Fertility of stallion semen frozen in 0.5-ml straws

Semen of 2 pony stallions was frozen by 2 methods in 0.5 ml PVC straws. The fertility of the frozen-thawed semen was evaluated by inseminating 60 mares during 69 oestrous cycles. An overall single cycle pregnancy rate of 55% was achieved. Freezing method, stallion, insemination during steroid-synchronized oestrus or insemination only every 2nd day during oestrus did not significantly influence pregnancy rates. Pregnancy rates were significantly improved from a mean 44% to a mean 73% when the mean number of progressively motile spermatozoa per insemination was increased from 175 x 10(6) to 249 x 10(6). It is concluded that the simpler freezing technique will yield satisfactory pregnancy rates when semen with a post-thaw progressive motility of 30% is used for AI. Starting when ovulation is anticipated to occur within the next 48 h mares should be inseminated every 2nd day with at least 220 x 10(6) progressively motile spermatozoa per insemination until ovulation is confirmed.