How IUDs can help wild horses

As the Bureau of Land Management carries out its mission to manage and protect wild horses and burros on public lands, we’re trying to develop and use a variety of humane fertility control methods that can slow herd growth and reduce the need to gather excess animals and find them good homes. Though we’ve significantly increased the use of GonaCon-Equine, a fairly new vaccine that can cause up to four years of infertility, having more tools is always better and gives our specialists more options for controlling herd growth and protecting these national icons on public lands.  

Starting in 2020, the BLM began using a new form of fertility control for wild horses that we anticipate will have safe, long-lasting effects: specialized intrauterine devices (IUDs). These flexible, soft, Y-shaped IUDs are made from medical-grade silicone and were specifically designed for use in horses. 

The IUDs were first tested and shown to be humane, safe and effective for horses in peer-reviewed research published in Animal Reproduction Science and in the Journal of Wildlife Management. Beyond their safety, researchers at Oklahoma State University found that these specialized IUDs could be an effective option for preventing pregnancy in wild horse mares for multiple years! If wild horses have the same IUD retention rates as were seen in pasture trials (75% for 2 breeding seasons), about half of IUD-treated mares could still be contracepted for up to 5 years later. The study also found that, similar to what is observed with Porcine Zona Pellucida fertility control vaccines, most mares with IUDs continued to ovulate, and mate with stallions.

Ultrasound is shown on a laptop screen.
The screen of this veterinary ultrasound machine shows imagery from a scan of a horse's reproductive tract, in the Oklahoma State University study that confirmed safety and effectiveness of soft, Y-shaped silicone IUDs for wild horses.
Horses grazing on a hill.
Horses treated with soft, Y-shaped, silicone IUDs, in an Oklahoma State University study of safety and effectiveness. Two groups of mares lived and mated with fertile stallions in a large pasture for two breeding seasons. During that time, about 75% of the IUDs stayed in the mares.

So how exactly does it work? To receive an IUD, a wild mare would first need to be gathered and placed in a working facility. Before a veterinarian places the IUD in a wild horse, the vet must confirm that the mare is not pregnant, using ultrasonography. After receiving an IUD, mares are typically observed for 7 or more days in captivity to be sure they are not showing any signs of discomfort before they are returned to public lands.  

Once back in the wild, the mare will behave similarly to any other mare in the herd, other than not becoming pregnant while the IUD remains in place. By temporarily delaying the birth of new foals, BLM herd managers are working to ensure that wild horses have enough food and water to thrive, even when conditions are dry. Reducing herd growth rates helps wild horses and burros stay healthy, protects the habitats that they and other animals rely on, and reduces the need for removal and adoptions.