The drama about the seedier side of horseracing will air the final two episodes of its first season now in progress, HBO said. But the series won't return for the second season that began production last month, it said.
"While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won't in the future," the channel said. "Accordingly, we have reached this difficult decision."
The move was made with David Milch, the show's creator, and Michael Mann, his fellow executive producer on the drama that brought film actor Hoffman to series TV. It was a high-profile project for the premium channel that stakes its reputation on such fare.
HBO said it was "immensely proud" of the series and those involved in it, and the producers said in a joint statement that they "loved this series, loved the cast, crew and writers."
On Tuesday, a horse was injured and euthanized at Santa Anita Park racetrack, and HBO agreed to suspend filming with the animals after the humane group that oversees Hollywood productions had issued an immediate demand "that all production involving horses shut down" pending an investigation.
The animal was being led to a stable by a groom when it reared and fell back, suffering a head injury, according to HBO. The horse was euthanized at the track in suburban Arcadia.
During season-one filming in 2010 and 2011, two horses were hurt during racing scenes and euthanized. HBO defended its treatment of the animals, saying it's worked with the American Humane Association and racing industry experts to implement safety protocols that exceed film and TV industry standards.
The American Humane Association's film and TV unit, the group sanctioned and supported by the entertainment industry to protect animals used in filming, called for a production halt at the Santa Anita Racetrack in suburban Arcadia after the second horse's death.
Racing resumed after new protocols were put in place and proved effective, Karen Rosa, the AHA unit's senior vice president, said in February.
On Tuesday, Dr. Gary Beck, a California Horse Racing Board veterinarian, said he had just examined the horse as part of routine health and safety procedures before it was to race later in the day. The horse passed the inspection, the AHA said.
"The horse was on her way back to the stall when she reared, flipped over backwards, and struck her head on the ground," Beck said in a statement. An attending veterinarian determined that euthanasia was appropriate, he said.
Dr. Rick Arthur, medical director of the state racing board, said such injuries occur in stable areas every year and are more common than thought. A necropsy will be conducted, he said, which is routine with all fatalities at racing board enclosures.
The first two horse deaths drew criticism from People for the Ethical Treatment of Animals, which said that safety guidelines used in filming failed to prevent the deaths "so clearly they were inadequate."
Kathy Guillermo, a PETA vice president, said at the time the group didn't consider the matter closed.
"Racing itself is dangerous enough. This is a fictional representation of something and horses are still dying, and that to me is outrageous," she said.
On Tuesday, Guillermo said PETA sent complaints about "Luck" to Arcadia police and an animal humane society in nearby Pasadena.
"Three horses have now died and all the evidence we have gathered points to sloppy oversight, the use of unfit, injured horses, and disregard for the treatment of thoroughbreds," Guillermo said, calling for an immediate halt to filming.