Increased levels of estrogen are produced by a maturing follicle in the cow's ovaries. Secretions along the reproductive tract allow for easier breeding and aid for sperm to travel through the uterus to the ovum. Standing heat usually lasts for 12 to 24 hours, after which ovulation occurs.;
, The mature follicle ruptures and the ovum travels through to the fallopian tube where it awaits the arrival of the sperm. Ovulation occurs in response to a surge of Luteinizing Hormone (LH) from the pituitary gland in the cow's brain. Ovulation occurs 12 hours after the cow goes out of heat.
 These cells regenerate and grow to create the corpus luteum (CL) in the area where the mature (now dead) follicle had ruptured and release the ovum.
 The growing CL produces high levels of progesterone which causes other follicles to regress and prevent from further maturation. During the early part of this phase, a portion of the lining over the caruncles (which are small protuberances on the inner uterine wall which the placenta attaches itself during the gestation period) becomes engorged with blood and bleeding from smaller capillaries may occur. Bloody discharge may be seen 2 or 3 days after the female has experienced estrus, which is caused by the sudden decrease in estrogen in her system. If you haven't seen the cow in heat, this is as good an indication as any that she had just been in heat a few days ago.
 The CL usually maxes out growth by the time days 15 or 16 arrive. This period is called Diestrus (or, "between estrus") because it is the longest phase of the estrous cycle. Progesterone secreted by the CL block any LH release from the pituitary gland, resulting in the ovaries being relatively inactive. No follicles are able to mature or ovulate. The cervix is closed up tightly and there are also no secretions from the reproductive tract during this time.
 Estrogen secretions stimulates the uterus to secrete prostaglandins, causing rapid regression of the CL.
 Very little progesterone is released, which means that progesterone and other reproductive hormones are no longer able to have any blocking effect on anything else. Several follicles on the ovaries begin to grow; one becomes dominant, secreting increasing levels of estrogen as it reaches maturity.
 An increase in estrogen and corresponding decrease in progesterone brings the cow back into heat again, starting the cycle back to 0.
Day 0 - Standing heat.
Ovulation.
Days 1 and 2 - Alteration of cells that line the Follicle.
Days 2 to 5 - Growth of the Corpus Luteum.
Days 5 to 16 - Continuation in Development of Corpus Luteum.
Days 16 to 18 - Follicles on the Ovaries begin growth again.
Days 18 and 19 - Corpus Luteum Becomes Non-functional.
Days 19 and 20 - Heat again.
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