We use PZP (Porcine Zona Pelucida) immuno-contraception. It can be darted in the field without capturing wild horses. It is the only acceptable form of birth control for wild horses, as it does not harm nor influence their hormones and therefore does not harm or influence their reproductive behaviors and herd dynamics. Any other form of birth control, such as geldings or overyectomies, are cruel, expensive, and will influence their hormones, which is why we do not support those forms of birth control for wild horses.
Birth control is important, because the herd is fenced in by civilization on all sides and their resources are limited; therefore they cannot grow exponentially. The PZP program will give the older mares a break from having babies and give the younger mares more time to mature before having a foal. All mares will still be given a chance to contribute their DNA to the herd. The goal of this program is for each horse born in the wild, to be able to live out its life in the wild.