and Swanson, L. (1972). Testicular and blood plasma androgens in Holstein bulls from birth through puberty. J. Anim. Sci. 34: 435–440.
26 26 Rawlings, N., Fletcher, P., Henricks, D., and Hill, J. (1978). Plasma luteinizing hormone (LH) and testosterone levels during sexual maturation in beef bull calves. Biol. Reprod. 19: 1108–1112.
27 27 McCarthy, M., Hafs, H., and Convey, E. (1979). Serum hormone patterns associated with growth and sexual development in bulls. J. Anim. Sci. 49: 1012–1020.
28 28 Lacroix, A. and Pelletier, J. (1979). Short‐term variations in plasma LH and testosterone in bull calves from birth to 1 year of age. J. Reprod. Fertil. 55: 81–85.
29 29 Chandolia, R., Evans, A., and Rawlings, N. (1997). Effect of inhibition of increased gonadotrophin secretion before 20 weeks of age in bull calves on testicular development. J. Reprod. Fertil. 109: 65–71.
30 30 Chandolia, R., Honaramooz, A., Bartlewski, P. et al. (1997). Effects of treatment with LH releasing hormone before the early increase in LH secretion on endocrine and reproductive development in bull calves. J. Reprod. Fertil. 111: 41–50.
31 31 Madgwick, S., Bagu, E., Duggavathi, R. et al. (2008). Effects of treatment with GnRH from 4 to 8 weeks of age on the attainment of sexual maturity in bull calves. Anim. Reprod. Sci. 104: 177–188.
32 32 Harstine, B., Maquivar, M., Helser, L. et al. (2015). Effects of dietary energy on sexual maturation and sperm production in Holstein bulls. J. Anim. Sci. 93: 2759–2766.
33 33 Dance, A., Thundathil, J., Wilde, R. et al. (2015). Enhanced early‐life nutrition promotes hormone production and reproductive development in Holstein bulls. J. Dairy Sci. 98: 987–998.
34 34 Byrne, C., Fair, S., English, A. et al. (2018). Plane of nutrition before and after 6 months of age in Holstein‐Friesian bulls: I. Effects on performance, body composition, age at puberty, and postpubertal semen production. J. Dairy Sci. 101: 3447–3459.
35 35 Byrne, C., Fair, S., English, A. et al. (2018). Plane of nutrition before and after 6 months of age in Holstein‐Friesian bulls: II. Effects on metabolic and reproductive endocrinology and identification of physiological markers of puberty and sexual maturation. J. Dairy Sci. 101: 3460–3475.
36 36 Curtis, S. and Amann, R. (1981). Testicular development and establishment of spermatogenesis in Holstein bulls. J. Anim. Sci. 53: 1645–1657.
37 37 Bagu, E., Cook, S., Gratton, C., and Rawlings, N. (2006). Postnatal changes in testicular gonadotropin receptors, serum gonadotropin, and testosterone concentrations and functional development of the testes in bulls. Reproduction 132: 403–411.
38 38 Kaneko, H., Yoshida, M., Hara, Y. et al. (1993). Involvement of inhibin in the regulation of FSH secretion in prepubertal bulls. J. Endocrinol. 137: 15–19.
39 39 Rawlings, N. and Evans, A. (1995). Androgen negative feedback during the early rise in LH secretion in bull calves. J. Endocrinol. 145: 243–249.
40 40 McCarthy, M., Convey, E., and Hafs, H. (1979). Serum hormonal changes and testicular response to LH during puberty in bulls. Biol. Reprod. 20: 1221–1227.
41 41 Brito, L., Barth, A., Wilde, R., and Kastelic, J. (2012). Effect of growth rate from 6 to 16 months of age on sexual development and reproductive function in beef bulls. Theriogenology 77: 1398–1405.
42 42 Lee, C., Hunt, D., Gray, S., and Henricks, D. (1991). Secretory patterns of growth hormone and insulin‐like growth factor‐I during peripubertal period in intact and castrate male cattle. Domest. Anim. Endocrinol. 8: 481–489.
43 43 Renaville, R., Devolder, A., Massart, S. et al. (1993). Changes in the hypophysial–gonadal axis during the onset of puberty in young bulls. J. Reprod. Fertil. 99: 443–449.
44 44 Renaville, R., Massart, S., Sneyers, M. et al. (1996). Dissociation of increases in plasma insulin‐like growth factor I and testosterone during the onset of puberty in bulls. J. Reprod. Fertil. 106: 79–86.
45 45 Renaville, R., Van Eenaeme, C., Breier, B. et al. (2000). Feed restriction in young bulls alters the onset of puberty in relationship with plasma insulin‐like growth factor‐I (IGF‐I) and IGF‐binding proteins. Domest. Anim. Endocrinol. 18: 165–176.
46 46 Spiteri‐Grech, J. and Nieschlag, E. (1992). The role of growth hormone and insulin‐like growth factor I in the regulation of male reproductive function. Horm. Res. 38 (Suppl. 1): 22–27.
47 47 Bellve, A. and Zheng, W. (1989). Growth factors as autocrine and paracrine modulators of male gonadal functions. J. Reprod. Fertil. 85: 771–793.
48 48 Cailleau, J., Vermeire, S., and Verhoeven, G. (1990). Independent control of the production of insulin‐like growth factor I and its binding protein by cultured testicular cells. Mol. Cell. Endocrinol. 69: 79–89.
49 49 Amstalden, M., Harms, P., Welsh, T. et al. (2005). Effects of leptin on gonadotropin‐releasing hormone release from hypothalamic–infundibular explants and gonadotropin release from adenohypophyseal primary cell cultures: further evidence that fully nourished cattle are resistant to leptin. Anim. Reprod. Sci. 85: 41–52.
50 50 Bagu, E., Gordon, J., and Rawlings, N. (2010). Postnatal changes in testicular concentrations of transforming growth factors‐alpha and ‐beta 1, 2 and 3 and serum concentrations of insulin like growth factor I in bulls. Reprod. Domest. Anim. 45: 348–353.
51 51 Bagu, E., Gordon, J., and Rawlings, N. (2010). Post‐natal changes in testicular concentrations of interleukin‐1 alpha and beta and interleukin‐6 during sexual maturation in bulls. Reprod. Domest. Anim. 45: 336–341.
6 Sexual Development and Puberty in Bulls
Leonardo F.C. Brito
STgenetics, Middleton, WI, USA
Introduction
Age at puberty is a major determinant of cattle production efficiency. The ability to breed animals at younger ages reduces generation intervals and increases genetic gains. However, reduced sperm production and poor semen quality due to immaturity are common causes of poor reproductive performance of young bulls and represent a serious loss of superior genetic stock. The ability to collect and freeze semen from younger bulls is also desired to reduce the time required for progeny testing and to accelerate the process of artificial insemination and sire selection. Therefore an understanding of pubertal changes and the factors that affect sexual development is required in order to promote the successful use of young bulls for reproductive purposes.
Testicular Development
Sexual development is associated with marked gonadal growth. Scrotal circumference (SC) is highly correlated with testicular weight (Figure 6.1) and is the most common endpoint evaluated to determine testicular development. The testicular growth curve in bulls shows an initial period of little growth followed by a rapid growth phase and then a plateau (Figures 6.2 and 6.3). Although the overall pattern of testicular growth is somewhat similar in all breeds, the characteristics of the growth curve are greatly affected by genetics. In general, the rapid growth phase is shorter and testicular growth plateaus sooner in bulls from breeds that mature faster (reach puberty earlier) than in bulls from late‐maturing breeds, resulting in marked differences in the curve slope. This is especially evident when Bos taurus bulls are compared with Bos indicus bulls, which in general reach puberty later than the former. The asymptotic value of the testicular growth curve, namely adult testicular size, also differs considerably among breeds (Figures 6.2–6.6) [1–8]. These same differences