From wikipedia:

The safety of the series' working environment was called into question by People for the Ethical Treatment of Animals (PETA), which criticized Luck over the injury and euthanization of two horses during filming for the pilot and the seventh episode. The American Humane Association (AHA) said both racehorses "stumbled and fell during short racing sequences" and that "the horses were checked immediately afterwards by the onsite veterinarians and in each case a severe fracture deemed the condition inoperable." HBO pointed out that precautions had been taken: Each horse was "limited to three runs per day and was rested in between those runs."^[7][8] On March 13, 2012, HBO agreed to suspend all filming involving horses while investigations took place over the death of a third horse.^[9] The AHA insisted the stoppage remain in effect until a comprehensive investigation was completed; it also noted the horse's injury did not occur during filming or racing.^[10] The following day, HBO canceled the series, saying that while it "maintained the highest safety standards throughout production ... accidents unfortunately happen and it is impossible to guarantee they won’t in the future."^[