Breeding myths...

[AndyPandy]

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Perhaps you could also clarify what brings a mare into season? I know day length is involved, but mine do seem to go on & off with the weather, and cycle oddly in some years.

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Day length is involved, yes. It's basically decreasing hours of darkness, and increasing average temperature.

During the transitional periods, follicles begin to mature but do not ovulate. This allows for extended periods of estrogen elevation, which is natures own spring cleaning process (it raises immunity in the uterus for long period of time, preparing it to carry a foal).

After continued increases in daylight, and temperature, mares begin to cycle fully. Of course, periods of bad weather (where it gets colder/darker) can disturb cycling and make cycles irregular.

Irregular cycles can also be due to uterine infection/irritation which causes v long or short cycles to occur dependant on the exact nature of the problem.

There are probably other players, but we do not know the details yet

[alleycat]

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I don't see how this holds up statistically:

If the Downs Syndrome egg were to be just a random event the chance of it being ovulated at any time would be the same for Super Menopause Avoider as for Schoolgirl Mother.


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So you're saying, if I had a huge bag with 3000 white balls, and one red ball, all the same weight, size and texture, and I picked a random ball, I would have just as much chance of picking the red ball here, as if I had a bag with 2 white balls and one red ball?

I don't think so

In the first instance, my chance is 1 in 3001, in the second instance, my chance is 1 in 3!!! If, in the first instance, I keep picking out balls, for every white ball I pick out, the chance of picking the red ball increases. Hope that makes sense (I'm assuming the red ball is equivalent to the Downs oocyte)

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This is true if the red ball isn't picked until you reach the last 3. But for every time this happens, for this to represent the true range of statistical chances, there would have to be 3001-3 other bags in which the red ball is removed first, 2nd., 3rd., 4th., etc. In these bags, the chance of the red ball being part of a 1 in 3 chance are: zero!

Another way of putting it is that every ball, at the beginning of the process, has a 3001 chance of being picked at the beginning and a 1 in 3 chance of being picked IF it makes the last 3. What you can't say is whether any particular ball will make the last 3...

Basically, you have assumed the red ball is going to be one of the last left. Real life bears this out with Downs Syndrome. Question is, why THAT egg and not one of the others? This suggests to me a mechanism which either makes the last eggs faulty, or keeps the faulty eggs until last.

[alleycat]

Thanks.. AND another question... is the one about a mare conceiving more readily if she is in rising condition (rather than poor, fat or a level plateau of good condition) likely to be true?

Thanks again in anticipation...

[MissIndependance]

Excellent post – the only one I was surprised at/hadn’t heard before wa the last one re worming… We haven’t had any problems with scouring foals previously but I think I will add ivermectin to my foaling kits now as a precaution…
Interesting re the iodine – we used Iodine last year on vets advice…But they did also say the blue spray which you use to treat thrushy issues in hooves was just as good…is that the same stuff you mentioned? I can’t remember the long name…!
Karynk that’s really interesting about the little TB stallion.
The Downs Syndrome theory statistically makes a lot of sense, and is something I’d never considered before, but it’s certainly an interesting thought… From how I interpret AP’s post he’s saying that although of course the red ball could be picked each time, that every time it is not randomly ‘picked/released’ that the next time the odds increase.
Ie start off with 4 balls, one of which is red and the first time you randomly pick a ball it is a 1 in 4 chance, but the subsequent times the odds that you pick that red ball (assuming it isn’t released first of course) increase, ie 1 in 3, and then 1 in 2 until you are 100% certain to get it…
Great to see a thread which has got everyone thinking!

[AndyPandy]

Let me try again... my analogy might not be that great, but I want to try to say where I'm coming from.

Let's say, at age 14, a girl starts menstruating, and cycles every month until she is 54. She has 480 eggs, and one of those has an extra chromosome, and so, if fertilised will create a downs syndrome baby.

For her first ovulation, the chance of that downs egg being ovulated is 1 in 480 (0.208%). Now, of course, because it is totally random, that downs syndrome egg could be ovulated first time, but it is unlikely that it will be.

So lets assume, for 10 years, that downs egg isn't ovulated.
Now we have 360 eggs left, and the chance of the downs egg being ovulated is 1 in 360 (0.278%), because those are the only eggs now available for ovulation.

At age 44, she only has 120 eggs left, so IF she still hasn't ovulated that downs egg, it's now a 1 in 120 chance (0.833%).

At age 50, she has 48 eggs left! So if that downs syndrome egg is still there, the chance of it ovulating is now 2.08%. And at age 53, if she still has that downs egg, the chance of it being ovulated is 8.33%, so the chance has rocketed up in those last few years.

Now look at the way my description matches up to the risk of a downs baby increasing with age here: http://upload.wikimedia.org/wikipedi...my21_graph.jpg

I think it fits very nicely. I just don't understand why you don't like my stats on this one If you can make it clearer for me, that would be great. Cheers

[AndyPandy]

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Thanks.. AND another question... is the one about a mare conceiving more readily if she is in rising condition (rather than poor, fat or a level plateau of good condition) likely to be true?

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It is indeed true for some mares - a rising plain of nutrition and body condition may be beneficial, however, using the body condition scoring index, a minimum of 5 and a maximum of 7 is desirable, and the rising plain is unlikely to make a big difference.

[alleycat]

I can see where you are coming from;and your maths is right: but it seems to me that though this is true of this one egg, it is also equally true for every other egg.

ANY egg that isn't selected (?) at any given ovulation has an increased chance of being picked next time.

My brain is spinning now. I shall go and follow up your link and try to get my head round it, and come back to you when I've thought it over.

Er....Anyone got 3000 white ping pong balls & one red one that I can borrow?

[AndyPandy]

I think the reason it works for me is that, yes any other egg could be picked, so the chance of any particular egg being ovulated is essentially the same. However, for the purposes of this discussion, every egg except the downs egg has the same property (that of being "normal"), whereas the downs egg is unique as it is the only one that carries the extra chromosome. So the chance of ovulation of a normal egg is always higher than a downs egg, because there are more of them

It's confusing, especially when you factor in that a woman is actually born with 2 million oocytes, and only 400,000 of them will ever mature to any degree, and 1000 oocytes are destroyed with every cycle... Aaaaargh! But I'm pretty sure the numbers still work, roughly.

My numbers match up to the graph better if you assume that only 1 in every 2 women has a single downs oocyte

MIND BOGGLING!

[AJBliss]

Isn't there some effect of a mare's age upon early embryonic development? I don't know that it would have an effect upon the resultant foal when all is said and done, but there was a significant developmental delay whilst the embryo was still within the oviduct. Also, when embryos from old mares (>/= 20yrs) were flushed and transferred to young (6-10yrs) recipients, only 31% (8/26) resulted in embryonic vessicles, whereas 92% (11/12) of embryos transferred from young mares resulted in embryonic vessicles. The conclusion would be that the subfertility noted in older mares could be due to the oocytes themselves, rather than solely the "environment" of the older mare. This was a study done waaay back in 1995 by Carnevale & Ginther ... has there been much done recently? I would be interested to hear anyone's thoughts!

[PapaFrita]

Thanks for that AP Very interesting.