The decision to breed a mare, either by AI or live cover, is usually dependent on when it is determined that the mare will ovulate. In most breeding operations today, multiple factors are used to predict ovulation in the mare. The goal of a recently published paper was to review 15 different parameters that can be used to predict ovulation in the mare. The list included things like reproductive history, the growth pattern of the follicle, diameter, shape, and wall thickness of the follicle, uterine edema, relaxation of the cervix, and interval from hCG or deslorelin administration, to name just a few. It was concluded, “no single criterion is consistently useful to predict interval to ovulation or impending ovulation.” “A combination of growth pattern and size of the dominant follicle, in conjunction with an increase in follicular wall thickening, development of an irregular shape, softening of the follicle, and a decrease in endometrial edema is suggestive of impending ovulation.” The use of hCG or deslorelin given at the correct time can also help greatly to predict the time of ovulation.
It is somewhat common knowledge that Friesian mares, on average, will ovulate a larger follicle than many other light breeds of horses. It is also important to understand and evaluate these other predictors of ovulation along with follicle size, as there are Friesian mares out there whose follicles may not “follow the rules”. Some Friesian mares' dominant follicle will ovulate in the mid-’40s to just around 50 millimeters rather than the 60-millimeter follicles that we may be more familiar with. It is helpful for breeders and veterinarians new to the Friesian breed to remember to use these other parameters as well so that we breed our Friesian mares efficiently and effectively, wasting neither time, money, or effort, resulting in a pregnant mare, a satisfied owner and less frustrated veterinarian.
Scientific Article: McCue, P. et. al. “Review of Techniques for Prediction of Ovulation in the Mare”. AAEP Proceedings 2014, vol. 60, pp. 41-44.