Do not be misled by this article that points the finger at GonaCon Equine.
PZP, the safe, proven and reversible fertility control vaccine endorsed by 98.6% of the wild horse advocates achieves the same result over a slightly longer timeframe.
As for the efficacy of GonaCon, Pendley indicated three years ago that it would inhibit the mares with one application.
Western Horse Watchers Association 