The aims of the study were to determine which of Days 5, 6 or 7 after the blood plasma
progesterone concentration (PPC) of bitches first reached 6–9 nmol/L (Day 0) yield the highest
fertility and whether day of insemination affects the gender ratio of conceptuses. Six
bitches were inseminated on Days 5 and 6 and 6 on Days 6 and 7. Ten million progressively
motile frozen-thawed sperm from each of 5 dogs were pooled for the first insemination.
The same number of sperm from 5 other dogs were pooled for the second insemination.
Only one batch of semen from each dog was used on all bitches, which largely prevented
any effect of male and semen. Twenty-three autosomal microsatellites and the amelogenin
gene were used to determine the paternity and gender of the conceptuses. Pregnancy rate
was 100%. Out of 103 ovulations 66 conceptuses were conceived (conception rate: 64%).
The proportion of available oocytes fertilised was 0.11, 0.56, and 0.27 for Days 5, 6, and
7, respectively. The odds of fertilisation was 16.7 and 4.2 times higher from insemination
on Day 6 compared to Day 5 (P < 0.001) and Day 7 (P = 0.013), respectively. The numbers of
male- and female conceptuses were equal (33 each) and gender was independent of insemination
day (P = 0.18). This study suggests that intrauterine insemination of bitches should
best be done 6 days after PPC first reaches a value between 6 and 9 nmol/L with a second
insemination one day later.