Fertility of cooled and frozen rabbit sperm measured by competitive fertilization

The fertility of rabbit sperm that had been cooled to 5 degrees C or frozen and thawed was determined by competitive fertilization. Treatments were identified by labeling sperm with fluorescein isothiocyanate (FITC) or tetramethylrhodamine B isothiocyanate (TRITC). Sperm from different treatments were mixed and used in a competitive insemination experiment. Does were inseminated 5, 10 or 15 h prior to ovulation. Time of ovulation was controlled by injections of luteinizing hormone. The functional sperm transport, as determined by the number of sperm transported to the site of fertilization and capable of fertilizing oocytes, was estimated by counting the total number of differently stained sperm that surrounded or fertilized each oocyte. The fertility of sperm cooled to 5 degrees C was not affected (p less than 0.05) as compared to fertility of uncooled sperm. Functional sperm transport at all times of insemination and fertilization ratio at insemination 10 or 15 h before ovulation were reduced (p less than 0.05) for frozen-thawed vs. cooled sperm. No difference in fertilization ratio (p greater than 0.05) occurred, however, when does were inseminated 5 h before ovulation. While sperm survival and capacitation time appeared to play roles in fertility of frozen-thawed sperm, the most important factor was reduced functional sperm transport. However, fertility of frozen-thawed sperm was improved when the time from insemination to ovulation was reduced.