How big will a foal or young horse get?

When I was looking to buy a weanling, I lusted after this lovely weanling in Riegelsville, PA (pictured left). He was a big boy at birth and large for his age when I was considering him. He was out of my price range, and the gods were smiling on me because now as a long yearling, it's looking like he will probably top out at 16.1 or 16.2. As a tall person (5 foot 10 inches) w/a 36" inseam, a tall horse is pretty much a necessity if I don't want my feet to drag on the ground!

There's not a heap of scholarly research on predicting horse height, at least not in the licensed databases I can access. So, I found what I could on the Internet - veterinary journals, forums, advice columns. I also talked to breeders, who gave candid evaluations of the likely mature height of horses I looked at. Below is a summary of the collected wisdom and the actual source info...

GENERAL OBSERVATIONS

• a mare's first foal tends to be born smaller than subsequent foals, and the first baby's mature height is slightly smaller.
• some claim that height is inherited more from the dam.
• look at sibling height to help predict mature height, esp any full siblings.
• look at height of parents through great-grand parents.
• Find out the dam and sire's reputation for "throwing size."
• overfeeding will speed up growth but will not affect final height.
  

SPECIFIC METHODS OF PREDICTING HEIGHT

1. MOST ACCURATE is to measure elbow to ergot/fetlock with a string. Then "flip" the string, rotating from the elbow and raising the bottom part of the string up, vertical and taut, aligned with the wither. This is the final horse height.
2. Second string test method: measure from the front part of the coronet band on the hoof right where the hoofwall meets hair, and pull a string straight up to the center of the horses knee (where the dip is in the middle of the joint). That measurement will co-incide with the horses final height. So if the measurement is 15 and 3/4 inches, the horse will be 15'3.
3. Evaluate your youngster's general conformation as a yearling or later. Is he or she extremely lanky/gawky, or are the proportions more similar to an adult/mature horse. If it is the latter, your horse may not grow much more.
4. Rough guesstimate at one year, take current height and add two hands.
5. The cannon bone has reached its full length at one year old, and it is about 98% its full length at birth. Compare a baby's knee height to its mom, or to others its age, or to other broodmares. Find an adult whose knees are roughly the same distance from the ground, the baby/youngster is likely to end up the same or similar height.
6. There are percentage growth estimates:

  Age in Months  Percent of Mature Height

          Birth              61 to 64

            1                67 to 70

            3                76 to 79

            6                83 to 86

            9                87 to 90

           12                90 to 92

           18                94 to 96

           24                96 to 98

           30                97 to 99

           36                98 to 100

           48                99 to 100

           60                100

For draft horses the estimates are different, reflecting slower growth...

   Age in months          Percent mature height
        6                           79
       12                           89
       18                           92

SOURCES









Equine Chronicle article: Growth rates in foals
http://www.equinechronicle.com/health/growth-rates-in-foals.html
 
String Testinghttp://ultimatehorsesite.com/info/stringtesting.html
Visual aids for string tests!!!

Horse Advice on weanling height
http://www.horseadvice.com/horse/messages/3/5218.html
This Web site has some free information and an article available for a fee. I found that the info from the fee based article is available freely on the Internet.

Today's Horse
http://todayshorse.com/determining-mature-height/
Great, sensible advice from Dr. Jessica Jahiel about predicting height.

Horse Express
http://animalscience.ag.utk.edu/horse/pdf/HorseExpress/HorseExpressSummer2006.pdf
This includes growth percentage charts for various breeds, and the reader might be able to guesstimate a particular breed (e.g., warmbloods) based on these charts.

Skeletal growth rates of weanling and yearling thoroughbred horseshttp://www.journalofanimalscience.org/content/73/9/2513.full.pdf
This is an interesting, if detailed article that discusses growth rates and weight for yearling thoroughbreds, bone by bone.

Estimation of Heritabilities for Weight, Height and Front Cannon Bone Circumference of Thoroughbredshttp://www.journalofanimalscience.org/content/47/6/1243.full.pdf+html
Discusses heritability of height. 

Horse Spine Growthhttp://www.equisearch.com/horses_care/horse-growth-continues-age/
Basically a tidbit about the growth rate of the spine, which continues after full height is achieved.
 
Horsedochttp://horsedoc_org.tripod.com/junior_measuring_up.htm
I'm unfamiliar with the site's author/editor, but it offered some interesting casual tidbits so I'm including it.