Abstract
The article reviews methods used for in vitro evaluation of sperm, with particular emphasis on frozen-thawed stallion sperm. The techniques, limitations of the methods and correlations with fertility results are discussed. Very few studies have tried to find correlation between fertility of frozen stallion semen and laboratory tests. It is difficult and expensive to inseminate an adequate number of mares to achieve statistically significant differences. Significant, but low correlations have been demonstrated between the foaling rate and subjective motility of sperm incubated for 2 h and 4 h at 37°C and hypoosmotic swelling test after 0 and 3 h of incubation. Significant correlations have been reported between the pregnancy rate and viability of propidium iodide-stained sperm assessed by flow cytometry as well as for glass wool and Sephadex filtration tests. No correlations have been detected between fertility and motility immediately after thawing. In spite of that, motility estimation by light microscope is the most commonly used method to evaluate frozen-thawed stallion sperm. Computer assisted automatic sperm analyzers have replaced light microscopy in research projects, but so far nobody has been able to demonstrate a correlation between fertility of frozen stallion semen and any of the motility parameters obtained by these instruments.
Highlights
There is considerable variation between individual stallions in how well their semen retains its fertilizing capacity after freezing and thawing
Filtration tests When stallion sperm were filtrated through cotton, glass wool (GW) and Sephadex (S) filters, the results indicated that spermatozoa with acrosome-damaged or -reacted sperm were trapped by GW filters
Subjective evaluation by a trained person using a good phase contrast microscope is adequate for routine evaluations
Summary
There is considerable variation between individual stallions in how well their semen retains its fertilizing capacity after freezing and thawing. When fresh stallion semen was subjectively evaluated, low correlations were found between fertility and the percentage of motile (r = 0.40) and progressively motile (r = 0.46) spermatozoa (Jasko et al 1992). This may be one reason why very little published data exist on the correlation of motility evaluated by light microscopy and fertility of frozen-thawed stallion semen.
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