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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 can be observed within breeds between early‐ and late‐maturing bulls (Figure 6.7), emphasizing the effects of genetics on testicular growth [9–11].
Figure 6.1 Regression lines for paired testes weight (PTW) according to scrotal circumference (SC). Holstein1 measurements obtained from mature Holstein bulls (n = 35); PTW = −1298.5 + (50.2 × SC) [116]. Holstein2 measurements obtained from Holstein bulls (n = 47) 19 to 184 months old; PTW = −654.4 + (34 × SC) [117]. Angus and Hereford SC measurements obtained from Hereford (n = 199) and Angus (n = 136) bulls 11 to 30 months old; PTW = −722.28 + (36.53 × SC) [3]. Angus and Angus × Charolais SC measurements obtained from Angus and Angus × Charolais bulls (n = 111) 14–16 months‐old; PTW = −1274 + (54.04 × SC).
Source: L. Brito, unpublished results.
Figure 6.2 Top: regression curves for scrotal circumference (SC) according to age in Holstein, Jersey, Nelore, and Guzera bulls. Holstein measurements (n = 9614) obtained from bulls 6 to 77 months old; SC = −11.75 + [56.7 × (log AGE)] – [15.3 × (log AGE)2].
Source: ASB Global Inc., unpublished results.
Jersey
