Timing of Service or Insemination
Artificial insemination or natural service lead to a pregnancy only if the spermatozoa are "at the right place at the right time." The egg is released from the ovary about 10 to 14 hours after the end of heat and can only survive unfertilized for six to 12 hours. In contrast, the spermatozoa may live up to 24 hours in the reproductive tract of a cow. A common recommendation for the best timing of artificial insemination is the "morning-evening" rule: cows observed in heat in the morning are inseminated the same evening, and cows observed in heat in the afternoon are inseminated the next morning.
In the case of natural service, the cow and the bull may be allowed to mate starting a few hours after the cow accepts mounting until the cow refuses to be mounted.