Big Brown with exercise rider Michelle Nevin at Belmont Park Race Track in June. (Al Bello/Getty Images)
The top three-year-old in the U.S. hurt his right front hoof in a six-furlong gallop on the turf at the Aqueduct track in New York.
Michael Iavarone, co-owner of IEAH Stables, said the injury was "a tremendous blow".
"There wasn't a lot we could do to prevent this," he told a teleconference. "It was just a complete fluke.
"We hadn't had issues with his feet. To have this come up like this is a shock to all of us especially since he worked fantastically today."
Big Brown had been due to take on Horse of the Year Curlin in the $5 million Breeders' Cup Classic at Santa Anita Park on Oct. 25 in one of the most highly anticipated races of the year.
The Kentucky-bred Big Brown, winner of seven out of eight career races and $3.6 million in prize money, hurt his hoof while working out with stablemate and Breeders' Cup Mile winner Kip Deville.
"They both worked very well," said Iavarone. "Everything seemed to go according to plan.
"They finished up together. They galloped out together. We were all excited."
It was later discovered Big Brown had taken off a "three-inch piece of flesh" from the back of his hoof.
No choice
"There's not a great deal of blood flow that gets down to that area," said Iavarone. "It takes a long time for that flesh to repair itself."He added the injury would take two to three months to heal and since Big Brown was set to end his career after the Breeders' Cup "we had no choice but to retire him".
Big Brown won his first five races, including the Kentucky Derby and Preakness.
However, the attempt by the son of Boundary to become the first Triple Crown winner since 1978 was derailed when he finished last in the Belmont Stakes.
Big Brown will now go to stud at Three Chimneys farm in Kentucky.
"The horse deserves a happy retirement," said Iavarone. "It rips right at the gut of us but we have to do it."
