Managing the Estrous Cycle
More uniform and consistent pregnancy outcomes achievable with greater understanding of the physiology of the estrous cycle.
In an opening presentation to the Applied Reproductive Strategies in Beef Cattle (ARSBC) Workshop hosted Aug. 29-30 in Manhattan, Kan. Kansas State University animal scientist Jeffrey Stevenson talked about the physiology of the estrous cycle. He said the application of artificial insemination (AI) in beef breeding herds has become much more practical due to advancements in timed AI programs. However, Stevenson also noted that an understanding of the limitations of methods used control the estrous cycle leads to more uniform and consistent pregnancy outcomes.
According to Stevenson, once the bovine female reaches puberty, she begins to experience estrous cycles — each being approximately 21-22 days in length — which continue unless interrupted by pregnancy. The cyclicity of estrous is attributed to repeated changes in the development of ovarian follicles, ovulation (release of an ovum or egg) and formation of a corpus luteum (CL) — all of which is controlled by the interactions of various hormones.
Ovulation generally occurs 24-30 hours after the onset of estrus, that period during the cycle when the female is receptive to mating.
The CL is a temporary aggregation of cells that forms at the site of a follicle after ovulation has occurred. It is maintained only if a pregnancy is established. However, while still present in a non-pregnant female, the CL secretes progesterone, which inhibits expression of estrus and ovulation.
“Estrus synchronization is largely about managing the CL,” stated Stevenson, explaining how synchronization programs involve administration of prostaglandin products, which induce regression of the CL and the subsequent removal of the influence of progesterone. Synchronization programs also call for administration of GnRH (gonadotropin-releasing hormone) to trigger a surge of luteinizing hormone, produced by the pituitary gland, which then stimulates ovulation to occur in about 30 hours.
Stevenson referenced the popular 7-day CO-Synch + CIDR (controlled internal drug release device) synchronization protocol, which is initiated with administration of GnRH and application of the CIDR device on Day 0. The CIDR is removed on Day 7 and prostaglandin is administered. Insemination occurs 60-66 hours later, at which time a second GnRH dose is administered. Ovulation response to GnRH occurs nearly every time, if CL regression occurs successfully, making the protocol practical for most beef cattle operations.
Stevenson suggested that results might be even better if the second dose of GnRH were administered approximately 16 hours prior to insemination, since that sequence more closely mimics what occurs at spontaneous estrus.
“Administering GnRH-2 (second dose) approximately 16 hours before AI allows for ovulation to occur 8 to 14 hours after sperm have formed a reservoir at the utero-tubal junction. This timing matches nicely what occurs naturally when cows come into heat spontaneously and ovulate in response to their own estradiol-GnRH-LH signals,” explained Stevenson.
Additional study is warranted, according to Stevenson, regarding the timing of the second GnRH dose, relative to the time of AI. To be practical, improvement in pregnancy outcome must outweigh time and handling costs.
Stevenson spoke during Tuesday’s ARSBC session focused on foundation principals. Visit the Newsroom at www.appliedreprostrategies.com, which features comprehensive coverage of the symposium, to view his PowerPoint, to read the proceedings or to listen to the presentation. Compiled by the Angus Journal editorial team, the site is made possible through sponsorship by the Beef Reproduction Task Force. To access video of the presentations, visit the Beef Reproduction Task Force page on Facebook.
The 2017 ARSBC Symposium was hosted by the Task Force and Kansas State University Research & Extension. Next year’s symposium will be Aug. 29-30 in Ruidoso, N.M.
Editor's Note: This article was written under contract or by staff of the Angus Journal, an Angus Media publication. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.