I don't have any mares so I don't know that much about colts.
However, I do work with a nutrition company, so perhaps I can help with
the nutrition aspect of it.
Leaving a colt on a pregnant mare too long can often cause problems with the colt she is bearing. When sucking colts are left on the mare too long, the nutrients go to the colt at side and not to the fetus. This can result in some wierd things like leg problems when the colt is born that she is pregnant with.
Another thing, mares need mineral supplement. They might get by without it, but at some point they are going to run out of steam and the
colt could be affected. Usually you won't notice much of a problem
until the mare is 7 or 8 years of age.
Hope this helps. I'd hate for problems to arise when they could be avoided.