The crowd of 157,770 was on its feet and cheering as the big, unbeaten, muscular bay crossed the line first, 4-3/4 lengths ahead of the filly Eight Belles.

Trainer Rick Dutrow was still celebrating, along with thousands of happy bettors, as Big Brown and the 19 other horses in the race galloped out around the first turn at Churchill Downs.

It took a few minutes to sink in, but anyone watching those horses soon realised that one of them had fallen to the track.

"It's the filly," someone whispered. She went down about a quarter mile past the finish line.

In just a few minutes, the joy of the Derby and the promise of a new triple crown season were upended when Eight Belles was euthanised by injection on the track.

She had broken both front ankles and could not be saved.

"This horse showed you his heart," winning jockey Kent Desormeaux said, "and Eight Belles showed you her life for our enjoyment today. I'm deeply sympathetic to that team for their loss."

Big Brown did everything his owner said he would do. An explosive finishing kick put away his rivals for his fourth consecutive victory.

Eight Belles, meantime, was attempting to become the fourth filly to win the Derby. Her owners chose to keep her out of Friday's Kentucky Oaks so she could run with the boys in the Derby.

Big Brown's start from the outside post did little to hamper his charge when the field turned for home.

Under the urging of Desormeaux, the 2-1 favourite cruised to an easy victory to become the seventh undefeated Derby winner. The last one was Barbaro in 2006.

That wasn't the only reason thoughts of Barbaro were hard to ignore on this Derby Day.

The breakdown brought back memories of the 2006 Preakness, where Barbaro shattered his right rear leg just after the start. The colt was euthanised months later, after developing laminitis from the catastrophic injuries.

In two weeks, Big Brown will race in the Preakness as the only three-year-old with a chance to become the first triple crown champion since Affirmed in 1978.

"We're ready to roll," Dutrow said.

All week, Dutrow told the world he had the best horse in the field - and the big bay colt justified every accolade tossed his way.

"I can't describe the feeling that all of us have right now," he said.

Big Brown became the second winner to start from the number 20 post. The gelding Clyde Van Dusen did it in 1929. Denis of Cork was third.

Larry Bramlage, the Derby's on-call veterinarian, said the filly's injuries were too severe to even attempt to move her off the track.

Associated Press