13, and 14 months of age were approximately 40, 50, 60, and 70%, respectively [62]. Similarly, only 48% of B. taurus beef bulls 11–13 months old in Sweden had less than 15% proximal cytoplasmic droplets and less than 15% abnormal sperm heads [63]. In Australia, sperm morphology evaluation of bulls of various breeds submitted to breeding soundness revealed a significant negative relationship between age and the occurrence of proximal droplets (Figure 6.10), whereas there was no association between age and other sperm abnormalities [64]. These observations have profound implications on the ability of producers to use yearling bulls and the ability of artificial insemination centers to produce semen for progeny testing at the youngest possible age.
Development of Accessory Sex Organs
Anatomical changes of the testicular vascular cone as the bull ages and starts to produce sperm are indications that the efficiency of the counter‐current heat exchange mechanism between the testicular artery and veins in the pampiniform plexus needs to improve in order to cope with increasing testicular metabolism. In B. taurus beef bulls the testicular vascular cone diameter measured by ultrasonography increases until approximately 13.5 months of age, or until 1–8 weeks before SC reaches a plateau [65] (Figure 6.11). In crossbred beef bulls, the length and diameter of the testicular artery in the vascular cone increased from 6 to 12 months of age (1.8 m and 1.9 mm vs 3.1 m and 3.5 mm, respectively), but did not increase significantly thereafter [66]. The testicular artery length and volume in the vascular cone were 1.6 m and 6 ml in 15‐month‐old Angus bulls and 2.2 m and 11.4 ml in 28‐month‐old crossbred bulls, respectively [57]. Other studies have reported that the testicular vascular cone length is approximately 10–15 cm and that testicular artery length varies from 1.2 to 4.5 m in adult beef bulls of several breeds [67–69]. In addition to the lengthening of the testicular artery, the distance between the arterial and venous blood in the testicular vascular cone also decreases with age as a result of thinning of the artery wall (317 and 195 μm at 6 and 36 months of age, respectively) and reduction of the distance between the artery and the closest veins [66].
The epididymis continues to grow until at least 6 years of age in Holstein bulls and epididymal weight increases from 9 g at 8 months of age to 15, 23, 27, and 38 g at 12, 18, 25–48, and 73–96 months of age, respectively [51, 70]. The greatest dimension of the epididymis tail measured by ultrasonography from an oblique plane near the distal pole of the testis in Friesian bulls increased from 0.9 cm at 3 months of age to 1.8 and 2.7 cm at 12 and 24 months of age, respectively [71]. The tube‐like vesicular glands in newborn calves increase in length and become lobulated during development. The weight of the vesicular glands increased until approximately 4 years of age in Holstein bulls, from 13 g at 8 months of age to 26, 35, 54, and 78 g at 12, 18, 25–48, and 73–96 months of age, respectively [51, 70]. The maximum diameter of vesicular glands in Friesian bulls increased significantly only from age 3 months (1.1 cm) to 9 months (1.6 cm), suggesting that further increase in gland weight is a result of increase in length [71]. Prostate diameter also only increases significantly until 12 months of age (0.85 cm from 0.49 cm at 3 months), whereas maximum diameter of the bulbourethral glands increased from 1.2 cm at 3 months of age to 1.5 and 2.1 cm at 12 and 24 months of age, respectively [71].
In B. taurus bulls, the sigmoid flexure of the penis begins to develop at about 3 months of age, penis length increases by up to five times by the onset of puberty, and length continues to increase until sexual maturity [72]. The penis in Friesian bulls 13–19 months old measured 73–89 cm [73], whereas the penis in Holstein bulls aged 25 months or older measured 95–106 cm [70]. First protrusion of the penis during mounting was observed at approximately 8 months of age, whereas complete separation of penis and sheath was observed at approximately 8.5 months of age in Angus, Charolais, and Hereford bulls [47, 74]. Complete sheath–penile detachment evaluated during electroejaculation was observed around the same time of puberty, whereas first completed service evaluated during libido testing was only observed approximately one month after puberty [48].
Development of Sexual Behavior
Development of sexual behavior has been studied in Hereford bulls. Mounting in response to an estrual female was first observed by 3, 6, and 9 months of age in 18.5, 26, and 48% of the bulls, respectively. By 12 months of age 59% of the bulls had their first ejaculations, whereas by 15 months 78% of the bulls registered a complete service. The number of services increased with age until 18 months of age [75]. In Angus, Brown Swiss, Hereford, Angus × Hereford, and Red Poll the first completed service was observed around 11 months of age [48]. Rearing seems to affect development of sexual behavior, since Hereford calves raised in individual pens had a greater number of services when tested for the first time than bulls raised in groups, although the differences quickly disappeared once these bulls were grouped together [76]. In another study, the influence of the presence of females during bull rearing was evaluated. In the first two hours of being exposed to females in estrus, males raised with females had 73% more services than bulls raised in isolation. However, bulls reared in isolation compared favorably in further tests, illustrating the fact that a learning process occurred rather quickly after exposure to females [77].
Age and experience have major effects on libido and serving capacity test results. An overall increase in test scores has been observed in several studies with yearling bulls when tests were conducted repeatedly over relatively short intervals, indicating that a maturing and/or learning process occurs rapidly after exposure of bulls to females. Exposure of yearling bulls classified as low serving capacity to estrual females for four days resulted in increased serving capacity score when bulls were tested within a week later; 85% of these bulls moved into the medium‐ or high‐serving capacity category [78]. No correlations between the numbers of services at 12 months of age with sexual performance at older ages were observed in Hereford bulls. Only when bulls reached 18 months did individual differences in serving capacity remain consistent from one age to another, i.e. 18, 21, and 24 months [79]. In one study, only 53% of 113 yearling bulls completed a service during 10 minutes of libido testing [80], whereas in a different study with yearling Angus and Hereford bulls the variance of the score obtained during eight libido tests conducted over a two‐month period ranged between 69 and 73% [81]. These observations suggest that attempts to predict sexual performance are likely to be unproductive until bulls can fully express their inherent sexual behavior and serving ability. When adult Angus bulls were evaluated, the time required to complete six services when exposed individually to 10 estrous females increased with age from 31 minutes in two‐year‐old bulls to 43, 55, and 67 minutes in three‐, four‐, and five‐year‐old bulls, respectively [82]. However, in another study no difference in libido score or servicing capacity test results between two‐ and three‐year‐old Angus and Hereford bulls was observed [83].
Effects of Nutrition on Sexual Development
In early studies evaluating the effect of nutrition from birth to maturity on sexual development and reproductive function in bulls, Holstein bulls receiving low nutrition (approximately 60–70% of requirements) were older at the time the first ejaculates containing motile sperm were collected and had smaller testes, whereas bulls receiving high nutrition (approximately 160% of requirements) had earlier puberty and larger testes when compared with bulls receiving control nutrition (100% of requirements) [84, 85]. Several recent studies involving manipulation of nutrition during prepubertal and pubertal periods in both beef and dairy bulls have provided unequivocal evidence that (i) most pronounced effects of nutrition occur during the prepubertal period, (ii) effects of nutrition during the prepubertal period cannot be compensated with changed nutrition during the pubertal period, and (iii) high nutrition during the peripubertal period hastens puberty and results in larger testes (SC and testes weight), whereas low nutrition produced opposite results. With optimal prepubertal nutrition, puberty can be hastened by 1–1.5 months and testicular mass at approximately 1.5 years of age can be increased by 12–18% (Table 6.4)
