I have never heard of that causing a problem - we never had any problems with our boys. Perhaps someone was confused and the worry is about them not developing immunities in time for strange horses coming in?
Any time a horse is vaccinated - stallion or mare - there is a possibility of a transient increase in temperature (fever) as a result of the body's response to the disease being vaccinated against. That transient increase, if it lasts long enough, can have a detrimental effect on stored and in-the-process-of-manufacture sperm, which must be stored at a few degrees below normal body temperature (hence testicles outside the body cavity rather than inside like the ovaries). It takes about 57 days to produce a sperm from scratch, so the suggestion of 60 days will allow the passage of all the potentially affected sperm.
Realistically, such reactions to a vaccine are comparatively rare. Should one vaccinate 60 days prior to the breeding season? Well, if you're planning ahead and have that option, sure, it's a good idea. If one gets within 60 days of the season should one then not vacccinate to avoid a possible sperm problem? No - the vaccination should definitely be done. The risk of a sperm problem is far outweighed by the increase in risk to the horse by not vaccinating.
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