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The oestrus cycle

obovis

The oestrus cycle

  • The cow has a 21-day oestrus cycle, but this can vary between 18 and 24 days.
  • Heifers will begin cycling at the onset of puberty and will continue to cycle until they are in calf.
  • The onset of puberty can be affected by various factors such as breed, nutrition, growth rate and disease.
  • After calving, cows will begin to cycle after 20 to 30 days.
    • This time may be extended in high yielding cows or those affected by disease post calving.
  • The oestrus cycle is governed by the complex interactions of various hormones that are produced in the brain and ovaries. The main hormones are progesterone and oestrogen.
  • The follicle (egg) grows throughout the cycle and ovulation (the release of the egg) occurs when the progesterone levels drop and the oestrogen level rises.
  • A corpus luteum then forms on the ovary and this produces progesterone, which helps to maintain the pregnancy.
  • Oestrus tends to precede ovulation in cattle, by approximately 12 hours. This gives rise to the “AM-PM” rule, whereby once a cow is identified as in oestrus it is inseminated in the afternoon or the following morning depending on the time of identification.
Image courtesy of Peter Jackson.
Image shows "bulling" string.

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