The Dual Threat to America's Wild Horses: Misplaced Darts and Genetic Erosion from Poorly Trained Darters and Chemical Sterilization
- William Simpson II
- Aug 6
- 7 min read
By: William E. Simpson II - © 2025 – All Rights Reserved

Image: A wild horse with a misplaced dart in its abdominal wall. Courtesy of Michele Cheeseman.
Introduction
Wild horses, roaming the arid rangelands of the American West, embody a legacy of freedom and resilience. Managed by the Bureau of Land Management (BLM) across Herd Management Areas (HMAs), these populations face a dual challenge: immediate welfare risks from poorly executed contraceptive darting and long-term genetic vulnerabilities exacerbated by chemical sterilization methods like Porcine Zona Pellucida (PZP) and GonaCon. A compelling photograph of a wild horse with a dart lodged in its abdominal wall underscores the dangers of inadequate training and poor marksmanship, while scientific evidence highlights how these practices, combined with small herd sizes and historical bottlenecks, accelerate genetic loss. Given the dire situation wild horses are already in—with populations bottlenecked, heterozygosity declining rapidly, and many HMAs below viable sizes—any form of contraception only worsens the crisis by imposing artificial selection and further eroding genetic diversity. This article explores these intertwined issues, rejecting contraception entirely and advocating for rewilding as the humane, science-based path forward.
The Image: A Stark Warning

The photograph depicts a sorrel wild horse—marked by its reddish-brown coat, white facial blaze, and white socks—standing in a dry, rocky landscape dotted with sagebrush. The horse appears calm, mid-step with its head lowered, but a small, brightly colored dart protrudes from its left flank in the upper abdominal wall, known as the paralumbar fossa. This area, posterior to the ribs and anterior to the hip, is far from the recommended hindquarter target site for darting.
The image (above), courtesy of Michele Cheeseman, serves as a visual testament to the consequences of poorly trained darters or inaccurate shots. Such misplacements highlight a critical gap in field practices, where lack of skill or experience transforms a supposedly humane tool into a potential hazard, further illustrating why contraception via darting should be abandoned.
Dangers of Poorly Trained Darters and Misplaced Darts
Proper darting requires precision, targeting the hindquarters—specifically the gluteal muscles or biceps femoris—for safe intramuscular injection of PZP or GonaCon. Guidelines from the BLM and wildlife veterinarians mandate training, often through certified courses, and teamwork (e.g., one darter, one observer) to ensure accuracy at ranges under 50 meters. However, when poorly trained individuals or poor marksmen handle dart rifles, the results can be disastrous:
1. Immediate Physical Harm
Misplaced darts, like the one in the photo, strike thinner areas such as the abdominal wall. The dart’s impact—delivered by compressed gas or powder charges—can cause bruising, lacerations, or penetration into the peritoneal cavity.
Risks include internal bleeding, organ damage (e.g., intestines, kidneys), or peritonitis, leading to colic—severe pain and gastrointestinal issues that can be fatal in horses without veterinary care. The horse in the image shows no immediate distress, but hidden injuries could develop if untreated.
2. Vaccine Delivery Failures
In non-muscular sites, darts may fail to fully discharge, resulting in under-dosing or leakage. Internal spillage of PZP or GonaCon can cause chemical irritation, reducing efficacy and potentially triggering sterile inflammation. This wastes resources and skews population control efforts.
3. Infection and Chronic Issues
Open wounds in dusty environments heighten infection risks, leading to abscesses or sepsis. Studies report injection-site reactions (e.g., swellings lasting months) in up to 79% of GonaCon-treated horses, with severity increasing in misplacements. Chronic pain or lameness could impair mobility, affecting foraging or herd dynamics.
4. Welfare and Management Challenges
Poorly trained darters may violate safety protocols (e.g., firing near people or in high winds), endangering both horses and handlers. Recovery of misplaced darts is often neglected, leaving environmental hazards and unconfirmed dosing. In remote HMAs, veterinary intervention is impractical, prolonging suffering and raising ethical concerns.
These physical risks are compounded by the fact that contraception itself is unnecessary and harmful, as it fails to address root causes while inflicting additional stress on already vulnerable populations.
Genetic Vulnerabilities: Bottlenecking, Heterozygosity Loss, and Small Herd Sizes
Beyond immediate physical risks, wild horse populations face genetic threats that poorly executed contraception worsens. Historical bottlenecks—due to habitat loss or past culling—have reduced genetic diversity, leading to inbreeding and fixation of harmful traits. Many HMAs, with fewer than 150 horses, fall below the recommended threshold for maintaining viable genetics (150-200 breeding animals). This results in low heterozygosity—critical for adaptability—declining 3-5% per generation in small herds.
Aspect | Recommended Minimum | Current Reality in Many HMAs | Genetic Consequences |
Herd Size | 150-200 breeding animals | <150 (often <100) | Accelerated allele loss, inbreeding depression |
Heterozygosity Decline | Minimal (<1% per generation) | 3-5% per generation | Reduced adaptability, higher disease susceptibility |
Effective Population Size (Ne) | >50-100 | Often <50 | Bottleneck effects amplified, fixation of harmful traits |
These conditions make herds prone to extinction vortices, where genetic decline fuels further population drops.
How Chemical Sterilization Amplifies Genetic Loss
PZP and GonaCon, administered via darting, aim to control population growth non-lethally by suppressing reproduction in mares. However, in genetically fragile herds, these methods intensify genetic erosion and should be rejected outright:
1. Reduced Effective Population Size
Contraception removes individuals from the breeding pool, shrinking the effective population size (Ne). In a 100-horse HMA with 20-30% of mares treated, fewer animals contribute genes, accelerating allele loss. Misplaced darts, as seen in the photo, worsen this by wasting doses, leading to uneven application and further skewing breeding dynamics.
2. Artificial Selection Pressures
Targeting specific mares (e.g., older or genetically valuable ones) can favor certain traits, reducing diversity. Models show faster allelic decline under contraception than heterozygosity loss overall. This may lead to unintended consequences like reduced fitness or behavioral shifts.
3. Prolonged Suppression Without Gene Flow
Unlike removals, which allow translocations to introduce new genetics, contraception keeps non-breeding horses in the herd, consuming resources without offspring. In isolated HMAs, this prolongs genetic stagnation. Studies warn of irreversible diversity loss in herds below viability thresholds.
Given the dire genetic situation—exacerbated by bottlenecks and small herd sizes—contraception represents selective breeding that violates the intent of the 1971 Wild Free-Roaming Horses and Burros Act, turning wild horses into managed zoo animals rather than allowing natural evolution.
The Synergy of Poor Darting and Genetic Risks
The image of the darted horse encapsulates this dual threat. A poorly trained darter’s misfire not only risks immediate harm—potentially causing colic or infection—but also undermines any pretense of humane management. Wasted or improperly delivered doses disrupt already flawed contraception plans, leaving some mares breeding while others are excluded, further distorting the gene pool. In small HMAs, where every breeding animal counts, such errors compound the loss of heterozygosity and exacerbate bottleneck effects, pushing herds closer to collapse. Supporting contraception in this context is untenable.
A Proposed Solution: Rewilding to Reach AML Using the Wild Horse Fire Brigade Plan
One innovative alternative to darting and contraception is the Wild Horse Fire Brigade (WHFB) plan, also known as the Natural Wildfire Abatement and Forest Protection Plan, developed by ethologist William E. Simpson II. This approach focuses on rewilding excess wild horses into vast wilderness and forest areas, achieving Appropriate Management Levels (AML) in HMAs while providing ecological, genetic, and welfare benefits—without the harms of contraception. More at: https://www.wildhorsefirebrigade.org/benefitswhfb
Key Elements of the Plan
Rewilding Methods: The plan involves humanely relocating intact family units of wild horses from BLM holding facilities, conflicted HMAs, and other areas to economically and ecologically suitable wilderness zones. These include approximately 353 million acres of privately-owned forest lands and 110 million acres of public wilderness areas in the western U.S., which are unsuitable for livestock due to terrain, predators, and regulations. Proposals target relocating around 48,447 excess horses to up to 100 million acres, as well as other state and county lands, utilizing laws like the Humane Transfer of Excess Animals and potential amendments to the 1971 Free-Roaming Wild Burro and Horse Protection Act to enable direct rewilding.
Reaching AML: By removing excess horses from areas where horses are in conflict with commercial enterprises in HMAs (where AML is often set low, e.g., 26,785 total across 177 HMAs; average of 150 horses/HMA), the plan reduces herd sizes to sustainable levels without roundups or sterilization. This alleviates grazing conflicts with livestock, which currently receive 80% of forage allocations.
Benefits
Wildfire Prevention: As keystone herbivores, rewilded horses graze 5.5 tons of grass and brush per year per horse, reducing wildfire fuels and mitigating catastrophic fires. This decreases toxic smoke (linked to 52,500–55,700 annual premature deaths) and related costs ($456 billion). Horses also reseed plants, enhancing ecosystems, as demonstrated in areas like the 2018 Klamathon Fire.
Genetic Preservation: Unlike contraception, which amplifies genetic loss by selecting for immune traits and reducing effective population size (Ne), rewilding supports large herds (2,500+ horses, Ne >500 per IUCN guidelines) under natural selection pressures like predation and weather. This avoids the 10-15% diversity decline from PZP/GonaCon, addresses bottlenecking in small HMAs (50% under 100 horses), and preserves heterozygosity through gene flow in expansive areas.
Animal Welfare and Cost Savings: Horses live freely in family bands, reducing confinement stress and darting risks. The plan saves about $140-million annually in BLM holding costs, with each horse providing $72,000 in wildfire abatement value. Pilot programs cost around $1.5 million.
As an alternative to fertility control, WHFB avoids behavioral disruptions (e.g., extended cycling) and roundups, which persist despite PZP use in HMAs like Onaqui and Little Book Cliffs. It promotes holistic management, potentially establishing large-scale wilderness grazing pilots.
Recommendations for Sustainable Management
To address these issues:
Training and Oversight: Mandate certified darting courses, regular skill assessments, and supervision—but ultimately phase out darting entirely in favor of rewilding.
Genetic Monitoring: Implement DNA sampling to track diversity and prioritize rewilding to enhance gene flow via translocations.
Reject Contraception: Cease all use of PZP, GonaCon, or similar methods, as they exacerbate genetic erosion and fail to resolve stakeholder/economic conflicts.
Post-Darting Care: Recover darts promptly and monitor for complications, even in wild settings, to assess welfare and dosing success—while transitioning away from these practices.
Implement Rewilding Initiatives: Pilot the Wild Horse Fire Brigade plan to test rewilding as a scalable solution for AML compliance, genetic health, and ecosystem benefits.
Conclusion
The photograph of this wild horse with a misplaced dart is a call to action. Poorly trained darters jeopardize immediate animal welfare, while chemical sterilization, in the context of small, bottlenecked herds, accelerates genetic loss—threatening the survival of these iconic animals. Given the dire genetic and welfare situation, no form of contraception should be supported. Instead, by embracing innovative solutions like the Wild Horse Fire Brigade's rewilding plan, we can protect both the health and heritage of wild horses for future generations.
Learn more here: https://www.wildhorsefirebrigade.org/benefitswhfb
Photos courtesy of Michele Cheeseman.
Article written by William E. Simpson II (© 2025) at 10:29 PM PDT on Sunday, August 03, 2025.
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