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BLM considers new horse population control

by Wyoming Livestock Roundup

As Wyoming BLM continues to try to manage their horse populations within the state’s borders, a new fertility control drug is being tested and considered for use in feral horses.
    The new treatment is Spay Vac, a liquid vaccine.
    “We’re entertaining the idea of trying it this fall as a research project,” says Wyoming BLM Wild Horse and Burro Specialist Scott Fleur. “It’s not being used anywhere else right now, and there is a study at a facility in Oklahoma where BLM is treating 90 mares in a five-year research project.”
    Of using it in Wyoming, Fleur says, “We were prepared to go with PZP (porcine zona pellucida) until last week, but if we get the go-ahead to use Spay Vac from the U.S. Geological Service and APHIS (Animal and Plant Health Inspection Service), and I can get the environmental assessment put together and the public is ok with it, I’d like to try Spay Vac.”
    Although there’s not a lot of research to date, Fleur says Spay Vac has shown promising effects.
    “It’s been shown to be about 86 to 87 percent effective,” he notes. “It either permanently sterilizes a mare, or provides four to five years of sterility.”
Results pending for PZP
    Currently PZP is the most popular choice for the agency’s reproductive control.
    “PZP acts as a foreign protein against which the treated mare produces antibodies and becomes infertile,” explains Fleur. “Over time, the antibody levels fall off and fertility returns. A booster injection should be given at 10 months, but we can’t catch the mares again, and ideally the mares with PZP have to be caught every two years to be treated, which is expensive.”
    On a typical gather, Fleur estimates the BLM is able to capture about 80 percent of the horses. The mares treated with PZP through time-release pellets are aged and described, a freeze brand is applied to the neck and hip and they’re turned back out.
    “We’ve used PZP since 2004 in a couple HMAs (Herd Management Areas), and now we use it in 14 of the 16 HMAs,” says Fleur.
    Fleur says he has flown the HMAs in July for the last three years to get a count on the foals born that spring.
    “Reproduction of 18 to 25 percent each year is the average, and we’re running about 18 percent, based on the number of foals born that we caught in relation to the total number of animals we caught,” he says.
   Although the Antelope Hills HMA is down in percentage, Fleur says he doesn’t know if that’s because of PZP, the weather or predation.
Additional strategies
    Other methods of reproductive control that the BLM has considered, in addition to PZP and Spay Vac, include gelding – which is in litigation in Nevada – and a three- to four-year PZP vaccine that is under study in Nevada.
    “There are 80 mares being tested, and if it works it would extend gathers from every two years to every four,” he said of the longer-lasting PZP.
    Spaying mares has been placed on hold, pending review by the National Academy of Science, with a decision on how to proceed expected next summer.
    GonaCon is a product that is being used in a deer study in New York. Fleur says it consists of a single shot that renders fertility control for up to five years and is effective with a booster injection.
    Of the self-regulation theory, Fleur says, “If you let them self-regulate, you’ll deplete your range. There aren’t enough predators that want to eat horse, or spend the time to chase one down. We know predation occurs, but it’s not signification enough to impact the population.”
    “The BLM is trying to be proactive in knocking production down,” says Fleur. “We’re getting there, but not quite fast enough.”
    Christy Martinez writes for  the Wyoming Livestock Roundup. Send comments on this article to

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