James Crisp / AP Dr. Larry Bramlage examines a thoroughbred at Lexington, Ky. The trend of short racing careers may be lucrative for breeders, but some in the industry question whether breeding for speed rather than durability is making the sport more dangerous.

LEXINGTON, Ky. — Big Brown figures to produce big green as a stallion, which is why the Kentucky Derby winner most likely will retire young, just as his parents did.

"We'll never know how sound he could be," said Ken Carson, a pedigree adviser in Pilot Point, Texas. "He could run 100 more times, but given what he's done at the racetrack, that won't happen. Genetically we won't know how many races he has in him."

Such is business these days. But while the trend has proved highly lucrative for owners who can fetch far more from breeding their young stars than racing them, some in the industry say durability is now becoming an afterthought. Those fears were heightened in the Derby when the second-place finisher, the filly Eight Belles, broke both ankles and was euthanized on the track.

Although the cause of the breakdown remains unknown, Larry Bramlage, the on-track veterinarian at Churchill Downs, says the thoroughbred breed is getting weaker in part because top stallions and mares are now judged largely by their ability to produce foals that win early races with colossal payouts. Little attention is paid to late-blooming horses with long racing careers that are steadier but a tad slower, he added.

"If we keep breeding the short careers together, we're going to get short careers," Bramlage said. "If we want durability, we have to select for it — and we really don't. We select for their ability to win."

Larry Jones, who trained Eight Belles — a large, muscular filly by anyone's standard — agrees times have changed.

"Are they more fragile?" Jones said. "When you take big — and long, spindly legs — and fast, that is somewhat of a deadly combination, but that is what wins races. Can a horse that size hold up and run? Well, yeah, they can, but if you took 100 of them like that, you would have a higher percentage of them breaking down."

A look at Big Brown's pedigree underscores the way the breeding business has focused on mating horses with top-heavy racing careers.

His sire, Boundary, now retired from breeding, ran only eight times — all of them sprints — before an injury ended his racing days at age 4. Big Brown's dam, Mien, made only two lifetime starts. And each of the Derby winner's grand-sires, Danzig and Nureyev, raced three times.

Compare those statistics with those from two generations earlier on Big Brown's family tree — Round Table made 66 career starts, Admiral's Voyage 52 and Nearctic 47.

Gary Knapp, Big Brown's breeder, says his decision to choose Boundary as a mate for his mare, Mien, was based not on what the foal might be able to do at age 2 but what it could do at age 4 or older. However, Knapp said, no breeder should have to apologize for discovering a colt that peaks early if it was bred for the right reasons.

"When I'm planning my matings, I'm trying to breed the best racehorses I can," Knapp said. "All of us in the horse business are in the business of entertaining people who want to wager on horse races. The better horse we can produce provides the most entertainment."

Should Big Brown win Saturday's Preakness, then the Belmont Stakes to become the first Triple Crown champion since Affirmed in 1978, he might be retired after six career starts as one of the world's most lucrative stallions. But even 3-year-olds with less impressive résumés leave the track for the breeding shed after winning a high-profile race.

It's not just the stars, either, but the entire breed seems to be experiencing shorter careers — and shorter seasons. According to The Jockey Club, the average U.S. racehorse made 6.3 starts in 2006 compared with 11.3 starts in 1960.

Alex Waldrop, president of the National Thoroughbred Racing Association, said the shorter racing careers are hurting the marketability of the sport because it's removing the recognizable names from the track early. As for shorter careers being responsible for more injuries, Waldrop says that will be studied. But he cautions there shouldn't be a rush to judgment after the high-profile breakdowns of Eight Belles and 2006 Kentucky Derby winner Barbaro.

"Some commentators have inadvertently been demonizing commercial breeders, as if that is some nefarious activity to breed horses for profit," Waldrop said. "Breeding for speed vs. breeding for durability is very complicated. If in fact it is a problem, and I'm not suggesting that I have an answer to that one, it's not one we can solve quickly."

For one thing, breeding is often more of an art than a science. Even stallions with a reputation for fragility produce some offspring with long careers. Both of Eight Belles' parents stem from Mr. Prospector, considered, Carson says, by some racing experts to be more known for speed than stamina. Many of Mr. Prospector's progeny, however, have enjoyed long careers.

Eight Belles' owner, Rick Porter, said nothing about the horse's background — despite her large size — suggested a susceptibility to injury.

"I wouldn't buy horses if there was a red flag out there," Porter said.

Dan Rosenberg, the former president of Three Chimneys Farm where Eight Belles was bred, said judging a horse's longevity by its size or physical attributes can be dangerous. For example, Point Given — one of the farm's top stallions — is a huge horse known for producing stable offspring.

Still, Rosenberg agrees that the average owner of a mare now is seeking stallions that can produce a winner at age 2 or 3 rather than a horse that can still be racing at 5 or 6.

"We're an instant gratification culture," Rosenberg said. "We want it right now."

One solution proposed by Bramlage is to somehow reward longevity, perhaps with larger purses for races featuring older horses. Breeding is just one of many safety issues — along with track surfaces and the use of a whip — being examined by the NTRA and The Jockey Club.

Scott Palmer, the on-track veterinarian that rushed to Barbaro's side after that horse broke down in the Preakness, says thoroughbred racing needs to use the recent public relations disasters involving euthanized horses to do something to make the sport safer.

"The climate is right," Palmer said. "The industry is sensitized. We can get some action."