224 THE EFFECT OF TREATMENT WITH VARIOUS DOSES OF A SYNTHETIC GONADOTROPIN-RELEASING HORMONE ON INDUCTION OF OVULATION IN LLAMAS

Abstract

Llamas require a copulation stimulus to elicit an LH surge that subsequently leads to induced ovulation. Gonadotropin-releasing hormone has been used to induce ovulation in llamas; however, results have been variable because of the different analogues and doses in previous studies (Bravo PW et al. 1992 Biol. Reprod. 47, 884-888; Ratto M et al. 2006 Anim. Reprod. Sci. 91, 299-306). Additionally, the effect of different doses and types of GnRH analogues on CL formation and function has not been described in detail. The purpose of the present study was to conduct a dose response trial to examine the effect of a synthetic analogue of GnRH, gonadorelin acetate (Ovalyse^®, Pfizer, Santiago, Chile) on ovulation and CL formation in llamas. Nonpregnant and nonlactating adult llamas (range 4-8 years) weighing 90 to 130 kg were examined once daily by transrectal ultrasonography using a B mode scanner with a 7.5-MHz linear-array transducer (Aloka^® SSD 500, International Clinics, Santiago, Chile). Llamas with a follicle ≥8 mm in diameter that had grown for 3 consecutive days were assigned randomly to be given an i.m. administration of gonadorelin at a dose of 1) 50 μg (n = 5); 2) 25 μg (n = 8); 3) 12.5 μg (n = 9); 4) 6.25 μg (n = 8), or 5) i.m. administration of 0.5 mL of PBS as a negative control group (n = 5). Llamas were examined by ultrasonography every 12 h from treatment (Day 0) to ovulation and thereafter on Day 8 to determine CL formation. Ovulation was defined as the sudden disappearance of a large follicle ≥8 mm that was detected during the previous examination and was confirmed by subsequent CL formation on Day 8. Ovulation rate was analyzed by chi-square analysis. Follicle diameter (at the time of treatment), interval from treatment to ovulation, and CL diameter on Day 8 were analyzed using one-way ANOVA. Mean ± SD diameter of preovulatory follicle did not differ (P = 0.4) among groups (11 ± 1.6, 10.6 ± 1.5, 11.5 ± 1.9, 11.7 ± 1.9, and 9.8 ± 2.9 mm for 50, 25, 12.5, and 6.25 μg of gonadorelin and 0.5 mL PBS, respectively). The proportions of ovulated llamas did not differ (P = 0.1) among GnRH-treated groups (100, 62.5, 77.7, and 62.5% for 50, 25, 12.5, and 6.25 μg of gonadorelin, respectively) but they did differ (P < 0.01) from the control group (0%). Also, the interval from treatment to ovulation in those ovulated females ranged from 36 to 48 h and it did not differ (P = 0.6) among groups. Corpus luteum diameter on Day 8 (12.2 ± 1.8, 10.2 ± 1.1, 11.7 ± 2.2, and 11.3 ± 1.8 for 50, 25, 12.5, and 6.25 μg of gonadorelin, respectively) did not differ (P = 0.3) among groups. In conclusion, the minimal dose of gonadorelin used in this study (1/8 dose) was as equally effective as the highest dose to induce ovulation in llamas without compromising CL formation.

The study was supported by Proyecto FONDECYT 11080141, Convenio de Desempeño en Investigacion (2007 DGI-CDA-04), Universidad Catolica de Temuco, and College of Graduate Studies, Faculty of Veterinary Sciences, Universidad Austral de Chile.