David Ramey, DVM offers two articles to address heat in mares. Part 1 describes how the heat cycle works. Part 2 discusses many of the different treatments that people use to try to keep their mare’s heat cycle under control.
Folks try to keep mares from coming into heat in any number of ways, some of which work, and many of which don’t . . .but still get used.
Source: Help! My mare’s in heat (I think)! – Part 1, David Ramey, DVM, August 23, 2018. Link.
Source: Help, My Mare’s in Heat – Part 2, Horse Network, July 14, 2019. Link.