A three-year-old colt has become the 32nd horse to die at Santa Anita since December after being euthanised when he sustained a catastrophic injury in a race.

Two-time Kentucky Derby-winning jockey Mario Gutierrez was tossed off in the incident on the second day of the autumn meeting at Santa Anita where the Breeders' Cup will be run in November.

Track officials said Gutierrez was not injured after landing near the inside rail.

Track veterinarian Dr Dana Stead said in a statement that Emtech had two broken front legs and she made the decision to euthanise the colt on the track.

Workers hurried to put up a green screen to shield the foundering colt from the crowd, his front legs unable to withstand his weight.

Dr Dionne Benson, chief vet for The Stronach Group, which owns Santa Anita, said a review would be opened to consider the factors that contributed to Emtech's injury.

She said the colt would have a necropsy at the UC Davis School of Veterinary Medicine, which is mandatory for all on-track accidents.

"There's an expected level of safety and accountability that is required to participate at a Stronach Group racetrack," a statement from the track owner said.

"If anything less is found which could have contributed to this incident, it will be addressed immediately."

The fatalities at Santa Anita since December 26 have raised alarm within California and the rest of the racing industry.

The majority occurred during the winter months when usually arid Santa Anita was hit with record rainfall.