Troy Dorchester had just rounded the first corner during the second heat of Monday night’s Rangeland Derby when his horse began to collapse.

The veteran driver, and winner of last year's Stampede sportsmanship award, was able to stop his team before any of the other horses were hurt.

"Everybody here, you love horses, that’s why you do this. It was a tough night and a tough morning walking in the barn," said Dorchester, his voice cracking with emotion. "We’re assuming it's a heart attack. We haven’t had the results back yet."

A team of veterinarians rushed to the collapsed horse on the track, but it died almost immediately.

The 14-year-old gelding was cleared by Stampede veterinarians ahead of Monday’s competition, and ran twice in Ponoka last week without any sign of distress.

The death is the first at this year’s Stampede.

A necropsy is planned to determine the cause of death, but Stampede spokesperson says given how early in the race the horse collapsed, and the lack of apparent injury, it’s unlikely anything more could have been done.

"We don’t, at this point, know what it was other than an internal, very serious internal medical condition," said Kristina Barnes of the Calgary Stampede.

The Jockey Club - an industry group based in the U.S. - tracks equine race fatalities in North America and says in 2018 the rate was 1.68 deaths for every 1,000 starts. Chuckwagon racing horse deaths at Stampede are less than half of that.

Dorchester says the horse was one of his family’s favourites and the same age as his son. "It was a hard night. It’s just like anybody you got a pet, you’re with them that long, they grow on you."

All horses competing at Stampede are microchipped and tracked for exertion and rest periods, as well as inspected by a veterinary team before they’re cleared to compete.

The Vancouver Humane Society is calling for the Rangeland Derby to be suspended until a panel of independent experts can be assembled to study how to make the races safer for the horses.

They say 61 horses have died in the event since 1986 - however 17 of those deaths happened in two separate incidents before drastic changes to the race format and health monitoring were brought in.