The Vancouver Humane Society (VHS) has asked the Calgary Stampede to eliminate chuckwagon races from its event roster after four horses were euthanized due to injuries sustained during this year’s competition.

The Calgary Stampede, held annually in Alberta, Canada, features rodeo-style competitions and livestock exhibitions, including the GMC Rangeland Derby chuckwagon race. The event pits four teams of horse-drawn wagons against each other in a series of elimination races over several days. In 2015 36 drivers and 216 horses competed for more than $1 million in prize money, the Calgary Stampede website said.

In recent years, several horses—including some used in chuckwagon races—have died in stampede events. In 2011, Calgary Stampede management announced new rules intended to increase safety for both equine and human chuckwagon race participants, however four chuckwagon horses used in the 2015 even died as the result of injuries sustained during races.

On July 4, a 10-year-old Thoroughbred sustained a fractured cannon bone during a race and was subsequently euthanized. A second chuckwagon horse was euthanized on July 7 after being injured July 6 when a driver’s wagon made contact with the lead horse of another team