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 0.5 mL straws at 500 × 106 cells/mL. Gilts ∼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 3 × 2 factorial treatment design to receive 1 × 109 motile FTS (n = 19), 2 × 109 motile FTS (n = 19), 4 × 109 motile FTS (n = 19) in a single AI at 32 h after onset of estrus, or 1 × 109 motile FTS (n = 18), 2 × 109 motile FTS (n = 17), or 4 × 109 motile FTS (n = 19) in each of the two AI at 24 and 32 h following onset of estrus. Ultrasonography was performed at 12 h intervals after estrus to estimate time of ovulation. Reproductive tracts were collected 28–34 days following AI. Estrus occurred at 139 ± 2 h (mean ± SE) after LFM and ovulation at 33 ± 1 h 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 2 × 109 motile sperm would produce the greatest number of piglets with fewest numbers of frozen sperm used, while double AI with 1 × 109 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.