America’s wild horses (Equus caballus), emblematic of the untamed West, are at the heart of a fierce debate over their management. The Bureau of Land Management oversees 83,000 horses across 175 Herd Management Areas (HMAs), enforcing a national Appropriate Management Level (AML) of 26,715, often below genetic viability thresholds, to prioritize livestock grazing (BLM Wild Horse Program). The Congressional Wild Horse Caucus, a bipartisan group led by Reps. Dina Titus, D-Nev., Steve Cohen, D-Tenn., David Schweikert, R-Ariz., and Juan Ciscomani, R-Ariz., champions fertility control — specifically Porcine Zona Pellucida (PZP) and GonaCon — as a humane alternative to BLM’s costly roundups and holding ($138.5 million annually, 48,447 horses confined). Nonprofits like American Wild Horse Conservation and the Humane Society of the United States echo this stance, claiming fertility control reduces population growth, ends roundups and resolves grazing conflicts with ranchers’ livestock.
However, mounting evidence suggests fertility control’s genetic and ecological flaws make it an unsustainable solution. Data from HMAs like Onaqui and Little Book Cliffs reveals that roundups persist despite PZP use, and sterilized horses compete equally with livestock for forage, failing to end grazing conflicts. In contrast, the Wild Horse Fire Brigade (WHFB), an all-volunteer, IRS-accredited 501(c)(3) nonprofit, proposes the Natural Wildfire Abatement and Forest Protection Plan, rewilding ~48,447 horses into some of the 100-million acres of vacant wilderness in western states to preserve genetic diversity, reduce wildfire fuels and mitigate conflicts. This article synthesizes new data with prior critiques, exposing the caucus’s misplaced faith in fertility control and advocating WHFB’s rewilding as a transformative, science-backed alternative.
Launched in May 2025, the Wild Horse Caucus aims to protect America’s mustangs through humane, science-based management, with fertility control as a cornerstone. PZP, derived from pig ovaries, prevents fertilization with 85–90% efficacy, while GonaCon, a GnRH vaccine, suppresses hormones for four to six years (50–100% efficacy), both EPA-registered pesticides. AWHC and HSUS, key caucus allies, cite successes like Assateague Island, where PZP maintains stable populations, arguing it reduces roundups and grazing conflicts. Rep. Titus’s 2020 amendment, allocating $11 million for PZP, and a 2024 letter by 77 representatives requesting 10% of BLM’s $140 million budget for fertility control in five HMAs, reflect this commitment.
Yet, scientific evidence exposes significant flaws. PZP and GonaCon select for mares with weaker immune responses, risking immunocompromised populations, as a 2023 study found genetic variations in immune genes influence efficacy. Nuñez et al. (2017) reported 3-5% heterozygosity decline per generation in herds <150 horses, with 50% of HMAs below 100, far from the IUCN’s 2,500-horse minimum (Ne=500) for genetic viability. BLM’s lack of stud-books or comprehensive genetic testing for small herds (500 viable with management) exacerbates risks, per the 2013 National Academy of Sciences report. Reduced foal numbers limit natural selection — critical for adaptation through predation, weather, diseases and competition — further threatening long-term evolution. Cassandra Nuñez’s research highlights PZP’s behavioral disruptions, like extended cycling and out-of-season births, impacting foal survival and social structures.
Claims that fertility control ends roundups are contradicted by data from key HMAs:
- Onaqui HMA (Utah): With an AML of 121–210 (~200,000 acres), Onaqui’s 586 horses in 2018 faced PZP treatment since 2012 (57 mares treated in 2012, $82,000 roundup) and GonaCon trials in 2023. Despite reduced growth (7% target in 2015), a 2021 roundup removed 415 horses to hit low AML, driven by sage grouse habitat and livestock grazing (thousands of cows/sheep), per BLM’s 2019 Environmental Assessment. Roundups continued, showing PZP/GonaCon’s failure to halt gathers.
- Little Book Cliffs HMA (Colorado): With an AML of 90–150 (~36,000 acres), this herd (~150–200 horses) used PZP since 2002, achieving near-zero growth by 2016 through Friends of a Legacy efforts. While roundups were less frequent (none 2009–2016), a 2016 gather occurred, and the 2018 EA proposed further gathers with PZP, citing AML exceedance and livestock grazing pressures. PZP extended intervals but did not eliminate roundups.
- Other HMAs: Pryor Mountains (AML 90–120) saw PZP reduce growth since 2001, but gathers occurred in 2015 and 2020 to enforce AML. Sand Wash Basin (AML 163–363) used PZP, yet a 2021 roundup persisted, driven by livestock’s seven to 11 times greater forage allocation. GonaCon’s limited use (e.g., Onaqui trials) shows no evidence of stopping roundups.
These cases reveal that fertility control, while slowing growth, does not prevent roundups, as BLM enforces low AMLs (often <150, risking 70–90% genetic erosion) to prioritize livestock grazing, contradicting caucus and nonprofit claims.
The caucus and nonprofits like AWHC argue fertility control resolves grazing conflicts by limiting horse populations. However, sterilized horses require the same forage (~20–30 pounds/day) and water (5-10 gallons/day) as natural horses, as sterilization does not alter metabolic needs, per the 2013 NAS report. Studies (e.g., Ransom et al., 2014) confirm no change in grazing or water intake post-PZP/GonaCon, with treated horses maintaining similar body condition. In Onaqui, 2020 Owyhee data showed no rangeland impact difference between treated and untreated horses, and Little Book Cliffs’ PZP-treated herd still competed with livestock.
Logically, sterilization does not end grazing conflicts, as both sterilized and natural horses vie for the same resources. BLM allocates 80% of forage to livestock (e.g., thousands of AUMs in Onaqui vs. minimal for horses), creating a structural imbalance, per AWHC’s own data. In Confusion HMA, livestock received 25,312 AUMs versus negligible horse allocations, showing rancher priorities drive roundups, not horse numbers. Fertility control’s population reduction is slow (e.g., 7% in Onaqui), leaving existing horses to compete, maintaining conflicts.
WHFB’s plan rewilds ~48,447 horses into 100 million acres of state, county, USFS, BLM, private forest, and wilderness lands under the HTEA, avoiding fertility control’s pitfalls. Key advantages include:
- Genetic Health: Maintains Ne >500 through natural predation, supporting large herds (2,500+, per IUCN) and natural selection via predation, weather, and competition, avoiding 10-15% diversity loss from PZP/GonaCon. Nine-generation success proves viability.
- Ecological Impact: Grazes 5.5 tons/horse/year, reducing wildfire fuels (two-thirds of deadly fires), mitigating smoke (52,500–55,700 deaths, $456 billion), and reseeding plants, per the 2018 Klamathon Fire.
- Grazing Conflict Resolution: Relocates horses to non-HMA wilderness, minimizing livestock competition, unlike fertility control’s failure to reduce forage needs.
- Cost-Effectiveness: Saves $50 million/year in holding costs, $72,000/horse in abatement, with a $1.5 million pilot, far below BLM’s $138.5 million.
- Humane Treatment: Preserves family units and natural behaviors, avoiding roundup trauma (e.g., 2022 Cañon City deaths) and sterilization’s disruptions.
Endorsed by Elko County (2024) and AM Best (2023), WHFB aligns with the 9th Circuit’s 2014 ruling recognizing horses as native and leverages post-Chevron (2024) legal opportunities to challenge BLM’s livestock bias.
The caucus’s endorsement of PZP/GonaCon, influenced by AWHC’s advocacy (e.g., Titus’s 2023 Freedom Award), overlooks fertility control’s limitations. While humane in intent, PZP/GonaCon risk 70–90% genetic erosion in small herds (<100, 50% of HMAs), select for weaker immune responses, and fail to stop roundups, as seen in Onaqui (2021, 415 removed) and Little Book Cliffs (2016 gather). The caucus’s focus on fertility control ignores its inability to resolve grazing conflicts, as sterilized horses consume equal forage (80% livestock vs. 1-2% horses), per AWHC’s data. WHFB’s rewilding, relocating horses to wilderness, supports large herds (2,500+), natural selection, and ecosystem restoration, aligning with IUCN guidelines and contradicting the caucus’s chemical approach.
The WHFB seeks $1.5 million for pilot programs in fire-ravaged California, and Oregon to test rewilding’s scalability, offering a humane, cost-effective alternative to fertility control’s genetic and ecological risks. As wildfires threaten lives and landscapes, and BLM’s livestock bias perpetuates conflicts, rewilding emerges as a visionary solution. The caucus’s well-meaning support for PZP/GonaCon falls short, ignoring the science-backed promise of WHFB’s approach. It’s time to embrace rewilding to protect America’s wild horses and their ecosystems for generations to come, as of May 11, 2025.
Editor’s Note: Simpson is an ethologist living among and studying free-roaming native species American wild horses. He is the award-winning producer of the micro-documentary film ‘Wild Horses‘. He is the author of a new study about the behavioral ecology of wild horses, two published books and more than 500 published articles on subjects related to wild horses, wildlife, wildfire and public land (forest) management. He has appeared on NBC NEWS, ABC NEWS, CBS NEWS, theDoveTV and has been a guest on numerous talk radio shows including the Lars Larson Show, the Bill Meyer Show, NPR Jefferson Public Radio and NPR National Radio, Global News, The Guardian and AM BEST TV.
