Luteal Insufficiency in the Mare

Insufficient function of the corpus luteum (the structure formed after the follicle that releases the egg, or ovulates, and then produces progesterone) has been proposed as a mechanism for early embryonic loss in the mare, but until now there hasn't been much research into this potential phenomenon. In her presentation "Luteal Insufficiency in the Mare: Fact or Fiction?" at the Hagyard-Davidson-McGee (HDM) Bluegrass Equine Reproduction Symposium, Karen Wolfsdorf, DVM, Dipl. ACT, of HDM discussed her field study on this topic.

"Progesterone is a primary secretory product of the corpus luteum (CL) and has been referred to as the 'hormone of pregnancy,' " she began. "The early conceptus (embryo) suppresses the release of prostaglandin from the endometrium, allowing the CL, progesterone, and therefore pregnancy to be maintained." Early in the pregnancy, before the embryo has attached to the uterine wall, progesterone is essential for its role in increasing the production and affecting the composition of uterine secretions by endometrial glands; these secretions provide nourishment for the embryo in its nomadic stage. Progesterone also influences uterine tone, which affects the mobility and implantation of the embryo.

Previous studies have suggested that different levels of progesterone should be sufficient for pregnancy maintenance in the mare (4 and 2.5 ng/mL), and usually these levels are checked at day 14 after ovulation, said Wolfsdorf. Before beginning her study, she hypothesized that it might be more effective to test progesterone (P4) levels on day 6 (P4D6) and begin treatment at that time (progesterone supplementation) if necessary. She suggested that this early testing would help determine which mares really need supplementation, hopefully decreasing widespread use of supplementation "just in case." She also wanted to prove whether there was any incidence of "luteal insufficiency" with low P4 levels.

In testing 156 estrous cycles, Wolfsdorf found that 9% of the cycles had a P4D6 of less than or equal to 4 ng/mL. All mares were considered reproductively sound, and all were managed on an Ovuplant or hCG (human chorionic gonadotropin) program for estrous cycle control. Since the mares belonged to commercial farms that didn't wish to risk losing the foals, the low-P4D6 mares were supplemented with progesterone as altrenogest (thus, it couldn't be determined if these mares would have lost their foals without supplementation). These mares had a 50% pregnancy rate at day 14-15, compared with a 66% pregnancy rate for mares with a P4D6 higher than 4 ng/mL.

Further analysis of the data and initial evaluation of this soon-to-be-published study will be in the scientific literature at a later date.

Wolfsdorf thus confirmed that luteal insufficiency with decreased P4 levels does exist in approximately 9% of estrous cycles, that measuring P4 at day 6 may detect if supplementation is necessary versus the usual 14-day measurement, and that starting supplementation at day 6 can improve pregnancy rates for these mares. She also recommends remeasuring progestogen levels at day 40 and progestogen (progesterone metabolites produced by the placenta) at day 120-150 for low-P4D6 mares to determine if continued supplementation is still necessary.

ne attendee asked Wolfsdorf about the accuracy of P4 assays; she answered that most commercial test kits only show a high (greater than 4 ng/mL) or low (less than 1 ng/mL) value, and that laboratory results were probably more exact. She also recommended testing these levels within 18 hours of sample collection, and cautioned against exposing samples to lots of sunlight or heat before testing.

Wolfsdorf will be looking into this topic further with a similar study that is currently underway. This one will compare the pregnancy rates of mares with low progesterone levels that were altrenogest-treated vs. non-treated mares.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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