Why do some thoroughbreds run better on grass than dirt?

Nice answer by Sandra. I'd like to elaborate.

Affinity for certain surfaces can be passed through the sire/dam families. Sometimes, the connection isn't immediately apparent, but every modern Thoroughbred has turf breeding within it's pedigree, to a greater or lesser degree.

Pedigree and conformation go hand in hand and you have to take your eyes off the paper and get up close to physically see what that pedigree looks like in the flesh. Conformation and stride are passed from the sire & dam, and to some extent, from the second generation. Certain sire lines produce typical physical/mental traits in the offspring, which can determine their surface preference.

There's a lot of controversy surrounding the conformation of turf runners and it varies from trainer to trainer. Most agree that pedigree is key and that one never knows if a horse will like the turf until they try it. Turf horses come in all shapes and sizes. One thing they share in common is the ability to quickly accelerate and the most successful ones are long striding.

Thanks for the excellent question!

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It's not necessarily true regarding offspring. This is tough question because we hear a lot of factors, (a number of things), some say turf horse have a bigger foot-size (the hoof that is larger and wider than average), better kickback, higher action, turn of foot, stride, and it could be but not necessarily offspring / pedigree etc... With that, yes conformation (the physical structure of the horse) does seem to weigh in...if you notice turf horses tend to be average size and/or smaller. However, most trainers would say the best way to know is to "try" the horse.Also, trainers state turf horses take to "all weather" better than dirt horses, meaning they are (arguably) more VERSATILE than dirt horses, and have the advantage on a sloppy track.

:)

General comment: if it was possible to know, based on pedigree and/or conformation, that a horse would be better on dirt or turf,

Turf champion John Henry wouldn't have made his first 17 starts on the dirt before someone figured out that maybe he'd run well on the grass.

Horse of the Year Cigar wouldn't have made start after start on turf before his connections figured out he was a better runner on dirt.

And you wouldn't have trainer after trainer, hall of famers and leading trainers, having to try their charges on different surfaces to figure out what surface they were most effective on.

While it's true that pedigree can give an indication that a horse might possibly be more effective on one surface than the other, it isn't an absolute indicator. It's interesting to note that Storm Cat, for example, who was not noteworthy as a runner on turf, was very successful in siring horses like Giant's Causeway, horses that did very well indeed running on the turf courses of England and Ireland; and that City Zip, a stallion whose own performances on the turf were unremarkable, and whose pedigree doesn't indicate any particular affinity for turf, is turning out to be a leading sire of turf runners.

It's also true that you'll hear some people say that a horse that has a low, "daisy-cutter" stride is more likely to have an affinity for the turf than a horse with higher action; and you'll also hear people talk about a horse having a "turf foot," specific conformation of the hoof and foot that supposedly makes it more likely for the horse to be effective on the turf.

But the truth about that is that if there was any conformational feature that was a true and absolute indicator of affinity for turf, then you wouldn't have trainers ever running a horse on the "wrong" surface. There are people in racing who have measured every angle on thousands of horses, done impressive work on stride analysis and studied the conformation of hooves, ankles, knees, hocks, stifles, length of back and ratios of various body parts to one another, and they STILL can't tell you with absolute certainty whether a horse will do well on turf or dirt (or artificial surfaces, for that matter).

Basically, the determination of whether a horse is given a shot at running on turf is still a trial-and-error game. Generally if a trainer suspects that a horse might be effective on turf, he'll try to get a chance to work the horse on a turf track with an experienced exercise rider or a jockey with experience riding turf horses up, and he'll ask the rider how the horse felt running over the turf. If the rider indicates that the horse seemed to take to the grass, the trainer will look for a race on the turf to try.

One thing is an absolute about turf runners here in the United States: in order to preserve the turf courses, racing secretaries generally don't card very many claiming races or starter allowance races on turf. They reserve the turf races for higher-class horses, so in general a horse has to be competitive at the higher claiming levels, at the very least, in order to get a chance to run on the turf. Races that are carded on the turf also tend to be longer races, on average, than races carded on the dirt, so a horse has to have an affinity for longer distances to be effective on turf races.

That's about as far as anyone can go in making an educated guess that a horse might like running on the turf. And an educated guess is the best that anyone can make about a horse's affinity for a particular surface. He might like it, he might be effective on it, it's worth a try. When it comes to the individual horse, that's ultimately the only way to know for sure: try him (or her) on the surface and see how it works out.