Recently reported trials demonstrate that treating mares with the gonadotropin-releasing hormone analog buserelin (Receptal®) 9 or 10 days after ovulation increases pregnancy rates.
Early embryonic loss is a major cause of sub-fertility in mares. Progesterone produced in the ovary by the corpus luteum maintains the early pregnancy. The embryo must signal its presence and ensure the corpus luteum is maintained in order to survive the first few weeks of pregnancy. In other species, luteal function in pregnant animals starts to diverge from that of non-pregnant animals between 11 and 14 days after ovulation. It is therefore, considered to be a critical time for the maintenance of the pregnancy.
John Newcombe and his colleagues carried out a series of trials over four years, to investigate the effect of the gonadotropin-releasing hormone analog buserelin on early pregnancy rates. Over 2000 Thoroughbred, cross-bred and pony mares were involved in the trials which extended over a four year period.
The mares were allocated into treated and control groups at the time of mating. All mares were examined by ultrasound scan to pinpoint the time of ovulation. The mares in the treatment group received a single dose of 20µg or 40µg buserelin 9 or 10 days after ovulation by sub-cutaneous injection. Mares were examined for pregnancy by ultrasound scan, starting at 12 - 14 days after ovulation, and continuing up to either 30 ± 2 or 40 ± 2 days.
Treatment with 20µg or 40µg buserelin 9 or 10 days after ovulation significantly increased the overall pregnancy rate by up to 10% in mares. The difference in pregnancy rates between treated and non-treated mares was maintained throughout the period of study. Both the 20µg and 40µg doses produced increased pregnancy rates. There was no significant difference in the effect produced by the two different doses.
The mode of action of the buserelin is unclear. The increase in pregnancy rate was noticed as early as 13-16 days after ovulation, which is before luteolysis occurs. Newcombe suggests that it is therefore unlikely that it has its effect by inhibiting luteolysis.
"We believe that improved pregnancy rated through the administration of buserelin are likely to be evident under practical field conditions." said Newcombe. "Further trials using buserelin in mares are indicated to determine the optimal dose and timing of treatment, and to confirm the effects on number of foals born."
The effect of the gonadotropin-releasing hormone analog, buserelin, on pregnancy rates in horse and pony mares.
J R Newcombe, T A Martinez and A R Peters.
Theriogenology, (2001) 55, 1619-1631