LORI.equine

Following the estrous cycle and breeding

Keywords: cycle, estrus, ultrasound, management

With the advent of ultrasonography, routine teasing has become less important than it was at one time. The author recalls how routine daily teasing formed the cornerstone of equine stud practice in the late '70s before ultrasound units were widely available. An exception to this statement is Thoroughbred practice where natural breeding is practiced exclusively and behavioral estrous is essential for breeding.

At top left it can be seen that the uterus is always palpated first, before ultrasonography is attempted. Performing ultrasonography in both cows and mares without establishing the palpable characteristics of the tract can lead to inaccurate conclusions and incomplete examination.

After establishing the dimensions and position of the tract all the palpated structures are viewed on ultrasonography (click and right click image for detail):

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In image A, a large pre-ovulatory follicle appears on the right side of the split image, with obvious uterine edema on the cross section of the uterus on the left side. Collectively, the large follicle and uterine edema are excellent indications of impending ovulation and the need to inseminate a mare.

If fluid is present in the uterine lumen as shown in image B (and is shown to be innocuous using cytology; which is very common) oxytocin is usually administered until the fluid is expelled.

In image C, a follicle is in the process of ovulating. In such cases insemination should have have taken place already. Otherwise, it should occur within the next 18 hours, the expected life span of an oocyte after ovulation. Close inspection shows that this follicle was ovulating at 10 am in the morning. That is slightly unusual because most ovulations occur late in the evening through early morning.

In image D, a corpus luteum (CL) has formed where the follicle once existed. Additional inseminations would no longer be required in this mare and a pregnancy diagnosis would be scheduled between 14 and 16 days after the intact follicle was last seen. The homogeneous echogenic appearance of a corpus luteum is common. However, considerable variation in the echodensity of the CL is possible. This is demonstrated in image E, a hematoma has formed instead of a normal CL. It is usually diagnosed by its large size (almost 6 cm in this case) and its non-homogeneous echogenicity.

Large post-ovulation hematomas are not very common and seem to have no endocrinological significance, usually disappearing within a few estrous cycles. Towards the end of the breeding season, it is not uncommon to enounter large, un-ovulated follicles filled with fibrin.