This brief article covers the average heat cycle of a sexually mature female cow or heifer. 

The general "heat cycle" period for all female cattle occurs every 19 - 26 days. With 21 days being the universal "norm" across all cattle breeds. heifers tend to have shorter cycles than cows. 

Heat cycles outside of these 19 - 26 day variations could be indicators of a problem. Such as ovarian cysts, which can cause females to cycle irregularly. 

Reproductive maturing, (when females begin to cycle) is highly genetic throughout generations of females. Early maturation can be bred for. Sexual Maturation also varies between breeds. Average first cycles of beef breeds is 8 - 20 months. 

The Estrus Cycle:

• Timing: Day 0 (lasts about 12 to 18 hours).
• What happens: Estrogen levels spike. This is the only time the cow is sexually receptive and will stand to be mounted by a bull or other cows (standing heat).
• This is when you would want to artificially inseminate your cattle. Semen can survive in the reproductive tract of a female for 12 -36 hours. 

• Timing: Days 1 to 5.
• What happens: The cow goes out of heat and ovulation occurs (she releases her egg about 10 to 14 hours after her heat ends). The spot on the ovary where the egg was released turns into a temporary gland called the Corpus Luteum (CL).
• This is why GnRH is often given after artificial insemination, as it stimulates the CL 

• Timing: Days 6 to 17.
• What happens: This is the longest phase. The CL gland produces progesterone, the "pregnancy hormone." Progesterone prevents her from going into heat, and prepares her uterus to nurture an embryo.
• This is why most embryos are put in to recips 7 days after witnessing standing heat
• CIDR's release progesterone to mimic this stage during estrus synchronization

Proestrus (The Reset Phase)

• Timing: Days 18 to 21.
• What happens: If the cow's body recognizes she is not pregnant, her uterus releases a hormone called prostaglandin. This hormone destroys the CL gland, causing progesterone levels to crash. Without progesterone holding her back, a new egg rapidly matures, estrogen spikes, and she returns to Day 0 to start all over again.
• Lutalyse or Estrumate serve this function

Artificial Hormones 

When breeding cattle with AI or putting embryos in, you can either breed on 1) Natural Standing Heat   or    2) Breed After putting cattle through a synchronization protocol. (artificial hormones that allow multiple animals to come into heat at roughly the same time. The most common ones are: 

Progesterone:

• most commonly found in the form of a CIDR

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• picture source

Prostaglandin (PG)

• most commonly found in the form of Lutalyse or Estrumate
• This destroys the corpus luteum (CL) on the ovary, which brings the cow back into heat

Gonadotropin-Releasing Hormone (GnRH)

• most commonly found in the form of Cystorelin or Fertagyl
• Its primary job is to act as a chemical messenger that signals the pituitary gland to produce and release two other key reproductive hormones: 
  • Luteinizing Hormone (LH)
  • Follicle-Stimulating Hormone (FSH) 
  • This surge causes the dominant follicle to ovulate and resets the cow's follicular wave

  

   

  

  above is a synch protocol commonly used on cows. (not heifers)