Objective.

Aimed to describe the characteristics of the onset of puberty in males in Colombian hair ram lambs (Ovino de Pelo Colombiano, OPC), and their crosses with Katahdin and Santa Inés sheep in a farm located in Villavicencio, Meta.

Materials and methods.

15 lambs of three biotypes: OPC x OPC (OPC), Santa Ines x OPC (SO) and Katahdin X OPC (KO) from four until 12 months old. Ram lambs were grazing and they had supplementation with commercial salt and water ad libitum. Monthly body weight (BW), scrotal circumference (SC), testicular volume (TV) were measured and testosterone level were determined by Elisa test, and ultrasound of the two testicles was performed to determine the presence of the testicular mediastinum, also evaluating the presence and detachment of the urethral prolongation and then electro ejaculation was performed to determine the macroscopic and microscopic characteristics of the semen.

Results.

Genotype effect was significant for evolution of body BW, SC, TV over time. The SO and KO crosses presented the highest values in BW, whereas OPC lambs maintained a lower value until the end of the assay. At six months old, the three biotypes presented a minimum concentration of 150 million of sperm per ml with 30% of individual progressive motility.

Conclusions.

In non-seasonal tropical conditions in Colombia (Orinoquia), depending on the variables included, body weight, testicular development, pennis morphology, semen quality, sperm concentration and testosterone levels, it is postulated that around six months of age, the onset of puberty is displayed in the three biotypes.

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