hCG is more effective than the GnRH agonist buserelin for inducing the first ovulation of the breeding season in mares

Abstract

Background Human Chorionic Gonadotropin (hCG) and Gonadotropin Releasing Hormone agonists (GnRHa) are routinely used to induce ovulation in mares. However, GnRHa efficacy in transitional mares has been suggested to be low. Objectives The aims of this study were as follows: (a) to compare the efficacy of hCG and GnRHa in inducing the first ovulation of the breeding season and (b) to evaluate the correlation between ovulatory response, uterine oedema and teasing score at the time of treatment during the early or late transitional phase. Study design Randomised controlled superiority trial. Methods Mares in winter anoestrus were treated with sulpiride when at least two follicles reached a diameter of 25 mm. The day after the follicle reached 35 mm in diameter, mares in oestrus were treated with GnRHa buserelin (N = 29) or hCG (N = 33) and checked daily for ovulation. Results More mares (30/33, 90.1%) ovulated when the first ovulation after winter anoestrus was induced with hCG, than with GnRHa, (11/29, 38.0%) (P = .0001). Ovulation rate was lower in mares that did not show uterine oedema and full acceptance of the teaser stallion for at least three days before the treatment (32/41, 78% vs 9/21, 42.9%) P = .01. Main limitations Plasma LH and oestrogen concentrations were not performed. Conclusions These results demonstrate that hCG was more effective than GnRHa for inducing ovulation in the first cycle after winter anoestrus. Uterine oedema and behavioural signs of oestrus, for at least three days before the treatment, were predictors for a positive response to ovulation induction.

Clinical insights: Assisted reproductive techniques: More than a solution to subfertility?

“…1 It is therefore interesting to note that in the case of barren mares pre-treated with the dopamine antagonist, sulpiride ® (to accelerate the development of the first ovulatory follicle), hCG was considerably more reliable than a GnRH analogue in ensuring ovulation within 48 hours (90% vs 38%). 2 This is presumably because hCG works directly on the follicle (in a luteinizing hormone (LH)-like fashion), whereas a GnRH analogue must elicit LH release from the anterior pituitary. This further supports the supposition that the primary factor limiting ovulation during the spring transitional period is pituitary ability to generate an adequate LH surge, rather than inadequate follicular responsiveness to LH.…”