Seasonality

In common with mares and many other mammals, the reproductive activity of the stallion is governed by season. He is, therefore, classified as a seasonal

Table 3.3. Change in daily spermatozoan production (DSP) g 1 of testicular tissue with age (Johnson and Thompson, 1983).

Age

DSP g"1 testes

(years)

parenchyma

2-3

6 X 106

4-5

18 X 106

6-20

20 X 106

breeder. The length of the breeding season depends upon many factors, including breed, environmental conditions and management. In the northern hemisphere the breeding season runs on average from March to November, and in the southern hemisphere from September to June. Coldblood and similar types of stallion tend to show shorter, better-defined seasons compared with hotblood and warmblood stallions, but in general the stallion's season is less distinct than the mare's season and, unlike her, given enough encouragement the majority of stallions are capable of breeding all year around. Spermatozoan production (unlike ova production) is a continual process, not governed by cyclical hormonal changes. However, season does affect various semen parameters and behavioural characteristics. Seminal volume, spermatozoan concentration, total number of spermatozoa per ejaculate, the number of mounts per ejaculate and the reaction time to an oestrous mare all suffer in the non-breeding season, making the stallion in general less efficient reproductively (Pickett et al., 1970, 1975b; Pickett and Voss, 1972; Johnson, 1991b; Clay and Clay, 1992). Figures 3.19-3.23 illustrate the effects of season on stallion reproductive performance.

Gel Free Volume Sperm
Fig. 3.19. Effect of season on the total volume of ejaculate produced (Pickett and Voss, 1972).
Ejeculation Amount
Fig. 3.20. Effect of season on the number of spermatozoa produced in the gel-free portion of a semen sample (Pickett and Voss, 1972).
Gel Free Volume Sperm
Fig. 3.21. Effect of season on the total number of spermatozoa produced per ejaculate (Pickett and Voss, 1972).
Fig. 3.22. Effect of season on the reaction time of a stallion to the presence of an oestrous mare (Pickett and Voss, 1972).
Gel Free Volume Sperm
Fig. 3.23. Effect of season on the number of mounts required prior to successful ejaculation (Pickett and Voss, 1972).

In addition to the above effects, breeding outside the natural season is reported to increase the percentage of spermatozoa with morphological abnormalities (Van der Holst, 1975). The reduction in a stallion's libido during the non-breeding season may be accounted for by a decrease in the number of Leidig cells and so by implication a reduction in circulating testosterone levels (Johnson and Tatum, 1989).

The seasonality of stallion reproduction is governed primarily by a daylength effect on the reproductive control axis: the hypothalamic-pituitary-testis axis (Fig. 3.24). This effect is mediated by the pineal gland. Other factors, including pheromones, nutrition and environmental temperature, also play a significant role. The importance of these other factors in the seasonality of reproduction is more evident in stallions kept nearer the equator, where daylength changes are more subtle and the seasonal variations in hormone concentrations less distinct (Lang et al., 1995).

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