Feral horse populations continue to grow, and management is focused on removal both at a substantial cost in the process and their long-term care. Fertility control has been identified as a means to provide a solution. Two EPA registered immunocontraceptives (Gonacon and ZonaStat-H) are available for equids, but both require an initial dose and at least one booster ranging from weeks to years later making this relatively impractical. Our aim was to develop a single dose long-lasting contraceptive targeting 2 oocyte-specific growth factors (OGF) GDF-9 and BMP-15. In our proof-of-concept study, GDF-9 antigen representing 14 amino acids of the mature protein, and a BMP-15 antigen representing 24 amino acids in the mature sequence were produced. Both were conjugated to keyhole limpet hemocyanin (KLH), and the experimental vaccine was manufactured using Pet-Gel A as an adjuvant. We reported that domestic mares were treated with an initial dose and 4 boosters at 6 week intervals. All treated mares developed antibodies and demonstrated infertility for an entire year, and 88% of the mares remained infertile for the following year. The current investigation was conducted to examine the effects of a single dose with a potentially long-lasting immunostimulant. We enrolled 32 mares and 4 stallions with proven fertility that had been removed from the Nevada range that year. All mares had produced foalsthe year before. In May, 16 of these mares were treated with a new OGF vaccine formulation using the same antigens conjugated to KLH and AdjuVac adjuvant containing liposomes. Another 16 mares received a sham injection of adjuvant alone. Serum samples were collected monthly for titer analysis via enzyme-linked immunosorbent assay and progesterone assays via radioimmunoassay. Pregnancy determination was also conducted monthly via transrectal ultrasonography. All mares were then housed with fertile stallions in August. Four treated and 4 control mares were housed in each of 4 pens with a single stallion. All (16/16) of control mares became pregnant. One treated mare was euthanized for reasons unrelated to treatment. Of the remaining treated mares 67% (10/15) became pregnant within 60 d while 20% (3/15) became pregnant much later (180–300 d). Two mares (13%) remained anovulatory for 2 years. The mares with delayed and inhibited fertility demonstrated the greatest (P < 0.001) and longest lasting (P < 0.0001) anti-OGF titers. While this specific vaccine attenuated ovulation it failed to sufficiently alter fertility as pregnancy rates were not different. Our goal is to now find an adjuvant that will result in a single long-lasting vaccine.