Top 5 things to know about wild horse and burro fertility control

The Bureau of Land Management is tasked with a very unique and challenging job that many may not even know about: keeping a rapidly growing population of wild animals, with virtually no natural predators, from running out of food and water in a very arid environment. How does the agency do that? 

The simple answer is: any way that is humane, legal, safe, effective and practical! That’s the approach the BLM is taking to solve the challenge of ever-growing herds of wild horses and burros that roam public rangelands, which can overgraze and disturb habitat if not carefully managed. One of the ways we’re protecting herds from overpopulation is through the use of wild horse and burro birth control, otherwise known as fertility control. 

Fertility control is when the BLM uses a method – in most cases a vaccine for mares – to prevent pregnancy, usually over a temporary period of time. But how does it work, and how effective is it? Here are the top 5 things to know about wild horse and burro fertility control. 

1. There are several forms of fertility control for wild horses and burros

The BLM primarily uses a vaccine called Porcine Zone Pellucida, or PZP, which has been approved by the EPA since 2012 for use in wild horses and burros. The BLM is also expanding its use of a newer vaccine called GonaCon-Equine, which works slightly differently than PZP, as well as soft silicone intrauterine devices. In FY2020 the BLM increased its use of GonaCon-Equine by 75 percent. 

Needles and syringes on a table.
A fertility control vaccine mixing station. PZP fertility control vaccines must be kept at a cold temperature until they are ready to be mixed and deployed.

2. Currently available fertility control vaccines have limits

The PZP vaccine is generally only effective for 1-2 years, after which an annual booster is required to keep it working. There is some evidence that GonaCon-Equine can last up to 5-6 years if boostered after the initial dose.

Woman mixes fertility control
A BLM specialist mixes PZP vaccine in the field in preparation for darting a wild mare.

3. Some wild horses can be remotely darted with fertility control vaccines

The BLM works, often with volunteers and partners, to identify, track and dart wild horses to help control growth in some smaller herds. Most wild horses in larger, more remote herds are not approachable enough to be darted and would need to be gathered for treatment. For those herds, the BLM must capture, treat, and release the animals back to public lands. 

A darter with wild horses on the range
A fertility control darter typically must come with 30-50 yards of a wild horse in order to effectively dart her.

4. Fertility control is most effective in smaller herds

Studies indicate that 75 percent or more of the mares in a herd must be treated to have a significant effect on herd growth. For those herds that are close to a sustainable size, the BLM is prioritizing fertility control treatments to slow future growth and reduce the need for removals. 

Horses running on the range.
A large wild horse herd in Eastern Nevada. Fertility control operations work best when a herd is at appropriate management level and the animals are approachable, identifiable and trackable.

5. The BLM supports research into improved fertility control methods

The need to frequently re-treat wild horses with vaccines is a barrier to more wide-spread use in larger, more remote herds where darting is not practical or effective. The BLM has funded a large number of fertility control research projects since the late 1970s to increase their effectiveness. The BLM’s recent focus has been on studies into humane methods with contraceptive effects that last for several years or more. Find more information about the BLM’s research efforts

Like all management methods for wild horses and burros, fertility control has benefits as well as some very real limitations, which is why the BLM considers it as one tool among several that we use to manage and protect wild horse and burro herds. Learn more about how the BLM manages wild horses and burros

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