by Troy Smith, field editor
Application of Basic Principles
Dr. Michael Smith, University of Missouri
DAVIS, Calif. (Aug. 17, 2015) — The dictionary defines the term “hormonal” as being of, relating to or effected by hormones. The reproductive cycle definitely is hormonal. Specific hormones induce the physiologic changes necessary for the establishment of pregnancy in the mammalian female. According to reproductive physiologist Mike Smith, an understanding of hormonal changes occurring in the bovine female can make estrous synchronization and artificial insemination programs for beef breeding herds more successful. The University of Missouri professor’s review of the bovine estrous cycle set the stage for subsequent speakers at the Applied Reproductive Strategies in Beef Cattle conference Aug. 17-18 in Davis, Calif.
Smith said the estrous cycle consists of three stages — follicular phase, estrus and luteal phase — which are regulated by the following hormones:
- Gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus
- Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secreted by the anterior pituitary gland
- Estradiol and progesterone secreted by the ovary
- Prostaglandin secreted by the uterus
“These hormones serve as chemical messengers that travel in the blood to specific target tissues, which contain receptors, and regulate the phases of the estrous cycle,” explained Smith. “Estrous synchronization protocols have been designed to mimic the physiological changes controlled by these [naturally occurring] hormones.”
Smith said the three types of estrous synchronization products include progestins, prostaglandin and GnRH. Commercially available progestin products used to manage the luteal phase include melangestrol acetate (MGA), which is administered orally, and the controlled internal drug release (CIDR®) device. Prostaglandin and GnRH are administered by injection. Numerous estrous synchronization protocols have been developed for heifers and cows, which prescribe the timing of product administration and insemination.
According to Smith, it is not uncommon to hear producers blame a particular estrous synchronization product or protocol for poor results. He reminded the audience that all of the commercially available products have been proven effective when properly stored and administered.
Unsatisfactory pregnancy rates can most often be attributed to cows or heifers being unsuitable candidates for estrous synchronization, poor choice of synchronization protocol or noncompliance with the chosen protocol.
“It’s critically important that synchronization products be administered at the correct time and at the correct dosage,” emphasized Smith. “Attention to detail is essential.”
Smith spoke during Monday’s ARSBC session focused on the basics of reproduction. For more information, visit the Newsroom at www.appliedreprostrategies.com to view his PowerPoint, read the proceedings or listen to the presentation.
Comprehensive coverage of the symposium is available online at www.appliedreprostrategies.com. Compiled by the Angus Journal editorial team, the site is made possible through sponsorship by the Beef Reproduction Task Force.
Editor's Note: This article was written under contract or by staff of the Angus Journal. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.