Executive producers David Milch and Michael Mann made the decision "with heartbreak," according to a statement from HBO.
"We maintained the highest safety standards throughout production, higher in fact than any protocols existing in horseracing anywhere with many fewer incidents than occur in racing or than befall horses normally in barns at night or pastures," reads the statement. "While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won’t in the future. Accordingly, we have reached this difficult decision."
The latest horse death came Tuesday, when a 5-year-old thoroughbred reared back and hit its head after a fall, sustaining a severe injury that led to its euthanization. The American Humane Association insisted that filming involving horses cease, but shooting on the second season of the series continued without horses while the investigation proceeded. Groups like People for the Ethical Treatment of Animals called for the show to halt all production.
"The two of us loved this series, loved the cast, crew and writers," Milch and Mann said in the statement. "This has been a tremendous collaboration and one that we plan to continue in the future."