The bovine estrous cycle
Keywords: estrus, estrous, progesterone, prostaglandin, estradiol, LH, FSH, bovine, cow
| Note: The use of estrus as an adjective is less common than estrous (1.23 million vs 840 thousand Google hits respectively). Nevertheless, estrus is often used as an adjective e.g estrus cycle, estrus behavior, estrus synchronization etc. Indeed, it has become so common as to be regarded as both a noun and an adjective. However, the author adopts the more common term estrous as an adjective in this entry. In the British commonwealth, the terms oestrus and oestrous are used for the noun and adjective respectively. |
The image shown here is useful for explaining the bovine estrous cycle*. It shows the rise of estradiol 17 beta that brings the cow into heat and causes a surge of luteinizing hormone (LH). Ovulation usually occurs about 16 to 24 hours after the onset of the LH surge i.e. when the cow is no longer showing signs of estrus.
After ovulation has occurred, a corpus luteum forms from the structure that was previously the follicle, producing progesterone (the yellow profile) for approximately 19 days. If the cow is not pregnant, at about 15 to 16 days after ovulation, the endometrium releases prostaglandin F2 alpha (PG) which causes luteolysis i.e. the demise on the corpus luteum. Immediately after ovulation has occurred a new wave of follicle growth begins because inhibin production by the previous large follicle, is no longer present. This results in an elevation of follicle stimulating hormone (FSH).
A dominant follicle is selected in each follicle wave, suppressing its cohorts. If that follicle does not ovulate, it undergoes regression and a new follicle wave begins. There are usually three such follicle waves during each estrous cycle but in young animals, there may be only two waves, resulting in an interovulatory interval that is a few days shorter than a conventional estrous cycle of 21 or 22 days.
Standing estrus last for about 8 to 12 hours in most dairy cows but may be much shorter (and difficult to detect) in high producing cows. Very short estrous periods are typical of zebu-type cattle as well.
*Readers are strongly encouraged to consult an excellent resource on the bovine estrus cycle developed by Dr Roberto Palomares, Kip Carter and colleagues at The College of Veterinary Medicine, University of Georgia. The presentation is available for Apple or PC use.
The Apple OS version has full animation. Lacking some animations, the PC (epub) version however remains valuable for all students of reproductive physiology. On most PC computers, the ePub version can be viewed without downloading ePub readers.
