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For Wild Horses Accustomed To Humans, Fertility Control Can Work

A volunteer with a darting gun used to treat horses with fertility control walks toward a band.
Kathleen Masterson
Steve Paige slowly approaches a band of wild horses to dart one of the mares with fertility control medication. The band is one of many on Blockchains' property at the Tahoe-Reno Industrial Center.

Wild horses roaming in the Virginia Range made national news when they were photographed on the campus of the Tesla Gigafactory. Though the photos are idyllic, the state says there are currently about 2,500 more horses than the land can sustain. Because this population is more accustomed to humans, that actually makes it a good place to try fertility control for managing the herd.  

On a sunny spring day, Deb Walker with the advocacy group American Wild Horse Campaign, drives along the USA Parkway with two volunteers who are trained in both vaccine preparation and rifle marksmanship. They’ll be shooting a dart filled with fertility control drug to inoculate certain mares. The birth control is called PZP, and if horses receive both a shot and a follow-up booster, it can prevent pregnancy for one year. 

Wild horse populations grow at a rate of about 20 percent a year, which means around 600 foals were born in the Virginia Range this spring.  

“Here's the thing, too, is the numbers that they're talking about, those would have been prevented had the program not been canceled," Walker said.

Walker is referring to an earlier agreement with the state that allowed the American Wild Horse Campaign to treat horses with fertility control. It was canceled abruptly in 2017 after just a few years of implementation. The horses have run basically unmanaged since then.

Unlike most wild horses in Nevada, which are managed by the Bureau of Land Management, this herd roams largely on private land, so the animals fall under state jurisdiction.  

The Nevada Department of Agriculture entered a new agreement this spring allowing the American Wild Horse Campaign to vaccinate the mares. The nonprofit funds the entire program, which, with the training of volunteers, will cost about $50,000 this year. 

A volunteer draws the vaccine into a dart which will be shot at a horse.
Credit Kathleen Masterson / KUNR
Volunteer Nancy Kilian prepares the PZP dose that will be used to vaccinate a mare. The vaccine must be defrosted, then mixed to activate and then loaded into a dart which will be shot into the mare's rear end.

An innovative 'Facebook' for horses

Given that there are about 3,000 horses spread over 400 square miles, finding the mares can be a challenging task. After driving down a dirt road to a small pond not far from Blockchains' offices, volunteer darter Nancy Kilian opens an iPad app with photos of horse faces, bodies and feet:

“So, they all have: a name, what's on their feet--if anything, their face, if they have a blaze, which way their mane goes,” says Kilian.

The database includes about 2,700 of the Virginia Range horses, all catalogued--and named--by the volunteers. The most important info for locating and identifying a mare is which stallion’s band she belongs to.


Kilian is a retired interior decorator who has been working as a horse darter for the last few years. I’m told she’s an excellent shot.


She says the darters work in pairs, one uses the book to identify the correct horse and look up any data about past vaccinations; the other preps the vaccine and shoots the dart.


Holding a paper version of the database, she flips through the horses: “We've got these sorted out by bays in the front, and then it goes to black horses, and so, if we see a horse out there and we see, oh, it's a black horse, he's got a blaze, he's got two rear white socks and a front right sock...then we know, 'Oh, it's Chogun.' ”  


It’s this intimate familiarly that the volunteers have with the Virginia Range horses that makes the likelihood of success with the PZP vaccine much higher, says Nevada state veterinarian J.J. Goicoechea.


“It works really well in smaller populations where you can get your hands on them very easily, and you know the horses,” says Goicoechea.


Challenges with vaccinating in the wild


The darter needs to correctly identify the horse both for the initial shot and then to make sure that the same mare gets a booster shot within a month or so.   


Another challenge: the darter must be able to get within 30 yards of the animal for the vaccine dart to have a chance at hitting the horse’s meaty backside and penetrating the skin. 

Goicoechea says that sometimes in the field, it can be hard to tell if the dart actually hit the target area: 


“We think it was administered where it needed to, we don't know if it hit a ligament. Did it hit a bone? Did it hit something else there? And so it isn't administered.”


He says there are a lot of variables to it, and, “obviously, if we had our choice, we would like to hand-administer these things because they work better. But we can't gather these horses every year. You can't do that to the horse, and you can't do it financially either.”


If a horse receives an inoculation and the follow-up booster, the fertility control is highly effective.  


Human encroachment on horse territory


While being accustomed to humans helps the darters get close to the horses and increase the likelihood of vaccination success, it’s also a problem.


“We're not naive to the fact of horse-versus-vehicle crashes that happen on USA Parkway, and neither the horse nor the person wins in those situations,” says Sarah Johns, a spokesperson for Blockchains. The tech company owns 67,000 acres in the Tahoe Reno Industrial Center and is working with the advocates to facilitate herd management.


“As we start to see the workforce grow out here, this is something that needs to be taken care of now, as a preventive measure, as a public safety measure,” Johns adds.


Jennifer Ott, the director of the Nevada Department of Agriculture, agrees.


“You know, when you have that many horses in a defined area like that, especially close to populated areas, then there's all sorts of, not just impacts to the natural environment, water resources, impacts to private land, but also horses on the roadways,” says Ott. “And the department is very concerned about it.”


A 2001 Natural Resources Conservation Service study found that the Virginia Range can sustainably support anywhere from 400 to 600 horses before the land will start seeing damage to watering areas and plant communities. 


The American Wild Horse Campaign says they are darting horses with the goal of reducing the population, but not all the way down to 600. They say that the 2001 study only analyzed part of the Virginia Range and didn’t consider all the existing habitat. 


Still, the nonprofit is concerned about the encroachment of human development that has reduced horse habitat and, in some cases, limited access to water.  

A volunteer horse darter checks the gun scope before inching closer to a band of wild horses.
Credit Kathleen Masterson / KUNR
Volunteer Steve Paige sights the scope of the gun before cautiously inching closer to the band of wild horses. He gets about 30 yards away before he shoots the dart at a black mare.

Back on Blockchains' property, volunteer darters Nancy Kilian and Steve Paige defrost the PZP vaccine and load the dart into the rifle. Kilian identifies two mares in this band that are due for a fertility shot.


Then Paige slowly and calmly walks toward the small band of horses led by a Stallion named Parker, all the while holding the rifle pointed up at the sky. The rest of the group hangs back, and Paige whispers some information about his approach strategy:


“So, we’re just stopping for a second so we don’t look overly aggressive and just come marching up to them, because if we do that, they’re just going to march right off,” says Paige. “Try to be nonaggressive and show a calm energy.”


When he gets closer to the band, the horses shift uneasily. Paige pauses.


“You can see we're already getting a little too close for these two groups now…We're 52 yards away and everyone's starting to move off. They sort of set the boundary for us.


Paige inches his way closer and then aims and shoots the dart at a black mare. The mare shifts a bit and the group trots off. I can’t tell whether the dart landed, but Paige saw it hit her back flank. He retrieves the dart from where it fell to the ground.


The wind has picked up and the group decides they won’t be able to accurately shoot any more mares today. As of this story, the group has vaccinated more than 360 mares in the Virginia Range. 


The group is targeting mares who haven't yet given birth. The vaccine doesn’t harm the in utero foal, and if the horse can be inoculated before she gives birth, she’s less likely to get pregnant. Mares can be bred with a new pregnancy within a week of giving birth.


But because the American Wild Horse Campaign got such a late start this year while waiting for state permission, many mares had given birth, and those mares are likely already impregnated with next year’s foal. 

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