How Big Will He Be?  

(Reprinted from Practical Horseman, March 2008)

Question:  Is there a reliable system for predicting a young horse's adult height?  If so, at what age does it work best to make the prediction?

I wish I could give you a quick answer in the form of an “equation” (as in, “put a tape on the horse's leg between two certain areas, then multiply by a number, divide by another number and, bingo, there's your answer.”)  Unfortunately, I've never found any scientific evidence proving that systems like this work reliably.  Predicting a horse's mature height can be difficult, not only because of the complicated nature of genetics but also because of the variability of individual growth rates

Different horses grow at different rates depending on a variety of factors, including breed, nutrition and heredity.  Some breeds seem to grow more slowly than others.  Whereas Thoroughbreds and Quarter Horses generally grow to their mature height within three to five years, warm blood often take six or more years to reach their full size.  Unless you know a youngster's history from birth, it's hard to predict where he'll top out just by judging his dimensions at any particular point during the growth process.  Even then, nature has a way of keeping us guessing.  Small yearling can surprise you with a late growth spurt, and giant weanling sometimes top out at average heights.

The only somewhat reliable system I've found for predicting a youngster's mature height is a careful analysis of his bloodlines.  Simply looking at the height of the dam and sire isn't adequate, because mares and stallions don't always reproduce their own size.  Two parents also rarely produce the exact average of their heights.  So breeding a small mare to a tall stallion won't necessarily result in average-sized offspring.  However, if you look carefully at the heights of the mare line (the dam, her parents and her grandparents) and the stallion line (the sire, his parents and grandparents) as well as the offspring of all of these individuals, you can sometimes find helpful patterns.  Certain stallions and mares seem to have a stronger “say” in whether or not their traits come through in their progeny.  Some mares, for example, tend to produce their own height no matter what size stallion you breed them to.  Other mares produce foals whose heights seem dictated by the stallion.

The best way to research a youngster's height heredity is to ask his breed registry if it records heights of stallions, mares and their offspring.  This information is well documented for some breeds.  Sweden, for example, has an excellent system for gathering inherit ability information, called the Best Linear Unbiased Prediction (BLOOP) index.  Because it's not uncommon for a stallion in Europe to book 300 mares in one year and most of his foals will be put through a testing program evaluating gaits, conformation, height, etc., you can glean a great deal of information about a Swedish Warm blood stallion's progeny through the BLOOP system.

If your horse's breed association doesn't offer such comprehensive inherit ability information or if you know nothing of his family background, you may not know how big he'll be for sure until he starts to fill out.  The old adage is “three days, three months, three years.”  What a foal looks like at three days theoretically gives you a good idea of how he'll look at three months and three years (or later, for the slower growers).  I recommend taking a photo of the horse at three days and three months and comparing back to them later.  When he appears to have regained the balance, proportions and “look” that he had at those earlier points in time, he may be close to the end of his growth.  However, if you notice that his knees are still “open” (with a slight depression in the middle of the front of the knee), he's still developing.  Also, if you're feeding him a consistent diet and he suddenly seems more “Sibby” than usual, he's probably going through a growth spurt.  Generally, by the time a horse is filled out – has lost the Sibby look and regained the three days/three-months balance – he's probably close to mature height.  But he may still surprise you!

In 1994, Deborah Borra transitioned from being a successful real estate agent and professional harpist to turning her breeding “hobby” into a 230 acres Swedish Warm blood (SWB) breeding facility, Normandy Manor Farm, in Genesee Valley, New York.  To date, she has produced 30 foals, four of whom have earned diploma status for 3 and 4 year old mares, and most of whom scored Class I at their SWB inspections.  In 2004, one of the farm's mares placed second in the two year old SWB class at Devon and another placed seventh in the Produce of Dam Class.  Deb offers breedings to several approved SWB stallions via fresh or frozen semen.