The effect of sperm number on fertility in blue fox vixens ( [formula omitted][formula omitted]) artificially inseminated with frozen silver fox ( [formula omitted][formula omitted]) semen

During the breeding seasons of 1989 and 1990, a total of 617 blue fox vixens aged 1 to 6 years (mean ± SEM, 2.6 ± 0.1) were inseminated with frozen silver fox semen with either 150 million (n = 213, 1989 + 1990), 100 million (n=172, 1990), 75 million (n = 119, 1989) or 37.5 million (n = 113, 1989) spermatozoa per insemination. Two intrauterine inseminations, each with an insemination volume of 1.0 ml, were performed at 24-hour intervals on the first and second days after maximum vaginal electrical resistance was measured. Conception rates were 87% (186 of 213) with 150 million spermatozoa per insemination, 85% (146 of 172) with 100 million, 77% (91 of 119) with 75 million and 68% (77 of 113) with 37.5 million. The mean numbers of cubs per litter ± SEM for the four groups were 7.6 ± 0.2 (168 registered litters), 7.5 ± 0.3 (115 litters), 6.4 ± 0.4 (86 litters) and 6.4 ± 0.4 (75 litters). A negative effect on both the conception rate and mean litter size at whelping was observed with decreasing sperm numbers (conception rate percentage: p = 0.0001, Chi-square, litter size: p = 0.02, Kruskal-Wallis Test). Only the two larger numbers of spermatozoa gave litter sizes comparable to those obtained by artificial insemination (AI) with fresh semen.