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Fluer explains controlling wild horse populations in Wyoming

by Wyoming Livestock Roundup

Riverton – “Wyoming has the second largest wild horse program in the nation behind Nevada,” said Scott Fluer, Bureau of Land Management wild horse and burro specialist. “Congress designates herd management areas where wild horses will be kept, and Wyoming has 16 herd management areas.”

Fluer spoke at the Fremont County Farm and Ranch Days in Riverton on Feb. 13. 

Management areas

“The bulk of the wild horses are in the southwest part of the state, but there are two herd management areas up by Meeteetse and Cody,” said Fluer.

When wild horse populations remain unchecked they can reproduce at a rate of 20 percent per year and can double their population every four to five years. 

“Unregulated growth on the population of the wild horses taxes our vegetation and our rangelands,” explained Fluer. “Under the Wildlife Roaming Act of 1971, Congress enacted the BLM to manage these herds.”

Fluer included that the BLM also ensures the sustainability and health of the wild horse herds and the public land that they roam on. 

Control efforts

The BLM has tried several methods to try and control wild horse herd levels, but the most effective one has been gathering the horses every few years and placing some of them in the national wild horse adoption program. 

“We do a gather and adopt out the horses that are ages four and under,” described Fluer. “Anything 10 and over, we put in a long-term holding facility in the Midwest and typically the five- to nine-year-old horses go back out on the range.”

“The horses that go back on the range are fertility treated,” he added. 

Currently there are 50,000 wild horses in the national adoption program and about 35,000 horses on the range. 

“Bottom line is we need to control the population numbers on the ground and in the field,” stated Fluer. “Cutting the reproduction level down to around five percent a year would be ideal.” 


One of the fertility measures the BLM is utilizing is an infertility vaccine called porcine zona pellucida, more commonly referred to as PZP. This vaccine tricks the mare into producing antibodies that makes her think she is pregnant.

“Mares are injected with PZP, and it allows for two to three years of fertility control, but then it’s supposed to drop-off and allow the mare to come back to cycling again to reproduce,” explained Fluer. 

The PZP vaccine costs around $325 per horse, plus an additional $500 to $700 for labor to catch a wild horse on the range – making it a total of approximately $1,000 per horse. 

When the wild horses are gathered, they are sorted by age and sex, as well as groups of horses to be adopted, horses to be shipped to long-term holding facilities and horses to be released back to the range. 

The PZP vaccine has to be mixed onsite where the horses are being sorted and treated. When administered, it has to be at a certain consistency that is similar to pudding. 

“It has to be delivered within 24 hours of being mixed up,” explained Fleur. “There are only nine of us in the U.S. that are trained to mix up the PZP vaccine.” 


Once the mares are in the chute and receive treatments, the hair is also clipped on the left side of their neck and hip. A HB brand is placed on the left hip to help designate which mares have been fertility treated. The brand was assigned to the BLM from the state of Wyoming. 

“Then we put a numerical number on the mare’s left side of her neck that helps identify the herd management area the mare is from,” described Fluer. “When that mare is retreated, a bar is placed underneath the number to help us keep track of how many times we have caught and vaccinated her.”

There have been very few side effects seen with the use of PZP. The product does wear off over time and if injected into an already pregnant mare, there have been no signs of it affecting the offspring. 

“If a mare is pregnant at the time of injection, she’ll go ahead and continue to foal normally,” reassured Fluer, “but the next year, she won’t be able to become pregnant.”

There has been some concern with the PZP vaccine of affecting the time of when foals are being born.

There has been an increase in amount of foals being born in January instead of in the spring. 

Fertility control measures 

Wyoming’s last wild horse gathering was in 2011, where 900 head of horses were gathered. The total number of mares caught was 357. At the time, 45 percent of them had a colt on their side, and the remainder 55 percent were dry. 

“These results are starting to tell me that maybe PZP isn’t working as well as it should be,” stated Fluer. “It’s not that 90 percent effectiveness we were hoping for.” 

Other fertility control measures the BLM has used have been time release fertility control pellets, another PZP vaccine called SpayVac that has the potential to last for four to five years of infertility, intrauterine contraceptive device (IUD) called GonaCon and darting horses with hormone implants.

Surgical procedures, such as spaying mares and either gelding or performing vasectomies on stallions, have also been considered. 

Due to increased public concern surgical procedures are not commonly practiced. 

Madeline Robinson is the assistant editor of the Wyoming Livestock Roundup and can be reached at 

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