and one July. Thought I was clever.

April mare carried 385 days!! May & June mare were within 2 weeks of each other…

All healthy foals, all normal pregnancies – but no one was following “the Plan” but me…

PS Per my vet and what I’ve read, mares DO ovulate the last 24 hrs. before going out of heat most of the time. However when we are using hormone treatments to force ovulation, etc., then the only way to really know is by ultrasound.

But in the days before all of this stuff you DID count from the last day the mare would accept the stallion.

[rodawn]

@Kyzteke;5653755

Also pretty much this. I go a year forward then count backwards 25 days.

One year I had 4 foals due, which is alot for me. So I carefully spaced out the breedings – one was due in April, one May, one June and one July. Thought I was clever.

April mare carried 385 days!! May & June mare were within 2 weeks of each other…

All healthy foals, all normal pregnancies – but no one was following “the Plan” but me…

hahaha… I got a good chuckle from this. Dang mares, why don’t they read the textbooks already! :lol::lol::lol:

[Jos]

[QUOTE=Kyzteke;5653755]
mares DO ovulate the last 24 hrs. before going out of heat most of the time. However when we are using hormone treatments to force ovulation, etc., then the only way to really know is by ultrasound.[/QUOTE]

Not true… well, the second part isn’t.

It is true that the majority of mares ovulate in the last 24-48 hours of standing (receptive) estrus. The reason that they cease to become receptive about 24-48 hours after ovulating is that the CL which forms in the area evacuated by the ovulating follicle starts to secrete the hormone progesterone - the hormone that causes resistance to males (:)) - and by 24-48 hours after the ovulation, the circulating progesterone levels in the mare have reached a point where the resistance happens. Conversely, if the mare is not pregnant, around 12 days later, you will see the mare go from “NO” to “Ho Hum” to Maybe" to “Yessireee!!!” if teased daily over the course of several days. This is the time when the prostaglandin release from the uterus has destroyed the CL and one is seeing the effects of decreasing progesterone levels.

So, in view of the above, using an ovulation promoter (hCG/Deslorelin/Histrelin/whatever) isn’t going to impact the time after the ovulation that the resistance starts - you’re still going to see that occurring in the 24-48 hours after the ovulation - so one can rely on that indication as effectively as if there had been no promoter used. What it may of course impact is the overall duration of receptivity, shortening that receptive time frame, as the ovulation occurs earlier in the receptive period.

Regardless however, if one definitively wants to know that the mare has ovulated, then ultrasound is required - promoter use or not - rather than relying on her no longer being receptive.

Hope this clarifies it!

[Kyzteke]

@Jos;5653949

Not true… well, the second part isn’t.

It is true that the majority of mares ovulate in the last 24-48 hours of standing (receptive) estrus. The reason that they cease to become receptive about 24-48 hours after ovulating is that the CL which forms in the area evacuated by the ovulating follicle starts to secrete the hormone progesterone - the hormone that causes resistance to males (:)) - and by 24-48 hours after the ovulation, the circulating progesterone levels in the mare have reached a point where the resistance happens. Conversely, if the mare is not pregnant, around 12 days later, you will see the mare go from “NO” to “Ho Hum” to Maybe" to “Yessireee!!!” if teased daily over the course of several days. This is the time when the prostaglandin release from the uterus has destroyed the CL and one is seeing the effects of decreasing progesterone levels.

So, in view of the above, using an ovulation promoter (hCG/Deslorelin/Histrelin/whatever) isn’t going to impact the time after the ovulation that the resistance starts - you’re still going to see that occurring in the 24-48 hours after the ovulation - so one can rely on that indication as effectively as if there had been no promoter used. What it may of course impact is the overall duration of receptivity, shortening that receptive time frame, as the ovulation occurs earlier in the receptive period.

Regardless however, if one definitively wants to know that the mare has ovulated, then ultrasound is required - promoter use or not - rather than relying on her no longer being receptive.

Hope this clarifies it!

Thanks, Jos. That makes perfect sense when you explain it that way. I am often “breeding blind” so to speak, as I have a mobile vet and she isn’t going to come out on weekends, etc. just to ultrasound a mare! I do my own AI and my own teasing and so far it’s been ok if I don’t use frozen.

Still, it’s helpful to know/watch other signs, from the days before ultrasound and drugs. After all, mares got pregnant then, too!

For instance, this last time was the Thurs.,about 2pm prior to Memorial Day wkend (naturally, mares prefer holiday ovulation…).

We checked the mare on Thurs. and she has a 40mm with 3+ edema. She was obviously in estrus.

Called for semen on Friday AM. Vet had left me with a shot of hcG, but I thought at a 40, why use it?

Semen came Sat. I AI’ed the mare Sat. @ 3pm and Sun @ 5pm and used oxy inbetween (this is a young mare who has no problems with clearance to my knowledge).

She wasn’t showing strong signs of receptivity Sun., but she didn’t resist to the AI.

I was surprised to see her go this long, but my guess is either she had another follicle come along and double ovulated, or ovulated one whooping big follicle!!

I guess we’ll find out Tues., when we check her…but when you ARE working sort of blind this way, it helps to fall back on the same “signs” the old timers (so to speak) used to use.

[Jos]

[QUOTE=Kyzteke;5654861]
Friday AM… 40… AI’ed the mare Sat. @ 3pm and Sun @ 5pm… She wasn’t showing strong signs of receptivity Sun… I was surprised to see her go this long, but my guess is either she had another follicle come along and double ovulated, or ovulated one whooping big follicle!![/QUOTE]

Progesterone trumps estrogen. In other words, even if she had a second follicle that ovulated, the rise in progesterone from the formation of the CL after ovulation of the first follicle would shut down receptivity prior to the second follicle ovulating if there were a day or more delay in the ovulation of the second follicle. Remember that mares can have diestrus ovulations - but it wouldn’t be diestrus if the estrogen trumped the progesterone, as it’s the estrogen that causes receptive behaviour!!

There are a couple of other things can be useful as indicators - but remember that mares are all individuals, so it’s not going to hold true for all mares all of the time, or even some mares some of the time!!!

• Check the cervix per vaginum with a speculum or clean gloved hand if you can’t tease with a stallion. The cervix will relax in response to estrogen, but tighten up in response to progesterone. It will be most relaxed around the time of ovulation, so if one AIs a mare with a very relaxed cervix, but the next day when going to AI again the cervix is less relaxed, then she has probably ovulated. The issue with this is that some mares have cervices that never really relax.

• Check the urine. You know how the mare's urine becomes thick, yellowy and stinky when she's in estrus? That's estrogen being secreted in the urine from the follicle. Those of you who really should get a life (instead of standing around watching your mares pee) may have noticed that towards the end of estrus, the urine often goes clear again. That's because estrogen levels peak about 24-48 hours prior to ovulation and therefore secretion levels peak at that time as well. This does give us a bit of an indication that if we haven't bred the mare (with fresh or cooled - no good for frozen) and the urine has gone from thick stinky to clear but she's still receptive to a stallion (or has a relaxed cervix), then we need to get cracking. The problem with relying on this is that if there is a second follicle present - which may not even ovulate in the end - it too will be secreting estrogen, so the primary (about to ovulate) follicle may have ceased estrogenic secretion, but that darned secondary follicle will keep making the urine thick and stinky. Consequently this is only a useful indicator in the event that you do see the urine go clear - hurry up and breed if you haven't already - but don't wait for it to go clear before breeding, as you might miss her completely.

Hope this helps!

[Kyzteke]

@Jos;5657677

Progesterone trumps estrogen. In other words, even if she had a second follicle that ovulated, the rise in progesterone from the formation of the CL after ovulation of the first follicle would shut down receptivity prior to the second follicle ovulating if there were a day or more delay in the ovulation of the second follicle. Remember that mares can have diestrus ovulations - but it wouldn’t be diestrus if the estrogen trumped the progesterone, as it’s the estrogen that causes receptive behaviour!!

There are a couple of other things can be useful as indicators - but remember that mares are all individuals, so it’s not going to hold true for all mares all of the time, or even some mares some of the time!!!

• Check the cervix [I]per vaginum[/I] with a speculum or clean gloved hand if you can't tease with a stallion. The cervix will relax in response to estrogen, but tighten up in response to progesterone. It will be most relaxed around the time of ovulation, so if one AIs a mare with a very relaxed cervix, but the next day when going to AI again the cervix is less relaxed, then she has probably ovulated. The issue with this is that some mares have cervices that never really relax.
• Check the urine. You know how the mare's urine becomes thick, yellowy and stinky when she's in estrus? That's estrogen being secreted in the urine from the follicle. Those of you who really should get a life (instead of standing around watching your mares pee) may have noticed that towards the end of estrus, the urine often goes clear again. That's because estrogen levels peak about 24-48 hours prior to ovulation and therefore secretion levels peak at that time as well. This does give us a bit of an indication that if we haven't bred the mare (with fresh or cooled - no good for frozen) and the urine has gone from thick stinky to clear but she's still receptive to a stallion (or has a relaxed cervix), then we need to get cracking. The problem with relying on this is that if there is a second follicle present - which may not even ovulate in the end - it too will be secreting estrogen, so the primary (about to ovulate) follicle may have ceased estrogenic secretion, but that darned secondary follicle will keep making the urine thick and stinky. Consequently this is only a useful indicator in the event that you do see the urine go clear - hurry up and breed if you haven't already - but don't wait for it to go clear before breeding, as you might miss her completely.

Hope this helps!

Cool info, Jos! Thanks!

Now I have another excuse for standing around doing nothing – I am waiting for my mare(s) to pee!

But I think I’ll have to forego the cervix testing. I have no stocks, so I must make sure the mare is ready and/or drugged. I’ve never had one protest or give me trouble as long as she is truly in heat, but then I am dealing with mares I’ve known all their lives – these are mares I’ve bred, so I feel pretty safe.

My last mare I did in the driveway, with her facing her pasturemates across the fence-line. She’s a darling…