Back to the actual topic-- here's an article by a Vet that may be in conflict with some opinions, ( I would not breed a mare at 2) but offers some good advice: EquiSearch’s Ask the Vet: Mare Breeding Age
It is certainly possible to breed a regular horse at the age of 2 for a foal at 3. That is if the filly is mature and on a good plan of nutrition. She needs to not be too fat or too thin and should have a good source of free choice minerals, high quality hay and grain as needed. You have to remember that being in foal and growing at the same time put additional stresses on the filly. Does she have good conformation and temperament? Does she have a healthy shape to her back or is it long and weak? Are you planning to try to ride and break her to saddle during this time? If you are going to breed a young horse, she really should not do much else during that time except get some light exercise and have plenty of pasture time.
If her temperament is poor, and you are hoping that breeding her will sort her out, it is best not to breed her. If she passes her temperament along to her foal, then you have two of them with difficult personalities.
This letter came to EquiSearch.com very late in the breeding season. Mares are at their most fertile in the spring and early summer and the gestation time is about 11 months. So a breeding in September or October can result in an August or September foal. It is safest to have a foal in the spring before the weather gets too hot. A foal has a fairly heavy coat naturally and the summer heat can be an extreme stress. Certainly foals can be born at other times of the year, but it takes much more careful management and the risks are greater. Since it is late in the year, it would be best to wait until next spring and be really ready to breed in late March or April.
Perhaps the ideal age to breed a mare for the first time is when she is 3 to foal at 4, or 4 to foal at 5. However, there are many factors that can change that ideal age. Mares that are starting a show career may be in training at that time and should not be asked to carry a foal, grow and train. Pick one or the other. She can be ridden and educated while in foal at that age, but should not be in hard training. Mares who are a bit older (5-6 years old), fit and working hard can be bred and kept in work until they are too heavy in foal to be comfortable working, but even then they can go for easy trail rides.