Pubertal development of Bos taurus beef bulls

The variability of onset of puberty, among and within breeds, has resulted in inconsistent reproductive performance of young bulls. To promote the successful use yearling bulls, it is very important to understand pubertal changes and factors that affect pubertal development. In the male suckling calf the seminiferous cords of the testes contain primordial germ cells and Sertoli cells that generate the seminiferous epithelium. A lumen is established in the seminiferous cords by about 5 mo of age and spermatogenesis is established by about 8 mo of age (1). An initial rise in FSH between 3 and 5 mo of age results in proliferation of Sertoli cells, seminiferous tubule lengthening and an increase in tubule diameter. At the same time, there is a rise in LH secretion resulting in increased testosterone production by the Leydig cells. Between 5 and 8 mo of age, FSH and LH remain low and then rise again with the onset of puberty. It has recently been shown that the greater the rise in LH at 3 to 5 mo of age, the earlier the onset of puberty and the larger that testis size will be at 1 y of age (2). In well-fed bulls testis growth is almost linear between 7 and 12 mo of age and scrotal circumference (SC) increases at a rate of 0.06 – 0.07 cm per day (3,4). The rate of testis growth declines after 12 mo of age, but by 24 mo of age, the testes will be approximately 90 % of mature size (5). Onset of puberty has been defined as the first time when the ejaculate has at least 50 x 106 sperm/ml with at least 10 % having progressive motility (6). Semen quality improves and achieves adult characteristics over a period of 3 to 4 months after the onset of puberty (7). Bulls within the same breed mature as much as 4 months later than the earliest maturing bulls (8). Approximately 33 % of beef bulls produce satisfactory quality semen (matur