A combination of GnRH-hCG in aged mares improves embryo recovery

Abstract

Older embryo donor mares typically have proven offspring, representing the largest population in most embryo transfer programs. However, aged embryo donors typically have poor embryo recovery due to uterine, cervical, and ovarian problems. In addition, older mares are thought to have deficient oocyte nuclear and cytoplasm maturation, leading to poor fertility. It has been proposed that better fertility results from combining LH-like (e.g. hCG) and GnRH-agonists to induce ovulation for potentially different mechanisms of action, as the LH-like molecule would presumably promote better nuclear maturation matching with the cytoplasm. Alternatively, the authors have been using a protocol in clinical practice, which consists of administering low doses of hCG and low doses of GnRH agonists before induction of ovulation to promote oocyte maturation; thus this study was undertaken to assess the usefulness of this protocol by assessing embryo recovery of commercial, aged embryo donor mares with subpar fertility. The study was conducted with six sub-fertile embryo donor mares (20±1.7, ranging 18-23yrs-old). In experiment 1, estrous cycles were assigned as control-cycles (n=13) or deslorelin-hCG cycles (n=12). Control-cycles received saline (0.5mL, i.m., q12h), and deslorelin-hCG cycles received deslorelin (125µg, i.m., q12h) starting when a dominant follicle achieved 20mm, and then once the same follicle achieved 25mm, mares received hCG (500units, i.m., q24h) until the dominant follicle achieved a periovulatorystage (i.e., ≥35mm with uterine edema ≥1). None of the mares received more than four doses of hCG. Then, ovulation was induced with hCG (2,500units). In experiment 2, thirty-cycles of the same mares were randomly assigned to control-cycles (identical to experiment 1, n=19) and histrelin-hCG cycles (n=11), which followed a similar design to experiment 1, except that instead of administering deslorelin, histrelin was given (125µg, i.m.). Ovulation was induced and confirmed by daily monitoring. In both experiments, mares were bred with cooled semen from fertile stallions. Embryo flushing was carried out eight days post-ovulation, and embryos graded. Statistical analyses were carried out with Fisher's exact test. Significance was set as P<0.05. Overall, 25% of cycles did not grow follicles in response to the treatment, double ovulation happened on three-treated-cycles. One mare had double ovulations in the control- and treated-cycles. None of the mares failed to ovulate after hCG. In experiment 1, the embryo recovery was 58.8% (7/12) for deslorelin-hCG cycles vs. 7.7% (1/13) for control-cycles (P<0.05). In experiment 2, the embryo recovery for histrelin-hCG cycles was 72.8% (8/11) in comparison with control-cycles 21% (4/19 cycles; (P<0.05). In conclusion, the results of this field study suggest that a combination of GnRH-agonist and hCG early in the cycle improves embryo recovery in sub-fertile aged mares and that the majority of cycles (75%) will respond to treatment namely grow follicles and ovulate.