Thirty-two lambs (8/breed) were used for puberty determination in Mora Colombiana, Criolla, Romney Marsh and Hampshire rams between 5 to 12 months old. Rams were maintained on pasture during the day and stabled at night. Semen was collected by electroejaculate method and its quality was determined by valuating: volume, normal sperm morphology, progressive motility, viability and concentration, and testosterone concentration determined by radioimmunoassay (blood samples were collected from the jugular vein). All samples were obtained monthly, the body weight and the scrotal circumference were measured in the same days of sampling.

The first appearance of spermatozoa in the ejaculate occurred at 26.5, 26,27.5 and 25 weeks of age when the lambs had attained a body weight of 28.3, 27.2, 24.7 and 27.2 Kg, and a scrotal circumference of 22.4, 22.8, 19.4 and 22.9 cm for Mora Colombian, Romney Marsh, Criolla y Hampshire breeds, respectively. Only scrotal circumference presented statistical differences between breeds (P<0.05). Puberty was defined when ejaculate reached: concentration >150 x 106 Spzs/ml, normal sperm morphology >50% and sperm progressive motility >30%. In Romney Marsh rams, puberty was presented at 33.5 weeks of age (body weight 31.2 kg and scrotal circumference 23.8 cm), Mora Colombian rams, at 30.5 weeks of age (body weight 29 kg and scrotal circumference 22.9 cm), Criolla rams at 33 weeks of age (body weight 26.9 Kg and scrotal circumference 20.8 cm) and Hampshire rams at 28 weeks old (body weight 29 Kg and scrotal circumference 23 cm). There were no significant differences in the age at puberty between breeds, but there were significant differences in body weight and scrotal circumference (P<0.05). In this experiment 50% of the rams showed puberty with a scrotal circumference between 22.4 and 24.4 cm. The second elevation of testosterone levels, at 32 weeks of age (0.78 ng/ml), coincides with the attainment of puberty.