A wide range of reproduction topics specific to stallions, mares, and foals were discussed at the Bluegrass Equine Reproduction Symposium Oct. 23-26 in Lexington, Ky., sponsored by Hagyard-Davidson-McGee. Following are reports of a few topics; more can be seen online in the Bluegrass Equine Reproduction Symposium category under Convention Reports at www.TheHorse.com.

Environmental Effects on Hormone Activity

Some natural and man-made compounds in the environment can disrupt the delicate reproduction hormone balance in many species, possibly including horses, says Cynthia Corbitt Gulledge, PhD, of the University of Louisville's biology department. Gulledge presented "Hormones and Anti-Hormones in the Environment: Relevance for Equine Reproduction."

"Chemical confusion can result when environmental chemicals trick the body into thinking that they're the natural ones," she said. Another problem is when environmental chemicals block the action of an animal's hormones. She cited documented instances of infertility, feminization of males, subfertility, and altered puberty due to environmental chemicals and/or hormones in other species. Her suggestion was that although this cause of reproductive dysfunction had not been studied in horses, it should be considered in cases of reproductive abnormalities with no apparent cause.

Sources of environmental hormones/anti-hormones include some plants (many plants contain phytoestrogens, or plant-made estrogens), agricultural runoff (crop pesticides and pharmaceuticals from feedlot runoff), treated sewage, industrial waste, personal care products (some sunscreens and hair care products), lawn care products, and insecticides.

Gulledge explained that chemical effects can depend on the animal's age when exposed (fetal exposure can have permanent developmental effects while adult exposure might result in lesser effects) and the specific roles of hormones in each species (which vary by species). For further information, she recommended the Center for Bioenvironmental Research's web site at www.som.tulane.edu/ecme/eehome/.

Luteal Insufficiency

Insufficient function of the corpus luteum 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 problem. (The corpus luteum is the structure formed after the follicle releases the egg. It produces progesterone, or P4.) In her presentation "Luteal Insufficiency in the Mare: Fact or Fiction?" 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 presence of an 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, P4 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. P4 also influences uterine tone, which affects the mobility and implantation of the embryo.

Previous studies have suggested that different levels of P4 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 (progesterone supplementation) at that time if necessary. She suggested that this early testing would help determine which mares really need supplementation, hopefully decreasing widespread use of hormones "just in case." She also wanted to prove whether there was any incidence of "luteal insufficiency" with low P4 levels.

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

For more information, see article #3929 at www.TheHorse.com.

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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