Table 3.2 illustrated the effect that testicular weight has on output of spermatozoa and how this is related to age. Testicular weight is correlated with testicular size, which can be more easily measured, either by calipers or by ultrasonic scanning (Rath and Brass, 1988; Love et al., 1991). No adverse effect of using linear scanning has been reported (Rath and Brass, 1988).
Using these methods, a linear relationship between testicular size and daily spermatozoan output (DSO) is evident (Fig. 4.23) (Love et al., 1991). A correlation between age and scrotal width (0.62) has also been reported (Pickett et al., 1989) along with scrotal width and spermatozoan motility
(0.34) (Sigler and Kiracofe, 1988). Such a relationship would be expected, as the amount of a product depends upon the amount of tissue producing it. Experiments conducted by Thompson et al. (1979a) produced various correlation coefficients between various size parameters of the testes and DSP and DSO. The best correlation (0.75) existed between total scrotal width (measured across the two testes from the point of greatest curvature of one testis to the point of greatest curvature of the other) and DSP. Testicular size is, therefore, a very good indicator of the spermatozoa-producing capacity of that stallion and may be advocated as a parameter when selecting for reproductive potential (Pickett and Shiner, 1994). Testicular size in bulls is highly heritable (65% or more) and is very likely also to be heritable in stallions. The correlation between testicular width and DSO at 0.55 is not as good, as factors additional to testicular size affect output of spermatozoa at ejaculation (for example, spermatozoa reserves, ejaculation frequency, etc).
The size of the testis indicates the volume of parenchyma within. The correlation between parenchyma weight and Sertoli cell number is reported to be 0.85 (Johnson et al., 1994). Hence an increase in testicular size indicates an increase in the number of Sertoli cells available to support developing germinal cells. Indeed, the correlation between parenchyma weight and DSP is 0.89 and between the number of Sertoli cells and DSP is 0.85 (Johnson et al., 1994). This correlation between testicular size and DSP has been exploited in the development of an index for testis shape (Kenney index)
which is reported to have a 0.62 correlation with DSP (Silva et al., 1996). The size of the testis also includes the tunica albuginea and it must be remembered that in older stallions the tunica albuginea tends to be thicker, and so the increase in testicular size with age is not entirely due to an increase in parenchyma (Table 4.11) (Johnson and Neaves, 1981).
In order for the correlation between testicular size and DSP to hold, it is necessary that as testicular size increases the volume of functional parenchyma also increases. Therefore, the functional ability of the parenchyma is important. Testicular function may also be indicated by testicular consistency, which can be assessed manually. Evidence of hardened areas within the testes indicates non-functional testicular tissue and, therefore, a decline in DSP. Abnormalities in the epididymal area may indicate a decrease in the ability of the epididymis to store spermatozoa, and so a reduction in the stallion's potential DSO. The presence of large testes is no guarantee that spermatozoa will be produced, nor that the spermatozoa produced will have acceptable motility and morphology. A stallion with average-sized testes may prove to be azoospermic (Pickett et al., 1977). Testicular size alone should not be used as an indicator of DSO, but it may prove to be a useful guide when used in association with a general semen evaluation.
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