Puberty heralds the start of reproductive activity in the stallion. The exact timing of puberty is unclear but has been defined as the time at which a stallion can produce 50 X 106 spermatozoa per ejaculate with greater than 10% progressive motility. Using these parameters, puberty is seen to commence at 21 months (Clay and Clay, 1992). The exact timing of puberty is known to be affected by several factors, including breed, management and the rate of stallion growth and development. Classically puberty is considered to commence between 17 and 22 months of age (Nishikawa, 1959b). Histological examination of testicular tissue indicates that spermatogenesis commences in utero with the migration of primordial cells into the fetal gonads (Everett, 1945). These primordial cells undergo division to produce gonocytes, which are found between the Sertoli cells. Puberty then heralds the further division of these to produce spermatogonia. Histological work on the changes within the testes, especially changes associated with the Leidig cells, suggests that puberty may occur between 12 and 26 months of age (Cornwall, 1972; Naden et al., 1990; Clay and Clay, 1992). Circulating hormone levels have also been used in an attempt to identify the timing of puberty. During early life, significant increases in circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels have been demonstrated, starting as early as 8-10 months. This is followed at 12 months by an increase in testes size; the appearance of spermatogonia and primary spermatocytes is evident at 12-14 months of age, followed by the appearance of round spermatids at 16 months and mature spermatids at 36 months. This heralds the start of spermatozoan production (Cornwall et al., 1973; Curtis and Amann, 1981; Johnson, 1991a). Changes in circulating testosterone levels are also evident, with a significant rise in concentrations being reported to occur between 16 and 21 months (Naden et al., 1990; Clay and Clay, 1992).
Regardless of the exact timing of puberty, it is evident that a stallion does not reach full adult reproductive ability and so is not capable of a full covering season until he is 5 years old. This is demonstrated in work by Johnson and Neaves (1981), Thompson and Honey (1984) and Berndtson and Jones
(1989) involving the assessment of several characteristics, including testicular weight, spermatozoan production, testosterone concentrations, and Leidig cell and Sertoli cell number and volume. Follow-on work by Johnson et al. (1991b) demonstrated that stallions of 3 years of age or younger are increasingly active spermatogenically and, therefore, capable of fertilizing mares. However, they only possess a limited sperm-producing capacity. Significant improvement in scrotal width, testicular tone, spermatozoan motility, the percentage of normal spermatozoa and the total number of spermatozoa produced is evident even between 2-3-year-old stallions (Sigler and Kiracofe, 1988). At 4 years of age stallions are reported to be able to produce adequate numbers of spermatozoa to cover the number of mares expected of a fully mature stallion, but they may not perform consistently well under heavy work loads. By 5 years of age they should be capable of consistently carrying out a full covering season (Johnson et al., 1991b). These apparent changes in reproductive activity in early life were also reported by Naden et al. (1990), who demonstrated changes in sexual behaviour and semen characteristics between 2 years of age and sexual maturity. However, their work indicated no change in time to erection, time to mount and the percentage of motile spermatozoa. It has been suggested that this improvement in reproductive ability is due not only to an increase in testis size and, therefore, function but also to an increase in the efficiency of spermatozoan production per gram of testicular tissue (Table 3.3).
Post maturity, reproductive function should continue, largely unchanged, until old age. However, reproductive function in terms of semen quality has been reported to decline after 20 years of age (Johnson and Thompson, 1983; Amann, 1993a,b). It has been suggested that this may be due to a reduction in output of spermatozoa (by up to 66%) evident after 15 years of age, accompanied by an increase in the incidence of morphological abnormalities (Van der Holst, 1975). This is disputed by other work which suggested that no such decline in performance is necessarily apparent (Johnson et al., 1991a).
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