The Calgary Stampede got off to a grim start on Friday when a horse had to be euthanized after breaking its leg during the festival's first night of chuckwagon racing.

The horse's death comes in the wake of a substantial overhaul to animal care standards at the Calgary Stampede, leaving officials to answer some pointed questions on rodeo safety.

"The leg injury was considered a catastrophic injury, so our veterinarian made the decision to euthanize the horse immediately," Stampede spokesman Doug Fraser said in a statement released after the horse was put down.

"Honestly, we're greatly saddened by what happened today. We take the care of our animals very seriously."

During a chuckwagon race, a driver -- in this case, Cliff Cunningham -- leads a team of thoroughbred horses around a circuit.

Fraser said the injury wasn't the result of a crash and happened between turns on the race course.

No riders or other animals were hurt during the race.

In the wake of Friday's chuckwagon incident, the Vancouver Humane Society (VHS) has called upon the Calgary Stampede to suspend the races.

"The Stampede's claims to have made the race safer have been proven false," VHS spokesperson Peter Fricker said in a statement issued on Saturday.

"It is totally unacceptable to continue this event when horse after horse is dying just to entertain a crowd."

In an attempt to fend off criticism after six horses died at the Stampede in 2010, the annual event's chuckwagon racing protocol has undergone significant changes including the implantation of a microchip in every horse scheduled to compete in the competition.

The chip tracks the horses during races and while they're at rest, allowing officials to monitor whether the animals' owners are complying with mandatory rest day rules.

Owners must also submit a list of horses competing daily by 4 p.m. The animals are then given a pre-race inspection, and if it is found not fit to compete, it is supposed to be pulled from the event.

Also, unlike years past when up to four outriders on horseback have accompanied each wagon as it thunders around the dirt track, the number has now been reduced to two.

On his blog, chief Stampede veterinarian Greg Evans said officials want to be careful not to race horses to the point of fatigue.

"Chuckwagon racing is such a unique sport, such a specialized sport, that there isn't a lot of scientific data available about what appropriate racing schedules are," he wrote.

"We feel like we are in a unique position, being the largest event and the longest event, to track some of that and generate our own data."

Evans said he hopes that new microchips and a strict schedule will help officials make educated decisions on when racehorses should rest.

But despite changes to animal care standards, several animal welfare groups remain ardently opposed to the annual rodeo, which many see as inherently cruel.

Last year, the protest spread to Britain. A group called League Against Cruel Sports called on Ottawa to end "the immense cruelty" in rodeos and suggested to British travel agencies that they refuse to offer vacation packages to the Stampede.

More than 50 MPs signed a motion in the British House of Commons that asked the Canadian government to improve how animals were treated at the rodeo.

Of the six horses that died at last year's Calgary Stampede, two suffered fatal heart attacks, two were destroyed after suffering injuries and another broke its back from bucking too hard.

The sixth died after experiencing health difficulties 40 minutes after a chuckwagon race.

With files from The Canadian Press