Читать книгу Bovine Reproduction - Группа авторов - Страница 94

After spermatogenesis is established, there is a gradual increase in the number of testicular germ cells supported by each Sertoli cell and an increase in the efficiency of the spermatogenesis, i.e. an increase in the number of more advanced germ cells resulting from the division of precursor cells. The yields of different germ cell divisions, low during the onset of spermatogenesis, increases progressively to the adult level [42–44]. Testicular histological changes and increasing efficiency of spermatogenesis are accompanied by increasing testicular echogenicity. Testicular ultrasonogram pixel intensity starts to increase approximately 12–16 weeks before puberty and reaches maximum values right around puberty [45] (Figure 6.8). If the initial changes in testicular echogenicity are associated with Sertoli cell differentiation and meiosis is not completed until formation of a functional blood–testis barrier, then 12–16 weeks seems to be the interval required for the gradual increase in the efficiency of spermatogenesis that eventually leads to the appearance of sperm in the ejaculate. That testicular echogenicity does not change significantly after puberty indicates that a certain developmental stage of the testicular parenchyma must be reached before puberty, a conclusion corroborated by the observation that testicular echotexture at puberty did not differ between early‐ and late‐maturing bulls [10]. In addition, testicular echogenicity did not change with age in mature bulls [46], suggesting that the composition of testicular parenchyma remained relatively consistent after puberty.

In general terms, puberty is defined as the process by which a bull becomes capable of reproducing. This process involves development of the gonads and secondary sexual organs and development of the ability to breed. For research purposes, however, puberty in bulls is usually defined as an event instead of a process. Most researchers define attainment of puberty by the production of an ejaculate containing 50 million or more sperm with 10% or more motile sperm [47]. The interval between the first observation of sperm in the ejaculate and puberty as defined by these criteria is approximately 30–40 days in B. taurus bulls [48, 49]. Age at puberty determined experimentally can be affected by the age that semen collection attempts are performed, the interval between attempted collections, the method of semen collection (artificial vagina or electroejaculator), the response of the bull to the specific semen collection method, and the experience of the collector(s). Moreover, age at puberty is affected by management, nutrition (see below), and genetics. Table 6.3 describes weight, SC, and age at puberty in different breeds. Although data from large trials comparing bulls of different breeds raised as contemporary groups are scarce, some liberties could be taken to make some generalizations. Dairy bulls usually mature faster and attain puberty earlier than beef bulls. Bulls from continental beef breeds (with the exception of Charolais) usually attain puberty later than bulls from British beef breeds, especially Angus bulls. Bulls from double‐muscled breeds are notorious for being late‐maturing. Puberty is delayed in bulls from tropically adapted B. taurus breeds and in non‐adapted bulls raised in the tropics. In general, B. indicus bulls attain puberty at considerably older ages than B. taurus bulls.

Table 6.3 Age, weight, and scrotal circumference (SC) at puberty (ejaculate with ≥50 million sperm and ≥10% sperm motility) in different breeds.

Breed	Age (months) a	Weight (kg)	SC (cm)	References
Angus	10.1	309	30.0	[47]
Bos taurus beef crosses	7.8–9.7	272–339	27.9–28.3	[6, 48, 116]
Brahman	15.9–17.0	350–430	28.2–33.0	[125–128]
Brown Swiss	8.7–10.2	233–295	25.9–27.2	[48, 49]
Charolais	9.4	396	28.8	[74]
Guzera	18.2	310	25.6	[119]
Gyr	17–19.2	315–346	26.2–27.9	[11]
Hereford	9.6–11.7	261–391	27.9–32.0	[9, 47, 48, 98]
Holstein	9.4–10.9	276–303	28.4	[49, 51]
Nelore	14.8–19.7	232–298	21.7–24.3	[10, 119, 129]
Red Poll	9.3	258	27.5	[48]
Romosinuano	14.2	340	28.8	[125]
Simmental	10.6–11.4	328–419	30.6–34.0	[98]

^a Transformed from days or weeks from original reports.

There is large variation in age and body weight at puberty across breeds and within breeds. Although on average B. taurus bulls attain puberty with SC between 28 and 30 cm regardless of the breed, the fact that there is still considerable variation in SC at puberty is sometimes overlooked. Interesting observations have been reported in studies evaluating differences between early‐ and late‐maturing bulls. Bulls that attain puberty earlier were generally heavier and had greater SC than bulls that attained puberty later; however, both weight and SC were smaller at puberty in early‐maturing bulls [9, 10, 50] (Figure 6.7). These observations not only indicate that precocious bulls develop faster, but also suggest that sexual precocity is not simply related to earlier attainment of a threshold body or testicular development. In fact, these thresholds seem to be lower in early‐maturing bulls, and late‐maturing bulls must reach a more advanced stage of body and testicular development before puberty is attained.

Spermatogenesis efficiency reaches adult levels at approximately 12 months of age in Holstein bulls [42, 51] and 2.5–3.5 years of age in B. indicus bulls [52]. Individual variation in spermatogenesis efficiency is relatively small and is not affected by ejaculation frequency; values between 10 and 14 million sperm per gram of testicular parenchyma have been reported for bulls [1, 6, 42,51–57]. Since spermatogenesis efficiency is somewhat constant among bulls, daily sperm production of a bull is largely dependent on the weight of the testes. Considering testicular weight at different ages, yearling B. taurus bulls are expected to produce around four to five billion sperm per day, whereas adult bulls are expected to produce around seven to nine billion sperm per day. Sperm output (number of sperm in the ejaculate) in bulls ejaculated frequently is essentially the same as sperm production [54]. One important difference between young and older bulls is the capacity of the epididymis to store sperm. Evaluation of sperm numbers in the tail of the epididymis in 15‐ to 17‐month‐old Holstein bulls demonstrated that sperm available for ejaculation corresponded to approximately 1.5–2 days of sperm production, whereas in 2‐ to 12‐year‐old bulls stored sperm numbers corresponded to approximately 3.5–5 days of sperm production [58, 59]. These observations are especially important for artificial insemination centers and indicate that more frequent semen collection is necessary to maximize sperm harvest from young bulls, whereas semen collection intervals of less than three days have smaller effects on increasing sperm harvest from older bulls. Sperm output increases with increased ejaculation interval up to the number of days required for epididymal storage capacity to reach its limit. Sperm that are not ejaculated are eliminated with urine or during masturbation.

Semen quality in peripubertal bulls is poor and a gradual improvement characterized by increase in sperm motility and reduction in morphological sperm abnormalities is observed after puberty. The most prevalent sperm defects observed in peripubertal bulls are proximal cytoplasmic droplets and abnormal sperm heads (approximately 30–60% and 30–40% at puberty, respectively; Figure 6.9) [9, 50, 60]. The difference between age at puberty and age at satisfactory semen quality (≥30% sperm motility, ≥70% morphologically normal sperm) was 110 days in B. indicus bulls [10] and 50 days in B. taurus beef bulls; 10% of the latter did not have satisfactory semen quality by 16 months of age [61] (Figure 6.9). In western Canada, the proportions of B. taurus beef bulls with satisfactory sperm morphology (≥70% morphologically normal sperm) at 11, 12, 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.