Swine fertility after AI with frozen-thawed spermatozoa varies between trials. As thawed spermatozoa have an extremely limited life span in the female genital tract, fertility of frozen-thawed spermatozoa depends mainly on the time of insemination relative to ovulation. The objective of this study was to evaluate whether the time of insemination relative to ovulation could explain the farm differences in fertility when frozen-thawed spermatozoa are used. Pooled sperm-rich fractions collected from three mature Pietrain boars were diluted in lactose/egg-yolk/glicerol/Orvus-ES-Paste extender, loaded in 0.5-mL straws (1 × 10⁹ cells/mL), and frozen under controlled conditions (Carvajal et al. 2003 J. Androl. 25, 389–396). Thawing was conducted in a waterbath at 37°C for 20 s. Inseminations were performed using the deep intrauterine insemination technique (Martínez et al. 2002 Reproduction 123, 167–170) with 1 × 10⁹ thawed spermatozoa (post-thaw motility >50%) diluted in 5 mL of Beltsville thaw solution (BTS). Ninety-seven and 82 weaned sows (parity 2–7) in farms A and B, respectively, were twice inseminated at 30 and 36 h after onset of estrus (estrus detection was performed twice a day by allowing females nose-to-nose contact with a mature boar and by applying back pressure). At insemination time, both ovaries were checked for ovulation by transrectal ultrasonography and sows were classified into three groups: F sows (follicles visible during the two examinations), O sows (ovulation visible during at least one examination), and C sows (corpora lutea visible during both examinations). Data were analysed with ANOVA and chi-square test, and are reported as % or mean ± SEM. Overall farrowing rates differed (P < 0.01) between farms: 70.1% (68/97) and 51.22% (42/82) in farms A and B, respectively. Litter size did not differ (P > 0.05) between farms (9.18 ± 0.24 and 9 ± 0.39 in farms A and B, respectively). Distribution of sows among F, O, and C groups differed (P < 0.05) between farms. Seventeen (17.52%), 70 (72.16%), and 10 (10.31%) sows in farm A and 33 (40.24%), 24 (29.27%), and 25 (30.49%) sows in farm B were classified as F, O, and C, respectively. Fertility in F, O, or C sows did not differ (P > 0.05) between farms. Farrowing rates and litter size in O sows (82.98% and 9.45 ± 0.23) were higher (P < 0.05) than in F (48% and 8.67 ± 0.54) and C (48.57% and 7.55 ± 0.62) sows. We can conclude that time of insemination relative to ovulation explains fertility differences between farms when frozen-thawed spermatozoa are used.

This work was supported by INIA (RZ01-019) and INFO (CARM).