Effect of timing of insemination, numbers of spermatozoa and extender components on the pregnancy rate in mares inseminated with frozen stallion semen.

Fertilization rate was highest in mares inseminated with frozen semen within 12 hr of ovulation. Foaling rate was improved (P less than 0-05) by increasing the number of motile spermatozoa inseminated from 40 X 10(6) to 80 X 10(6) but was not further improved by increasing the number to 160 X 10(6) or by increasing the frequency of insemination from once to twice daily. The fertilizing capacity of spermatozoa frozen in one of the hydrogen ion extenders studied was dependent upon relative osmotic pressure and method of freezing (ampoules or pellets). Adjusting glycerol concentration from 7% to 2%, addition of glycerol 15 min before freezing and freezing 2 hr after extension all enhanced the fertilizing capacity of spermatozoa. Pregnancy rate per mare service period was higher (P less than 0-01) for mares inseminated with unextended semen (75%) than for extended semen containing no glycerol (50%) or extended semen containing 7% glycerol (35%). Although wide variability in the fertilizing capacity of spermatozoa from individual stallions was observed when semen was extended or extended and frozen, pregnancy rate was depressed in all stallions. It was concluded, therefore, that hydrogen ion extenders depress fertilizing capacity of stallion spermatozoa immediately after extension and show little promise as semen extenders for short- or long-term storage of stallion semen.