Effects of the number of sperm and site of uterine semen deposition on conception rate and the number of embryos in weaned sows receiving a single fixed-time insemination.

Reducing the number of sperm needed to produce a litter with artificial insemination (AI) allows greater use of higher genetic merit boars. Induced ovulation with single fixed-time artificial insemination (SFTAI), combined with intrauterine (IUI) or deep uterine insemination (DUI), could improve fertility with low numbers of sperm. The objectives of the study were to determine the fertility effects of sperm numbers and the site of insemination. At weaning (0 h), sows (n = 534) were assigned by parity and estrus induction method (equine chorionic gonadotropin [eCG] or Control) to receive 1,200 × 106 sperm by IUI; 600, 300, or 150 × 106 sperm by IUI or DUI; or 75 × 106 sperm by DUI. At 80 h postweaning, sows received OvuGel and 26 h later a SFTAI using pooled semen. Sows were exposed to boars once daily and ultrasound was performed to determine follicle size and time of ovulation. Following SFTAI, sows were slaughtered 27 d after AI to determine pregnancy and litter traits. Data were analyzed using different models to test for effects of estrus induction, interaction of three levels of sperm (600 to 150) with two levels for site (IUI vs. DUI), and the overall effects of AI method (eight treatments). There was no effect (P > 0.05) of estrus induction on estrus (93%) within 5 d of weaning or on follicle size (6.1 mm) at OvuGel, but wean-to-estrus interval (3.8 vs. 4.0 d) was slightly reduced (P < 0.01) as was AI-to-ovulation interval (15.9 vs. 17.0 h, P = 0.04) for eCG and Control, respectively. There was no effect (P > 0.05) of estrus induction on pregnancy rate (78.6%), number of corpora lutea (CL; 21.7), or number of viable embryos (12.2). There was no effect of number of sperm or site of insemination and no interaction (P > 0.05) on pregnancy rate (range: 80.9% to 70.5%), but AI occurring after ovulation reduced the pregnancy rate (P < 0.02). The total number of embryos (range: 16.5 to 10.3) was not affected by estrus induction, number of sperm, or site of insemination (P > 0.05), but was influenced by AI treatment (P < 0.01). Treatments with a higher number of sperm (1,200 and 600) had more embryos compared with those with a lower number of sperm (300 to 75). The numbers of embryos also increased with the number of CL (P < 0.0001). These results suggest that the lower number of sperm affects litter size more than the pregnancy status. Acceptable fertility can be achieved with low numbers of sperm when using a SFTAI and uterine deposition, but AI-to-ovulation interval and ovulation rate influence final fecundity.