The latest we've had is covering in August, to foal in July. We have excellent winter turnout areas though, so it wasn't a problem - and the foal is now a rather large three year old!
Yes and no - we hate early foals here. The little bugg#rs dont grow a winter coat till April then sweat all summer.
We dont think its fair on a mare to get too fat on spring grass, its enough strain on their body carrying a foal around for nearly a year!
The latest I would breed would be beginning of May, for an early April foal - so much fun & stress, to be had showing....... For racing, then I'd say the same - if they get too hot in the summer then body clip them.......
Well ideally i'd have foals in April/May but unfortunately things don't go to plan and so my 2 foals are due end of June and beginning of July.
I've known lots of people have foals in August and they seem to be completely fine. They have become strong before the Winter and have the rich Autumn grass. Howver, i do think you need good facilities and good quality forage if having an August foal.
In the wild they breed at all times. I would go for it. Hopefully you will have a nice big loose box for mare and foal when it gets cold. Foal rugs are plentiful if needed.
It is only us humans that need to decide a time, nature has a wonderful way of sorting itself out.