Quality of foals relating to the number of foals a mare has had
I've been wondering...you hear that a maiden mare might have a smaller foal for their first foal. But does anyone feel like there's a quality difference? Do mares produce more quality as they have more foals and move into their "peak"? Does the quality reduce as the mare gets older?
A mare might be a better mom, ie more relaxed. But genetically she has the same chance to pass on genes every single time she is bred. I can't think of any epigenetic issues that would be related to number of times bred.
Smaller first foals at least makes some sense as a hypothosis, however if proper nutrition was offered to the foal, mature height shouldn't be affected (height being genetic)
The thoroughbred industry has studied this very question and I'm sorry I have no link but perhaps if you google around you can find it. I wondered about this several years ago as I had a young horse from a long term broodmare and I was searching for info. They concluded that the 4th foal was overall the most successful in racing and then the quality/success rate dropped with each successive foal.
I think most of our mare's foals have gotten better as we've gone along, but, I would attribute that to making improved breeding choices based on what she seems to be reliably passing or what consistently needs improvement according to the ideal we have in our head. After 3-4 foals, it becomes much easier to make a good choice for her, IMO.
Also the first foal thing doesn't seem to really apply either, it seems. Of our mares, the first foal has not necessarily been the smallest at all. At birth perhaps (THAT could make sense biologically), but they have caught up to be whatever size they are genetically programmed for. And sometimes a subsequent foal is smaller in size than a previous one at first but mature in the same range.
This is totally anecdotal and not at all objective (as my "better" is not like assessing a racehorse or a jumper), but I have a couple of mares that have had quite a few foals. With one, of her total of nine foals, I would assess the first and the ninth to be her best of the lot. With another mare who has had eight, I'd have to give the third, fourth and last top billing.
I have not found this to be the case with my mares, esp. when bred to the same stallion. Now it's only natural if your an astute breeder that the more offspring a mare produces the better one would be at choosing her next mating.
The old wives tale is that the third foal is the best, and I've heard that repeated a lot over the years by many respected horsemen. But I don't know what the reasoning is behind that theory. I also haven't found it to be noticeably true.
It's always hard to objectify "best", also. What I prefer in a mare's offspring may not be what someone else prefers, so to them, her "best" may not be my pick. We have one mare who's had seven foals for us, and while they all are similar and very nice, my favorite one is not necessarily a friend's favorite due to the high level of subjectivity in the hunter world.
Maybe in racing, say, it's easier to judge this because there is a clear way to measure one being "better" than the other.