Fertility of Ram Spermatozoa Frozen by the Pellet Method

Summary. The transport of fresh and frozen spermatozoa through the genital tract of the ewe was studied in four experiments. Extremely low numbers of spermatozoa were recovered from the Fallopian tubes of ewes after insemination with either fresh (diluted) or frozen spermatozoa when both were used at a concentration of 0·4 ×109 motile spermatozoa/ml, but the fertilization rate was higher with the fresh semen. High doses of oxytocin did not affect sperm transport, but significantly depressed the fertilization rate. Concentrating the thawed semen before insemination resulted in increased numbers of spermatozoa recovered from the Fallopian tubes. Fresh semen resulted in even higher numbers in the tubes and an increased proportion of ova recovered with spermatozoa on the zona pellucida. Ewes were slaughtered 30 min after insemination with either fresh or frozen semen of varying concentration. The number of spermatozoa recovered from the cervix after insemination with frozen semen was dependent upon the concentration of motile spermatozoa in the inseminate. At equal concentration, fresh semen was superior to frozen semen. Uterine insemination at varying periods before ovulation showed that a proportion of the pellet-frozen spermatozoa survived in the genital tract with retention of fertilizing ability for 18 to 35 hr.