ARSBC 2018: Physiology of the Estrous Cycle, Application of Basic Principles

Michael Smith
University of Missouri

The estrous cycle averages 21 days, with a range of 17 to 24 days. 

There is variation in how long cows express estrus. Most cows express estrus for more than 11 hours. However, there is a significant number of cows who express estrus for less than 10 hours. These cows are often the ones that are missed in heat check programs. The success of artificial insemination programs is a function of both estrus detection and pregnancy rate.

The presence of progestorone prevents ovulations and the expression of estrus. We use this principle in estrus synchronization procotols to synchronize estrus and ovulation.

There are three catagories of horomones used in estrus synchronization:

• Progestins/Progesterone
• Prosteglandins
• GnRH

Progesterone blocks estrus, blocks ovulation, preserves oocyte quality, and prepares for pregnancy. The two progestorone products used in the US are MGA and CIDR.

MGA feeding for 14 days syncs heifers to all express estrus at the same day. However, some of those heifers would have ovulated much earlier. During this time, the oocyte (egg) ages during this MGA feeding period. This is why we use to estrus cycles in 14-day protocols.

Prostaglandin F2alpha has no effect if the heifers or cows are not cycling. Also, it is not effective for corpus lutuem that are less than 6 days old. 

Females that express estrus by the time of AI have much higher pregnancy rates than those heifers that do not. 

The rapid rise in progestorone after ovulation is necessary to promote pregnancy.

From day 6 to day 8 after fertilization there is differentiation of cells that will become the calf and cells that will become the membranes. From day 8 to day 16 there is dramatic elongation of the membrane cells. 

Take Home Messages:

• Specific hormonal changes during the estrous cycle are required for pregnancy to be established.
• Estrous synchronization protocols mimic the preceding hormonal changes.
• Therefore, estrous synchronization products must be administered at the proper dose and at the correct time.
• Attention to detail is critical for success.

Note: this post was live blogged and may contain errors.