A Canadian humane group is calling for an immediate halt to chuckwagon races at the Calgary Stampede in Alberta, Canada, after three horses died and another was injured during this year’s event.

The Stampede’s GMC Rangeland Derby chuckwagon race pits four teams of horse-drawn wagons against each other in a series of elimination races over several days. Traditionally, each team consists of four wagon horses and four horses carrying outriders.

In recent years, several horses have died in separate Stampede rodeo events, including Thoroughbreds used in the chuckwagon races. In 2011 Stampede management announced new rules intended to increase safety for both equine and human chuckwagon race participants, including veterinary inspections to determine whether horses are fit to compete in the race. Those deemed unfit are scratched from the event while those deemed fit to compete also undergo post-race veterinary inspections. The rules also prohibit any horse from running in the event for more than three consecutive days, and require all horses used in the chuckwagon races to rest at least one day in every four.

On July 12, during the fourth heat of the day’s Rangeland Derby race, driver Chad Harden’s left lead horse collapsed resulting in an accident involving all four wagon horses. Stampede spokesman Doug Fraser said that three of those horses died as a result of the incident, including the right lead horse and an outrider horse that collided with the wagon. The right lead horse and the outrider horse were humanely euthanized due to the severity of their injuries, he said. A necropsy will determine the cause of the left lead horse’s collapse, Fraser said. Another horse located on the wagon’s right wheel sustained non-life-threatening injuries requiring surgery, Fraser said