The fifth heat of the opening night of the Calgary Stampede's chuckwagon races was the fastest of the night but it will be remembered more for an accident which prompted the euthanizing of a horse.

Rookie driver Cliff Cunningham's right wheel horse went down with a broken leg entering the tracks second turn .

This year, Calgary Stampede officials made sweeping rule changes aimed at preventing chuckwagon accidents. An opening night horse death was about the last thing it wanted to see.

"We are deeply saddened by what happened last night," says the Stampede's Doug Fraser. "It was a catastrophic injury by a horse who misstepped."

Animal welfare advocate Temple Grandin is in Calgary to meet with Stampede officals to discuss the care of it's livestock. She maintains injuries like the one in Friday's chuckwagon race are a result of selective breeding and not dangerous race conditions.

"The Stampede has done everything it can to prepare the track and change the rules," says Grandin. "Wagons are not smashing into each other and they are not hitting the rail. It is a genetic problem and it needs to be corrected."

Chuckwagon drivers maintain many of the animals would have been put down already if they had not become chuckwagon horses, and claim the safety of the animals is their primary concern.

"It's unfortunate this horse slipped and fell," says driver Rick Frazer. "Our stock comes first. They eat first, they sleep first, and they get water first."

The Stampede maintains chuckwagon races are safe for both the horses and the humans on the field.

A sample of the euthanized horse's blood has been sent for toxicology tests to determine if the incident was more than a simple accident.