Here's HBO's statement:
"It is with heartbreak that executive producers David Milch and Michael Mann together with HBO have decided to cease all future production on the series 'Luck.'
"Safety is always of paramount concern. We maintained the highest safety standards throughout production, higher in fact than any protocols existing in horse racing anywhere with many fewer incidents than occur in racing or than befall horses normally in barns at night or pastures. 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.
"We are immensely proud of this series, the writing, the acting, the filmmaking, the celebration of the culture of horses, and everyone involved in its creation."
And Milch and Mann issued this statement:
'The two of us loved this series, loved the cast, crew and writers. This has been a tremendous collaboration and one that we plan to continue in the future."