After filtration, the semen is poured into a warm, graduated cylinder (a size of 100 ml will usually suffice) so that the volume of the gel-free fraction of the semen sample can be determined. It is normal practice that the collecting vessel attached to the AV is also graduated so that an indication of the total volume of the ejaculate can be obtained when in-line filters are not used. The volume of gel produced can then be calculated by subtraction.
The normal volume produced by a stallion varies with many factors. As with all evaluation it is ideal to have information on expected values for each stallion to act as a reference point or standard to refer back to if abnormalities are suspected. In general, the total volume of a sample should be between 30 and 250 ml. On average, stallions will produce 100 ml in total, of which 20-40 ml will be gel, leaving 60-80 ml of useful gel-free semen (Dowsett and Pattie, 1982; Pickett et al, 1988b; Fayrer-Hosken and Caudle, 1989; Ricketts, 1993). Significant variations in these volumes are seen which may be due to nothing more than individual stallion variation. However, they may be indicative of abnormalities, a change in the breeding season, age, workload, length of teasing, etc. (Pickett et al., 1988a,b). The availability of a standard for a stallion will help to ascertain the reason for any change. Work by Pickett and Voss (1973), Squires et al. (1979a), Pickett et al. (1988a,b), and Johnson and Tatum (1989) illustrated well the effect of some of these factors, and a summary of some of their findings is given in Tables 4.2, 4.8 and Fig. 3.19.
The effect of season on the volume of seminal plasma has been demonstrated by several workers. Differences of up to 50% have been reported between the height of the breeding season and the non-breeding season. These differences are largely accounted for by changes in the volume of gel produced (Pickett et al., 1976; Pickett and Voss, 1972; Pickett and Shiner, 1994).
The effect of workload (that is, ejaculation frequency) has been investigated by several authors (Pickett and Voss, 1972; Kenney, 1975; Pickett et al., 1975c; Sullivan and Pickett, 1975; Swierstra et al., 1975). In summary, it is apparent that the major effect of increasing the ejaculation frequency of a stallion is on the volume of gel fraction produced, with the first ejaculation invariably having a higher gel fraction than subsequent ejaculations (Pickett et al., 1976). When frequency of collection throughout a week was investigated, it was concluded that for the majority of stallions a frequency of collection ranging from one per week to six per week had no significant effect on seminal volume, either total or gel-free (Pickett et al., 1975c). However, a second ejaculation within 24 h did show a significant drop in total volume (Dowsett and Pattie, 1987; Pickett and Shiner, 1994). No significant effect should be seen in the great majority of stallions with a collection frequency of less than one ejaculate per 24 h.
Age has also been demonstrated to have a significant effect on seminal volume. Squires et al. (1979a) showed an increase in seminal volume from 2 to 16 years of age. The increase was particularly evident between the ages of 4-6 and 9-16 years, where volumes were shown to increase with age by 120%. It was also apparent that older stallions produced a higher gel fraction, though the difference was not significant.
The length of time that a stallion is teased prior to ejaculation has also been shown to have an effect on the volume of seminal plasma produced. The longer a stallion is allowed to tease an oestrous mare, the greater is the volume of semen produced, largely due to an increase in the gel-free fraction (Ionata et al., 1991b).
As will be discussed in later sections of this chapter, significant increases in volume may be the result of contamination with urine (urospermia), blood (haemospermia) or pus (pyospermia). These are normally picked up initially as changes in the appearance of a sample and are discussed in the next section. In addition it has been reported that conditions such as testicular rotation may also be implicated if seminal volumes are lower than might be expected (Pickett et al., 1988a,b).
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