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Management of overabundant free-roaming horse (Equus ferus caballus) populations has become a divisive and challenging effort for natural resource managers worldwide. Controlling these populations requires land managers to balance socio-political issues, multiple use management concepts, and endearing affection of horses to some stakeholders against negative ecological alterations created by excessive densities of these animals. One potential solution to this dilemma is controlling the fertility of female horses. During 2013–2020, research was conducted to develop and test a prototype dart configuration system to deliver an effective dose of GonaCon-Equine to free-roaming horses. This remote dart delivery system was demonstrated to be safe and reliable in free-ranging horses with a 90% (71/79) success rate for first attempts and provided the opportunity to design an experiment to determine the long-term effectiveness, reversibility, and physiological side effects of different reimmunization schedules. Five experimental groups of mares were established consisting of a control (n = 25), and a 4-yr reimmunization interval (n = 25) of mares from a previous experiment that were treated with this same vaccine by hand injection in 2013. Additional booster intervals of horses were (2-yr (n = 11), 1-yr (n = 15), and 0.5-yr (n = 14) and vaccinated by remote dart delivery during 2013–2016. Weekly ground observations were utilized to estimate foaling proportions, foal survival, body condition, and injection site reactions related to treatment groups. All GonaCon-Equine booster intervals reduced (P ≤ 0.001) foaling proportions between treated and control mares for all years of the experiment. Effective contraception was also achieved when GonaCon-Equine was applied by remote dart delivery at post-primary intervals shorter than 4 years. Reversibility of GonaCon-Equine immunization occurred in all treatment groups and averaged 0.40 (26/65). The only detectable adverse side effect of vaccination was intramuscular swelling observed in 89% (115/129) of mares and occasional draining abscesses at the site of vaccine injection. However, regardless of method of vaccine delivery (hand vs dart), none of these mares displayed any evidence of lameness, altered gait, or abnormal range of movement during the 6 years of this study. These results support previous findings that effective contraception of free-roaming horses will require a primary vaccination followed by reimmunization but that timing from 6 months to 4 years result in similar success and together can achieve greater reduction in population growth rates.