I don’t know the details, but there is occasional “reinforcement” breeding in addition to live cover. It is allowed by the JC. My vet requested it specifically for my difficult maiden mare back in 2012. She was deep horn inseminated at the breeding shed immediately after she was covered (successfully). It worked. Maybe the AI was unnecessary, but on May 29 near the end of the season we were taking every option available.
The stallion in question was Sightseeing, who stood at Richland Hills in Midway, KY.
I am not sure what exactly the “reinforcement” was…I assumed thawed frozen semen, but I didn’t know at that time that frozen must be given within 6hrs of ovulation. And I have no recollection that we timed her that precisely.
Most big time breeding sheds have well-equipped labs on site. It is customary for the “dismount” dribble to be collected in a cup and immediately examined by a vet under a microscope to determine good motility for that cover. It would not surprise me at all if there was semen stored in the lab too, for “back up” purposes.