| | Day 0 | Days 1-2 | Days 3-5 | Days 6-16 | Days 16-18 | Days 18-19 | Days 19-20
This section describes the series of hormonal and physiological changes which occur during the cow's normal estrous cycle. The estrous cycle in the cow averages 21 days in length, but can vary between 17 and 24 days and still be considered normal. The length of the estrous cycle is measured as the time between two consecutive estrus or heat periods. The physiological and hormonal changes which occur in the female over the estrous cycle prepare the reproductive tract for estrus (the period of sexual receptivity), ovulation (release of the egg) and implantation (attachment of the fertilized egg to the uterus).
Day 0
The cow is in standing heat or estrus. High estrogen levels, produced by the granulosa cells lining the inside of the ovulatory follicle, act on the hypothalamus to initiate standing estrous behavior and release of the ovulatory gonadotropin surge from the pituitary. Also under the influence of estrogen, the cervix dilates slightly and cervical mucus becomes less sticky to allow easier penetration of spermatozoa. Uterine and oviductal contractions, stimulated by estrogen, increase to aid in sperm transport through the uterus and egg transport down the oviduct. Between 16 and 30 hours after the onset of standing heat, the ovulatory follicle on the ovary ruptures and the egg is released into the oviduct. Once the surge of gonadotropins has occurred, the cells lining the inside of the follicle cease estrogen production. After release of the egg, the granulosa and theca cells remaining within the collapsed follicle begin rapid division, accompanied by infiltration of blood vessels and connective tissue.
Days 1 - 2
Granulosa and thecal cells, now referred to as a corpus luteum, continue to rapidly divide and are changing from estrogen to progesterone secreting cells. The egg is at the ampullary-isthmic junction of the oviduct where fertilization occurs.
Days 3 - 5
The corpus luteum is now growing rapidly in both size and progesterone secreting capability. Progesterone stimulates the uterus to produce uterine milk which nourishes the fertilized egg or embryo. Progesterone also quiets oviductal and uterine contractions and increases the stickiness and viscosity of cervical mucous. The embryo reaches the uterus between days 3 and 4.
Days 6 -16
The corpus luteum continues to develop and reaches its maximum growth and function by day 1 0 to 12. Circulating levels of progesterone, produced by the mature corpus luteum, are high and act to inhibit development of an ovulatory gonadotropin surge. Follicles continue to grow on the ovary during this time, but fail to reach maturity and will become atretic or die due to the high levels of progesterone and the high amplitude. low frequency pattern of LH secretion.
Days 16 - 18
If the animal is not pregnant, the corpus luteum begins regression or death due to secretion of prostaglandin F2a by the uterus. Progesterone level is rapidly decrease in the blood stream.
Days 18 - 19
Progesterone levels in the blood are low and progesterone's blocking effect on the hypothalamus and pituitary are released. Gonadotropin secretion changes to a high frequency, low amplitude pattern which stimulates ovarian follicle development to maturity. Usually in the cow, only one follicle is present on the ovary in the right stage of growth to respond to the change in gonadotropin secretion. This one follicle becomes dominant and will go on to ovulate, while the remaining follicles become atretic.
Days 19 - 20
Rising estrogen levels, produced by the rapidly growing follicles, stimulate reproductive tract preparation for standing estrus. Estrogen also stimulates the hypothalamic/pituitary axis to release greater quantities of hormone at a higher frequency of release. As estrogen levels peak in the blood, standing heat is again initiated, along with the surge of gonadotropins to induce ovulation, and the cycle begins again at Day 0.
The timing given for the preceding events is only approximate, and would differ for different cycle lengths. Also, the discussion of events occurring during the estrous cycle was based on a full cycle in which pregnancy does not occur. If the egg is fertilized and begins development in the uterus, the corpus luteum does not regress but continues to function by secreting progesterone. No follicles develop to maturity and ovulation does not occur. Progesterone keeps the uterus quiet and thus provides the most favourable conditions for the developing fetus. Cows are considered to be spontaneous ovulators which means that ovulation occurs at a certain time during the estrous cycle whether mating occurs or not. Cows normally exhibit heat more than once per year and are called polyestrous. However, some breeds of cattle may be susceptible to seasonal subfertility (e.g., Brahman). Species that are considered to be continuous breeders (such as the cow) are not without periods of anestrus during which the estrous cycle stops. For example, anestrus is commonly observed in cows, especially young cows, when nursing calves and subsisting on low planes of nutrition. Estrus is not always accompanied by ovulation, nor ovulation by estrus. Heat without ovulation (annovulatory estrus) will not result in pregnancy even though the female is bred. Annovulatory heats, which occur due to high levels of estrogen, are common in heifers before reaching puberty and may even be observed in pregnant cows. Ovulation without the external signs of heat (quiet or silent heats) is not uncommon in cows, especially the first few weeks after calving. Such females usually will not accept service. Typically, the first estrous period after calving is accompanied by an abnormally short estrous cycle (7 to 11 days in length). Short cycles are due to premature regression of the corpus luteum, but causative factors are currently unknown.
Adapted from Beef Herd Management Reference Binder and Study Guide 405-1 |
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