When the breeding questions get specific—such as regarding age of the mare, or how to breed miniature horses-it's always best to go to an expert.
Here, Dr. Joyce Harman answers questions from a reader.
Question: At what age is it safe for a filly to breed? My filly is 2 years old. Also, at what age can miniature horses be bred?
Answer: Miniature horses should not be bred until they are 4 years old, in my opinion. They can have many foaling problems, so they should be as mature as possible before attempting to breed. Have a vet check her out before breeding to be sure she is in good shape to breed, and have the vet on call at foaling time in case there are any problems.
This is also true of other breeds, not just miniature horses--always keep your vet in touch with the pregnant mare. 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 me 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. On the other end of scale, while we are looking at breeding ages, the question often comes up as to how old can you breed a mare, especially for the first time.
In part it depends on the health of the mare and whether she has ever had any unsuccessful attempts at previous breedings. If the mare is healthy, usually it is quite safe to breed her up to about 15-16, though I have seen mares conceive for their first up to about 18 years old. Have your vet palpate and ultrasound her uterus to check its condition. In many cases a mare that has never had a foal may have had an infection early in life and may have scarring or damage that could prevent her from getting in foal.
With the older mare, if you go through a breeding season from March or April to June or so, with a fertile, proven stallion and have had no luck, I would give it up, personally. You can spend a lot of money and stress the mare but the likelihood of getting an older mare in foal goes down with each failed attempt.