A vaginal swab is taken for cytology to determine the optimum time to breed. The type of cells alter over their entrus cycle. Usually swabs are taken over a period when in heat, ideally 1 early to determine a base line & then around days 10 to 14.
The swab with the vaginal cells are then analysed under a microscope.
This is a useful tool especially if you are having to travel long distances for the stud.
Also if a bitch has had unexplained misses it can be useful to do frequent tests during this period as they may have varying days to “normal”