Bovine Estrous Cycle
The estrous cycle is the series of events that occur from one period of estrus
to the next. Behavioral estrus (heat) is commonly referred to as day zero in the
cycle. It is the period during which the cow is receptive to the bull, where she
will "stand" to be bred, hence the term "standing heat." If the cow does not
become pregnant, estrus will recur on average about every 21 days (ranging from 18 to 23 days)
The functional rhythm of the estrous cycle mirrors cyclical changes in the
ovary. Follicles mature under the influence
of follicle stimulating hormone (FSH) released by the pituitary (see
Bovine Reproductive Hormones). A growing follicle releases an egg (ovum) during
ovulation, under the control of luteinizing hormone (LH). The mature ovum passes
through the oviducts to the uterus (see Reproductive Anatomy), where fertilization may occur.
Meanwhile, the cavity in the ovary previously occupied by the follicle fills
with clotted blood. During the next 5 days, the clot is replaced by luteal
tissue and becomes the corpus luteum, which secretes progesterone. This hormone
maintains pregnancy should fertilization occur.
Toward the end of the estrous cycle, the uterus releases prostaglandin F2alpha
(PGF2alpha), which initiates the regression of the corpus luteum if
fertilization has not occurred. Regression of the corpus luteum is called
luteolysis. It sets the stage for the release of FSH and the development of a
new wave of follicles.
Phases of the Estrous Cycle
The bovine estrous cycle can be divided into four phases that merge into one
another. The table below describes these phases. Notice that the estrous phase
is the shortest phase, making it very easy to miss estrus even in normally
cycling cows.
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Phases of the Estrous Cycle
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Phase
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Approximate
Length
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Associated Hormone(s)
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Events
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Proestrus
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3 days
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FSH and increasing
amounts of estradiol
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Growth of follicle and increased
vascularity in uterine mucosa
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Estrus
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12-18 hours
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Estradiol
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• Sexual desire and acceptance of the male
• Increased blood supply to uterus
• Mucus may be visible on vulva
• Ovulation 12 to 14 hours after the
end of estrus, 3 hours earlier in heifers
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Metestrus
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6-8 days
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LH and progesterone
(which inhibits FSH release
by the pituitary)
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• Rapid growth of corpus luteum
• Slight postestrual bleeding may
occur
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Diestrus
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9 days
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Progesterone
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• Mature corpus luteum
• Thick uterine endometrium and
relaxed uterine muscle
• Scant and sticky vaginal mucus
• Corpus luteum regresses late in
this phase
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The absence of an estrous cycle is called anestrus. For the producer, anestrus
represents a significant problem because a cow must be in estrus before she can
be bred.
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