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Posted 03.28.05

Ovarian Follicle Maturity Key to Successful Hormone-Induced Pregnancy in Cattle

COLUMBIA, Mo. - Knowledge of procedures that control the timing of ovulation in human beings and other mammals are valuable in fertility treatment. When introducing fertility hormones to livestock to increase chances of achieving pregnancy, researchers have examined the size of the follicle, or blister-like structure within the ovary that releases the egg, as the key indicator of female readiness to ovulate and establish a pregnancy. Now, researchers at the University of Missouri-Columbia and the U.S. Department of Agriculture (USDA), have discovered that follicle maturity, not size, is more important in achieving a successful pregnancy, a finding that could have implications for fertility treatment in humans. The research will be published in this week's issue of Proceedings of the National Academy of Sciences.

"We have known for a while that follicles ranging in size can be induced to ovulate in response to a particular hormone," said Jonathan Green, assistant professor of animal sciences in the MU College of Agriculture, Food and Natural Resources. "The researchers thought that follicle size at the time of ovulation would have important effects on whether or not a subsequent pregnancy would be successful. In light of common protocols used in livestock breeding, scientists wanted to see if inducing ovulation also might contribute to pregnancy success or failure."

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For efficient, large-scale livestock production, it is common for animals to be given gonadotropin-releasing hormone (GnRH), a hormone sometimes used in human in vitro fertilization procedures. This hormone stimulates the release of two other hormones, known as gonadotropins, which cause the follicle to grow and release the egg. This process usually is performed on many animals at once to "synchronize" them for breeding.

Examining animals at the USDA Agricultural Research Service's Fort Keogh Livestock and Range Research Laboratory in Miles City, Mont., the researchers found that when the body naturally ovulates, known as spontaneous ovulation, follicle size matters little because the body intuitively knows when a follicle has reached maturity. However, when inducing ovulation, follicle size becomes important because introducing GnRH too early may induce a follicle that appears to be the proper size but is not physiologically mature.

"The study demonstrated that inducing ovulation of follicles that have not reached physiological maturity substantially decreased the chances for an animal to become pregnant," said Michael F. Smith, professor of animal sciences at MU. "Second, even if a pregnancy did occur, the likelihood of embryonic death increased."

Smith said the research might result in altered protocols for synchronizing ovulation in cattle.

"I suspect that more attention may be directed toward increasing the proportion of physiologically mature follicles at the time of ovulation," he said. "We may find that some minor adjustments to current synchronization techniques will help decrease the incidence of pregnancy loss arising from inducing the ovulation of immature follicles."



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