timing of AI and the number (and location) of normal sperm introduced into the
mare’s uterus. There is, nevertheless, considerable between-stallion (and
ejaculate) variation in the “freezeabilty” (Vidament et al, 1997). For this
reason, testing of the ability of a stallion’s sperm to withstand the freezethawing process is essential before acceptance onto a commercial semen
freezing programme; approximately 20% of stallions fail to meet the criteria for
adequate freezability (Loomis and Graham, 2008). Moreover, since there is
marked between-stallion variation in the response of the sperm to different
extenders, cryoprotectants and packaging concentrations, it is not sufficient to
test a single freezing extender/technique on a single ejaculate and conclude
that a stallion does not meet the grade. Instead, most commercial stallion
semen freezing centres now customize the freezing protocol for each
individual stallion on the basis of initial tests, ideally performed on an
ejaculate(s) collected after depletion of the extra-gonadal sperm reserves, to
establish the most appropriate extender and freezing protocol (Loomis and
Graham, 2008). Regular semen collection and analysis in the initial period will
also give an indication of a sensible collection frequency; there is no need to
collect semen every day for freezing, but the between-collection interval
should not be too long because semen quality will drop (Loomis and Graham,
2008); collection frequencies of 1-3 times per week are the norm. With
regards to level of semen quality considered to indicate adequate freezability,
values in excess of 35% progressively motile sperm post-thaw are generally
considered to offer a reasonable chance of good fertility using a standard AI
dose of 250 x 106 progressively motile sperm (Sieme, 2009). While frozen
semen is rarely accompanied by a description of morphological normality, a
minimum of 35% morphologically normal sperm would a reasonable additional
parameter to indicate adequate quality.
Concluding remarks
The major drawback of any BSE, including examination of suitability for AI via
cooled or frozen semen, is that satisfactory semen quality is no guarantee of
adequate fertility. This is largely because the range of tests for semen quality
performed during a standard semen analysis (percentage motile, percentage
morphologically normal) does not cover the full range of characteristics that a
sperm must possess if it is to reach and fertilize an oocyte to produce a viable
embryo. In this respect, a number of additional tests have been developed
over the past decade to analyze other aspects of sperm normality or
functionality, and thereby improve our ability to identify stallions or ejaculates
that do not meet the criteria for ‘a reasonable chance of normal fertility’.
With regard to AI, it is apparent that specific methods of semen preservation
can induce specific types of sperm damage, in some stallions. For example,
chilled storage in the presence of a high proportion of seminal plasma has, in
some stallions, been reported to lead to dramatic increases in the percentage
of sperm with DNA damage (as measured by the sperm chromatin structure
assay: Love, 2005); this reduction in the ability to induce normal fertilization
would not be apparent by routine microscopic examination. In the case of
cryopreservation, freezing and thawing induces damage to the acrosomal cap
and/or capacitation-like changes in the sperm membranes (Thomas et al,
2006) that will, respectively, reduce fertilizing capacity or longevity; identifying
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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