Efficacy of buserelin for induction of ovulation in mares

Abstract

The rational and controlled management of equine reproduction in commercial programs promotes the development of drugs that help to regulate certain physiological processes in mares, such as luteolysis or ovulation. Human Chorionic Gonadotropin is used predominantly in commercial and experimental programs showing optimal ovulatory results in multiple studies. However, a lower inducing effect has been reported in old mares, which are highly represented in the commercial breeding population. Some studies with human formulations of buserelin, one of the oldest GnRH synthetic analogs, have demonstrated similar efficacy to other analogs in mares (Newcombe and Cuervo-Arango. Journal of Equine Veterinary Science. 2016; 41, 57). In Argentina, until now, there are no specific buserelin formulations for mares. The aim of this clinical prospective study was to compare the ovulation induction efficacy of two doses of a new buserelin formulation in mares of different age groups. The study was carried out in a commercial breeding program in the subtropical north of Argentina using healthy Quarter Horse and Criollo mixed breed mares randomly assigned to treatment groups. When uterine edema, a relaxed cervix and 35 mm follicles were detected, mares were induced once, with a single intravenous (IV) injection of 0.2mg of buserelin (Busereleq,Laboratorio CIMOL, Argentina) for group 1 (G1, n=77) and 0.4mg for group 2 (G2, n=54). The response was considered positive when absence of the dominant follicle and presence in the same ovary of a corpus luteum were detected by ultrasound, 48 hours after treatment. In both groups, mares were classified according to age in <10; 11-15; >15 years old (yo). The data were analyzed by the chi square test for categorized data using Infostat Software. The follicular size, degree of edema and age were compared in the different groups at the time of treatment by means of a non-parametric Kruskal-Wallis analysis of variance. The results showed that 87.01% of the G1 (0,2 mg) and 70.37% of G2 (0.4mg) mares responded positively (p<0.05). No significant differences were found between follicular size, degree of edema and age of the mares at the time of induction among treated groups (p >0.05). Efficacy of the different treatments by age group in G1 and G2 was 77% and 68% in <10 y.o; 96% and 71% in 10 -15 y.o; 90% and 73% in >15 y.o respectively. A significant difference among treated groups was detected in middle age (10-15 years) with more efficacy in G1 (p=0,017). Our results suggest that a low dose buserelin (0.2mg IV) can be used to induce ovulation with acceptable clinical efficacy, even in older mares.