Deep anestrous mares under natural photoperiod treated with recombinant equine FSH (reFSH) and LH (reLH) have fertile ovulations and become pregnant.

The most common equine breeding practice to decrease the time to the first ovulation of the year is to use artificial lights starting December 1 in the Northern Hemisphere. It can take 60-90 d for this lighting regimen to induce a fertile ovulation. The success rate for pharmaceutical compounds to carry out the same process has been variable. One compound that did induce an early ovulation was recombinant equine follicle stimulating hormone (reFSH), but neither pregnancy nor cyclicity was established in that study. Starting on December 1, 20 deep-anestrous mares of light horse breeds (4-15 y old) with follicles ≤ 20 mm in diameter and progesterone < 1 ng/mL were maintained under natural photoperiod while 10 control mares were maintained under artificial photoperiod. Starting on February 6, treatment mares were randomly assigned to one of two groups: reFSH (n = 10) or reFSH/reLH (n = 10). Jugular blood samples were collected daily from all mares, and luteinizing hormone (LH), FSH, progesterone (P), estradiol-17β (E) and immunoreactive (ir)-inhibin were analyzed by radioimmunoassay (RIA). When the largest follicle reached ≥32 mm in diameter, reFSH treatment was discontinued in both groups while reLH treatment continued in the reFSH/reLH group until a cohort of follicles reached ≥35 mm in diameter. Human chorionic gonadotropin (hCG) was administered intravenously (iv) to induce ovulation, and mares were bred to a fertile stallion every other day until ovulation. Mares receiving either reFSH or reFSH/reLH developed follicles ≥35 mm within 5-6 d of treatment compared with 15.8 ± 3.4 d in the control group. Both reFSH and reFSH/reLH induced ovulation in 100% of the mares within 10 d after treatment resulting in an 80% conception rate and a 70% pregnancy rate for both groups. Conception and pregnancy were designated as either presence of a 14 d old embryo (n = 16) or a fetal heartbeat at 24 d (n = 14), respectively. Only three mares in the control group ovulated within the same treatment period. Later ovulations in the control group resulted in 100% conception and pregnancy rates. At 25 d post-conception, treated mares that were pregnant (n = 7 per group) were administered prostaglandin (PGF2) to terminate the pregnancy and later returned to estrus. Treatment with reFSH or reFSH/reLH given to deep-anestrous mares under natural photoperiod induced fertile ovulations that resulted in pregnancy and cyclicity when pregnancies were terminated.