culturing

The mare's own uterine infection fighting processes are at their highest level during estrus. If a pathogen is isolated this means that the mare is unable to get rid of it herself.
Since the uterine infection fighting processes are at their highest, if a pathogen is inadvertently introduced during sampling, it she may well be able to get rid of it herself.
The cervix is relaxed during estrus making access easier for the technician and less traumatic for the mare.