I hope everyone had a great week and that you came out to say “Hi!” to the Springhill Equine folks at the Alachua Spring Festival. I know the staff and Doctors always look forward to the festival and its great vendors and activities. And continuing on our Spring theme, this week I am going to discuss the changes mares go through in Spring and Fall and during a normal cycle. These changes are very closely monitored by our Doctors by ultrasound to help them determine the best time to breed.
Last week I covered that horses are seasonal breeders during long daylight hours. Determining when that optimal amount of daylight happens to have our mares cycling is the tough part! To begin with our Doctors rectally palpate the ovaries and uterus. Normal ovaries are about half the size of a human fist, getting slightly small when the mare is out of heat and larger when in heat. During the winter the ovaries shrink down to the size of walnuts. The uterus on a normal cycling mare varies in tone during an estrous cycle (the period between ovulations). When your mare is in heat the uterus is very loose and floppy, when she is out of heat it is firm like a well toned muscle. If a mare is not yet cycling the uterus feels loose and floppy but the ovaries are small. So palpation is the first clue.
Next an ultrasound machine is used to look at the ovaries and uterus. Again our Doctors find that mares who are not cycling will have small ovaries with very small (less than 10mm) follicles and a uterus that feels like an in heat uterus but looks like an out of heat uterus (more on the difference later). If in doubt our Doctors will pull a progesterone level. A small amount of blood is taken and tested to determine if these levels are low, high or in the middle. Mares who are not cycling yet will be very, very low.
Now comes the tricky part….Before mares actually ovulate for the first time in a season they build a few big follicles which don’t ovulate, the uterus starts to have tone and everything looks like it is headed in the right direction only to stop just before ovulation occurs. This can mean several ultrasounds a few weeks apart to determine when your mare is finally ready for breeding!
Dr. Lacher and Dr. King have determined your mare is cycling and we can now work on breeding her. What the heck does that mean and what are they looking for to determine the perfect time to order semen or send her to the stallion? Three things have to happen for everything to be perfect: a follicle of 35-38mm in size, a uterus with a hint of edema, and an open cervix.
The ultrasound image below would be what Drs. Lacher and King would be looking for. They would measure across the black circles to determine when one reaches that magic 35-38mm.
They will also be looking at the uterus to see if there is any edema present. Edema is a very normal part of the cycle. The edema is seen in the image on the right and makes the uterus look like a wagon wheel or orange slice. Once at least a small amount of edema is present, a follicle of the correct size is seen, and the Doctors palpate an open cervix then an ovulatory timing drug such as deslorelin is given.
Drugs such as deslorelin help us precisely time ovulation. Once given 90% of mares will ovulate within a 36-48 hour window. This makes ordering semen from far away, making appointments with stallion owners, and timing of frozen semen insemination much easier.
Next week we will talk even more about the joys of the cycling mare and the fun of breeding! Until then may your litter box be clean and your food bowl full!