I used to work at a very large throughbred farm where they often used nurse mares. There were a few mares that would not take to their foals, or if a mare died they would use a nurse mare to raise the foals. At this farm these nurse mare babies are taken for their maother at around 4 months of age and bucket fed. Although their growth tends to be slowed down they do eventually catch up. I raised a foal that was a nurse mare baby and he turned into a very strapping handsome man! I also think it's sad. They seem so young and too little to be away from mom! The mare owners,however, often donate their mares in order to get a free breeding to a nice stallion and the foal are generally well cared for ( at least coming out of the farm I worked at.)