Despite an extensive overhaul to its animal care standards, the Calgary Stampede got off to a rocky start Friday night when a horse had to be euthanized after breaking its leg during the chuckwagon races.

"The leg injury was considered a catastrophic injury, so our veterinarian made the decision to euthanize the horse immediately," said Stampede spokesman Doug Fraser."Honestly, we're greatly saddened by what happened today. We take the care of our animals very seriously."

Fraser said the accident was not the result of a crash and happened between turns on the chuckwagon race course.

No riders or other animals were hurt.

How the chuckwagon races are run, as well as standards for other events at the rodeo, have been greatly overhauled in the last year since six horses died at the Stampede's 2010 edition.

Two died of 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.

One change made in the wake of those deaths sees veterinarians implanting a microchip in every horse that is scheduled to compete in the chuckwagon races.

Stampede officials have said the chip will allow the horses to be tracked and monitored both during competition and at rest.

The biggest change to the chuckwagon races has seen the number of outriders on horseback who accompany each wagon as it thunders around a dirt track reduced to two from four.

Drivers must now submit a list of horses competing daily by 4 p.m. for a thorough pre-race inspection. If a horse is found not fit to compete, it is supposed to be "scratched" for that evening.

All horses will also be inspected as they leave the track, and the Stampede will impose a mandatory rest of two days for any horse that races four days consecutively.

"Keep in mind that when we introduced the large number of animal care changes over the past year, we did so in an effort to make our races safer," Fraser said Friday. "So we're particularly saddened by tonight's circumstances.

Our animal safety program is the leader in the world. There are thousands and thousands of racetracks, and do these things happen at other racetracks? Yes, they do."

Nonetheless, the Stampede has become a lightning rod for a growing number of animal welfare organizations who say rodeo is simply cruelty masquerading as entertainment.

Last year, the protest spread to Britain. A group called League Against Cruel Sports called on Ottawa to end "the immense cruelty" inherent 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.

The Vancouver Humane Society said more than 50 chuckwagon horses have died since 1986.