Effect of interval between inseminations when using frozen-thawed boar sperm on fertility and fetal paternity in mature gilts

Multiple artificial inseminations (AI) are used in swine to compensate for variation in time of ovulation and following each insemination, a uterine immune response occurs to remove dead and damaged sperm. There are more dead and damaged sperm inseminated with use of frozen-thawed-semen (FTS) and closer insemination intervals are required to compensate for the reduced lifespan of the sperm. Therefore, the objectives of this experiment were to evaluate the effect of interval between inseminations and estrus to ovulation interval (EOI) on fertility and the impact of each AI using fetal paternity in mature gilts when using FTS. Semen from 11 boars was collected and frozen in 0.5 mL straws at 1.4 × 109 cells/mL. Time of estrus was synchronized in gilts (n = 191) using Matrix® and assigned at estrus to AI treatment: (1) AI at a 4 h interval (34 and 38 h); (2) AI at an 8 h interval (30 and 38 h); or (3) AI at a 16 h interval (22 and 38 h). Ultrasonography was performed to determine time of ovulation. In each treatment, the first and second AI were from unique sires to allow for paternity identification using microsatellite markers. Each sire was represented across first and second inseminations. Gilts received an AI with 2 × 109 motile FTS in 80 mL of extender. Pregnancy rate and litter size were assessed at slaughter on Day 33 following AI. Interval between inseminations affected pregnancy rate (P < 0.01) with AI intervals of 8 (80%) and 16 h (75%) greater than the 4 h interval (55%). The EOI also affected pregnancy rate (P < 0.001) with increased pregnancy rate for gilts having ovulations by 36 h (85%) compared to those having ovulations by 24 (55%) and 60 h (47%) but did not differ from those having ovulations at 48 h (74%). Total fetuses (10.7), number of normal fetuses (10.2) and embryo survival (70.4%) were not affected by interval between inseminations or EOI (P > 0.05). There was an interaction of treatment with EOI (P < 0.05). Gilts inseminated at 8 or 16 h intervals and with 36 or 48 h EOI showed changes (P < 0.05) in the proportion of offspring sired by the second AI while gilts inseminated at a 4 h interval did not. The results of this study indicate that intervals between inseminations and time of ovulation each affects pregnancy rates but not litter size, but interacts to affect the proportion of offspring sired by the second insemination. This study suggests no detrimental effect of reduced amount of time between inseminations on the fertility of sperm used in a second insemination. For practical application, in a twice daily estrous detection and double AI, the greatest pregnancy rates occur when inseminations are spaced at 8 or 16 h intervals to compensate for variation in ovulation.