How tall will my foal get?
Perhaps the most common question we get as breeders is: "How tall will he be?"
Can you actually predict the future height of a foal?
The answer is yes you can predict its future height if you are willing to accept a range. There is however NO methods that can give you a definite precise height (16.1 1/2!) and we will see why.
You will find a few rules of thumbs here and there (the string method and the cannon bone method are the most commonly used and mentioned- I will touch on that later) and a whole lot of anecdotal evidence of this and that. Some will talk of taking the average height of the parents or to look at siblings but those are often less accurate than the rules of thumbs. If you ever met two human siblings you will understand that height is not always constant in a family 🙂
What we seem to know for sure is that horses will grow to their genetic potential-not that it helps much because no one can truly tell what that genetic potential is. Unless absolutely starved, and we are talking about life-threatening starvation here, horses will mature to their genetic makeup pretty much no matter what.
No matter how much you feed that shetland pony it will not grow more and you can leave a Percheron to fend on prairie grass and it will grow into the tall horse he was meant to be. So breed and antecedent certainly have a part to play in this guessing game.
However, studies have been done and they found no statistical difference in final height between a cohort of youngsters that had a restricted diet versus one that was fed adequately. Now, other problems arise from a poor diet, but you can put away the story that is often mentioned: "this horse is small because it did not get XYZ diet when it was younger." There seems to be no scientific proof for this. Anecdotes are fun and we remember them better but they do not make facts.
Speaking of facts, here is a solid source of information. Dr. Harold Hintz did a multi-year study, carefully measuring thousands (1,992 to be exact) of foals as they matured and published the result. The growth rate curve below is the result.
The orange line represents the average and the vertical line the variation. As you can see there is quite a range in the first few months, this reflects that some foals grow quicker than others. Therefore a foal that is 5 months old could be 84% of its final height or at 81%.
At about 10 months of age, the range is reduced and then it remains about the same until the 3 years mark where it is reduced even more.
Now you understand why it is that with estimating mature height in young horses one must speak in range. If you want a definite number wait until the horse is about 5 yrs old 😉
In case you are wondering if this growth curve is particular to well-fed Thourougbred kept in an ideal setting, a very similar study was done on the hardy Icelandic horses. The curve was identical.
This brings us to the fact that all horses breed mature at the same rate. Something that was clearly expressed in Dr. Deb Bennett seminal article that I reference at the end of this article. There are no slow-maturing horses (or quickly maturing one for that matter). All horses mature and grow at a very similar rate, with allowance for those ranges indicated earlier.
So how tall will my weanling be at maturity based on the growth curve?
With simple math, one can therefore use the growth curve validated with 1000 data points to extrapolate the final mature height of a young horse. Keeping in mind that the younger the animal, the wider the range of potential results.
I made two charts for you to refer to.
The first one tells you that if you are hoping your young horse to reach a particular height it should measure within an inch of those values at the age indicated.
Again, please remember to think in terms of range. If one inch (or two for that matter) will make a huge difference to you, you might want to wait and measure a bit later before putting your youngster up for sale...On the other hand, be skeptical of the ads suggesting that the 15.3hh 3 yrs old will likely finish 16.2hh-(assuming the horse is correctly measured in the first place!).
The other chart below tells you what range of height to expect of 6 months old foals.
I picked 6 months because that is often the age at which foals are weaned and sold. For more accuracy in foals under 12 months of age, it would be better to look at their measurement at around 10 months (we will see a bit later why that is) as that is when we see a narrowing of the range of expected heights, but let's face it who can wait ?!
To know if your particular foal will be on one end of the range or another take a look at its parents and at its general look. Is one parent smaller than the other? That might pre-dispose it to mature a bit on the smaller end of the range. Does it look very leggy or more proportioned than other foals it's age? The leggier, weedy looking youngster will likely be taller than its more proportional buddies.
Other methods to estimate mature height.
Two other methods are usually used to predict mature height in young horses:
- the knee to coronet band method
- the string method
The knee to coronet band method ( valid once the foal is over 8 months of age).
Based on the reality that the first bones to stop growing are the ones in the lower limbs and that mature horses generally have a measurement from the knee to the hoof about 1/4 of their height this method can be handy.
You simply measure from the middle of the knee to the coronet band in a straight line. The number of inches will give you the final height of the horse in hand +/-one inch or two. Again it's about as precise as the other method as long as you get a good measurement. The horse should be standing square, with its leg underneath its elbow and with its weight on the leg you are measuring.
The string method is more convoluted. It requires a string and it is based on the idea that the mature horse will have a general balance of legs and girth. The ideal mature horse having about the same distance between elbow and ergot (the back of the fetlock) as between elbow and top of wither. You use a string to measure first from elbow to ergot and then you rotate the string up to see where the wither should be at maturity. Like I said it requires a patient horse, a helper, a string, a good hold of the horse's elbow.
Once you have that first measurement you double it to get the total height (presumably in inches that you then divide by 4). It just seems overly complicated to me but some people swear by it.
In either case, I suggest you practice on fully mature horses and convince yourself that your measurement is correct.
While you are at it why don't you measure your mature horse. (Generally speaking, horse owners are about 2 inches off the real measurement of their mature horses when they simply estimate it from how it "looks"- a topic for another day.)
As mentioned the technique above works for horses older than 8 months old and the reason is shown in the chart below (taken from Dr. Bennett study), as mentioned above that is the age at which the key bones of the legs are done gaining in length.
So why the variation of 1 to 2 inches?
As you see any method of prediction gives results accurate to one or two inches.
For some, that is a big difference and it certainly can be. Standing between two accurately measured horses that have two inches differences in height one realizes just what those two extra inches can feel like a lot!
The main reason there is so much uncertainty with the final height of the horse is that we measure the horse at its wither. The wither is simply the point of the spinous process of the thoracic vertebrae of the horse and they are the last bone to mature and reach its final length.
That is why horses can seem to gain 1 or two inches in height toward the end of their growth. Since the spinous process length is related to the weight of the head you can see why the bigger horses with the bigger head will keep "growing" a bit longer. In reality, all the major bones are done growing but the spinous process keeps growing until the horse is 6 or 7 yrs old.
Some horses have very prominent withers that will easily gain them 2 inches on the height measurement while their back is relatively low, others will have lower relief to their withers and therefore have a back more in line with it.
The reality is that the convention of measuring a horse at its wither is here to stay and so we have to accept that the final height of a mature horse will be subject of much speculation until the horse is fully grown at 6 or 7 yrs old.
Growth rate study in Icelandic horses
The seminal article by Deb Bennet on growing horses. A good read but a long one.