This is one question that people ask me again and again. Is it safe to breed a large, heavy stallion to a lightweight, smaller mare? Obviously natural mating a 17 hand stallion to a 10 hand mare, although it has been done, is probably not wise. However, there are basically no concerns here with the use of AI. It has been proven again and again that the old adage "the mare takes care of it" is absolutely true. A mare will not have a foal that is larger than she can bear because of genetics.
Many years ago, studies were done in England by Ron Barely where pony mares were bred to draft horse stallions. These mares foaled pony size foals, which developed into draft size adults. They then bred draft horse mares to pony stallions. These mares foaled larger than the average pony sized foal that developed into smaller than draft size adults.
The mare limits the size of the fetus to her own uterine capacity and therefore pelvic capacity. Fetal oversize is a very rare occurrence in horses and is usually associated with brain abnormalities (tumours or abnormal development) rather than to breed differences. As a mare has more and more foals, the uterus becomes more stretched, and so fetal size may increase over several years during the mare's breeding career. However, fetal size usually reaches a peak after several foals and then only genetics will play a role.
Studies have even been done where full-blooded shire embryos were transferred into Shetland pony mares. Each of the mares foaled, without difficulty, these purebred shire foals! Again the size of the fetus at birth was determined by the mare's uterine capacity than by genetics, although once the foal was on the ground and nursing, genetics kicked in and the foal quickly outgrew the mare.
So, while it is not generally recommended breeding draft horses to ponies, you can do it without worrying about injury to the mare. For maiden mares, it is generally safer to avoid breeding to stallions that tend to throw larger foals, to allow for ease of the delivery of its first foal. This makes the foaling experience a little easier for the maiden mare.
Here is an exert from the old Black Forest Shires website. I personally had a talk with Jeff Barko on this subject years ago and still believe to this day he made some very valid points that I could have not said better.
“Occasionally we have clients ask us why we breed our fillies at 2 years of age. I am writing this page to help our website visitors and Gypsy Horse breeders understand this.
To start with, we at BFSGH do not breed our fillies at 2. However, many Gypsy people overseas do breed their 2-year olds. In fact, I cannot recall any Gypsy families who do not allow 2-year-olds to be bred. The Gypsy practice of breeding horses at this age goes way back. It's not so much that the Gypsy breeder goes out of his way to put 2-year olds in foal, but rather that's the result of the way many Gypsy breeders manage their herds. This type of breeding management has been practiced by Gypsies for longer than we've had horses in the Americas. So one could say we're a little out of place suggesting to the Gypsies that they're "doing it wrong". Here in the USA, we don't have to breed our 2-year olds if we don't want to, that is personal choice, however we'd be out of place telling the Gypsies that they shouldn't.
Considering how widespread the practice is, it is quite likely that the mares you own, or plan to own, were themselves bred as 2-year olds (foaling at the age of 3). It's also very possible that one or more of the horses you own is the offspring of a mare bred at 2.
Many arguments can be made that support the practice of waiting until your mare is 3 or even 4 before breeding her. Despite this, we cannot immediately conclude that this Gypsy breeding practice is 'Bad'. In fact, as we look back over the years we've spent with the Gypsy breeders, I cannot say I've seen any higher incidents of foaling or foal-growth problems in foals from mares bred at 2 than I see from older mares. What we see, is what one might expect given the circumstances. The Gypsies have created a wonderful breed of horse with many outstanding qualities. Among them is the fact that they breed well at a young age, a quality which has been bred into the breed by decades of natural selection doing what it does best. Now I wouldn't run out and start breeding my Quarter Horses and Appaloosas at 2 years old,- I'd certainly be asking for trouble. These breeds don't have that quality which the gypsy horses possess, the ability to breed safely at an early age.
So while it may not be in your plans to breed 2 year olds, it's important to realise that the Gypsy people always have, will continue to do so and the breed has shown little, if any, ill effects from doing so.”
Additional comments from Brightwater Gypsy Vanners. On my last visit to the UK I made a point of talking to a lot of gypsy breeders about their breeding practises. In fact many of them breed them even younger than two. The biggest comment that kept coming back was the yearlings were put in with the rest of the herd that was running with the stallion. Some got pregnant, had a foal and showed no ill effects, while others did not get pregnant but had no issues conceiving the following year. Logic dictates that if nature did not intend them to be breeding at such an early age than they would not begin cycling. We see this common misconception in other species, even humans. Today we are shocked when a 12-13 year old girl becomes pregnant, yet only 150 years ago it was common practise for a 13 year who came of age to be to married off and pregnant as soon as possible. If you did not have children by the age of 17-18 you were seen as barren and no man would have you, as having children to help on the farm was conducive to your survival. We breed cows, sheep and deer as early as one and yet we see this as normal. Why has it become abnormal for horses then. I believe we have humanised the horse due to their beauty, service and companionship and like the dog they have become dear to us rather than a source of food. This is admirable, but we must remember they have differing needs and instincts than humans and we must accept that as normal for their species. In the wild they would breed at one or two depending on the environmental circumstances.
The comment that Jeff Barko made re the gypsy vanner breed being raised as feral is so true and left to their own they cope just fine. It is human interference and ignorance that has done more harm. It makes sense when one thinks about it. These horses are kept in pretty much a feral environment, with little to no human intervention. Nature is allowed to dictate the lives of these animals, and will ultimately decide when the time is right to breed, and if the pregnancy will go to term. It's this 'feral' lifestyle that has created so many of the qualities in the breed that we treasure. It was once said to me by an American whom would like to remain anonymous "Unfortunately, here in America, most people will not follow natures path, and I fear we will ultimately breed-out many of these qualities that we have come to love in the gypsy vanners." I hope he is wrong and only time will tell.