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The Basics of the Estrous Cycle

Understanding the basics of the estrous cycle lays the groundwork for a successful synchronization and artificial insemination program.

RUIDOSO, NEW MEXICO (Aug. 29, 2018) — “It is important to understand the basic principles of the estrous cycle so you can make better decisions about which protocol will fit a group of cattle,” Mike Smith, University of Missouri Curators’ professor in animal science, told cattlemen gathered for the 2018 Applied Reproductive Strategies in Beef Cattle workshop in Ruidoso, N.M., Aug. 29-30. “You will also understand the importance of administering the protocol exactly how it is designed and how to fix it when something is not done correctly.”

“The key to any protocol is synchronizing the follicular wave and controlling the life of the CL. This can increase pregnancy rates significantly,” said Mike Smith.

The typical length of an estrous cycle is 21 days, with the opportunity for two or three follicular waves, depending on the animal. The variation in length of standing heat, estrus, stems from the follicular waves.

“The release of an egg is about 30 hours after the cow first shows estrus,” Smith said. “A majority of cattle release between 11 and 20 hours while 15% release less than 10 hours after they show estrus. Even if you check twice a day, you could miss those shorter-cycling cattle.”

Pregnancy rate is a function of estrous detection. During the estrous cycle, a corpus luteum (CL) is formed on the ovary. If a cow is pregnant, the CL will eventually form the calf. If the cow is not pregnant, the CL will be regressed and she can come back into heat.

“There are five hormone changes occurring for the cow to get pregnant,” Smith explained. “Progesterone is elevated in the blood, that comes from the CL on the ovary after ovulation. The rapid decrease in circulating progesterone initiates the release of prostaglandin F2-alpha. This will induce the regression of the CL, which will cause a rise in estradiol from the dominant follicle. Then the sharp release of luteinizing hormone (LH) will start the ovulation process.”

Administering progesterone before breeding or starting a fixed-time artificial insemination (AI) protocol will block standing heat and subsequent ovulation. This ultimately preserves the quality of an oocyte.

Almost all protocols require a shot of gonadotropin-releasing hormone (GnRH) to ovulate a follicle. This synchronizes the follicular waves among a group of cows.

“The key to any protocol is synchronizing the follicular wave and controlling the life of the CL. This can increase pregnancy rates significantly,” Smith added.

Melengestrol acetate (MGA) or a controlled internal drug release (CIDR®) device can reduce the quality of an oocyte. This greatly impacts pregnancy rates.

“Because the level of progesterone from these two products is low for a sustained period of time, the quality of the egg can be impacted,” Smith said.

This is why a protocol including MGA or a CIDR recommends breeding at the second standing heat instead of the first. Since it is impossible to know where the animal was in their follicular wave once the protocol started, the age of the CL is hard to predict.

“Giving a shot of GnRH at the start of a protocol will start a new follicular wave, and the oocyte will not be aged,” Smith said. “This will greatly increase fertility and pregnancy rates.”

Protocols are built to mimic the complex hormone changes of the estrous cycle. This makes timing absolutely critical to the success or failure of a program.

To read the proceedings, review the PowerPoint presentation or listen to this presentation, visit the Newsroom at

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