Follicular Development, Oocyte Maturation and Ovulation in Gonadotrophin-Treated Prepuberal Calves

Twenty pre-puberal calves, ranging from 85 to 140 kg in body weight, were divided randomly into five groups of four. Group I served as an untreated control. Group II received 1,000 IU pregnant mare’s serum gonadotrophin (PMSG) 5 days prior to slaughter. Group III received 1,000 IU human chorionic gonadotrophin (HCG) 3 days prior to slaughter. Group IV was treated with 1,000 IU PMSG, followed 5 days later with 1,000 IU HCG and slaughtered 30 h later. Group V received 200 μg of a synthetic gonadotrophin-releasing hormone (GnRH) twice daily for 5 days prior to slaughter. All injections were given intramuscularly. PMSG alone and in combination with HCG increased the number of follicles > 5 mm. Only the PMSG and HCG combination resulted in multiple ovulations (mean 12.5 CL per animal) and follicular oocyte maturation (41% in 2nd metaphase plate). Fewer than 1% of oocytes surrounded by a compact cumulus underwent nuclear maturation, whereas 81% of those with an expanded cumulus underwent nuclear maturation (P < 0.05). Alone, neither GnRH nor HCG caused any appreciable follicular development, oocyte maturation or ovulation.