Effect of number of motile, frozen-thawed boar sperm and number of fixed-time inseminations on fertility in estrous-synchronized gilts

There are advantages for use of frozen-thawed boar sperm (FTS) as a tool for preservation and transfer of valuable genetic material, despite its practical limitations. It was hypothesized that increasing the number of motile FTS and number of fixed-time artificial inseminations (AI) would improve pregnancy rate and litter size. Semen from six boars was frozen in 0.5mL straws at 500x10⁶ cells/mL. Gilts approximately 170 days of age, were induced into estrus with PG600 and synchronized using MATRIX (synthetic progestagen). Following last feeding of MATRIX (LFM), gilts were checked twice daily for estrus. At onset of estrus, gilts were randomly assigned in a 3x2 factorial treatment design to receive 1x10⁹ motile FTS (n =19), 2x10⁹ motile FTS (n =19), 4x10⁹ motile FTS (n =19) in a single AI at 32h after onset of estrus, or 1x10⁹ motile FTS (n =18), 2x10⁹ motile FTS (n =17), or 4x10⁹ motile FTS (n =19) in each of the two AI at 24 and 32h following onset of estrus. Ultrasonography was performed at 12h intervals after estrus to estimate time of ovulation. Reproductive tracts were collected 28-34 days following AI. Estrus occurred at 139±2h (mean±SE) after LFM and ovulation at 33±1h following onset of estrus. Dose and number of inseminations did not interact or individually influence pregnancy rate at slaughter (73±4.2%) or numbers of normal fetuses (10.8±0.5). However, number of fetuses tended (P =0.14) to increase with double AI but not with dose. Boar did not affect pregnancy rate but did affect number of normal fetuses and embryonic survival (P <0.01). Longer intervals from insemination to ovulation reduced pregnancy rate (P <0.05), number of normal fetuses (P <0.001), and embryonic survival (P <0.01). Ovarian abnormalities at slaughter were associated with reduced pregnancy rate (P <0.001). The results of this experiment indicate that a double insemination using 2x10⁹ motile sperm would produce the greatest number of piglets with fewest numbers of frozen sperm used, while double AI with 1x10⁹ motile sperm would be most practical for pig production with limited genetic resources. Fertility was also influenced by boar, interval from insemination to ovulation, and gilt ovarian abnormalities.