Two horses have died at the Calgary Stampede, bringing to three the number of animals that have died since the annual competition began Friday.

A chuckwagon horse, belonging to Doug Irvine's racing team, died of a suspected heart attack during a training session on Monday morning.

Then a bucking horse named Cinder Mountain broke its back during rodeo competition in the afternoon and had to be euthanized. The horse tossed Steed Cline during the novice saddle bronc event.

"It's what they refer to as an empty buck when the horse has already thrown its rider, or it's already off its back anyway, and the horse bucks again and unfortunately it breaks its back," said Stampede spokesman Doug Fraser.

Nolan Jans of Maple Creek, Sask., was taken to hospital as a precaution after a steer fell on him Monday. (CBC)

"We're constantly reviewing our practices, because I know there will be criticism, and we'll continue to review those practices and make those improvements that are necessary."

The incident happened minutes after Brittany Mercier of Airdrie made history by becoming the first woman in Stampede history to ride in the event.

On Sunday evening, another horse suffered a suspected heart attack during the team cattle-penning competition and fell on its rider, sending her to hospital.

Amy Carver is in intensive care in a Calgary hospital with a traumatic head injury and a broken shoulder blade.

A junior steer rider will be OK after a steer fell on him during competition on Monday. Nolan Jans of Maple Creek, Sask., was taken to hospital as a precaution.