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Signs of Heat

It is essential to understand the primary and secondary signs of heat in order to achieve accurate and efficient heat detection.
Standing and mounting activity.

Standing and mounting activity.

Primary sign

A cow standing to be mounted is the most accurate sign of estrus. Standing heat is the most sexually intensive period of the estrous cycle. During this period, cows stand to be mounted by other cows or move forward slightly with the weight of the mounting cow. Cows that move away quickly when a mount is attempted are not in true estrus. In order for standing behavior to be expressed, cattle obviously must be allowed to interact.

The expression of heat is due to the elevated level of estrogen in the blood when progesterone is very low. Occasionally cows in early pregnancy, approaching the end of pregnancy, or with ovarian follicular cysts have similar hormonal relationships and may express signs of heat.

The average duration of standing heat is 15 to 18 hours, but heat duration may vary from 8 to 30 hours among cows. An estrous cow usually stands to be mounted 20 to 55 times during her estrous period. Each mount lasts three to seven seconds. Factors that affect expression of heat are discussed in a later section.

Secondary signs

Secondary signs vary in duration and intensity. These signs may occur before, during, or after standing heat and are not related to time of ovulation. Dairy producers should use these signs as clues or watch the specific cow more closely for standing behavior. To determine whether cows exhibiting secondary signs will stand to be mounted, they may be isolated with a sexually active cow or teaser animal.

Mounting Other Cows

Cattle that exhibit this behavior may be in heat or approaching heat. Mounting activity is performed much less frequently by cows in midcycle. Although mounting cannot be used as a true primary sign of heat, cows exhibiting such behavior should be watched closely for standing behavior.

Mucus Discharge

As an indirect result of elevated estrogen levels, mucus is produced in the cervix and accumulates with other fluids in the vagina before, during, and shortly after estrus. Long viscous, clear elastic strands of mucus generally hang from the vulva. Sometimes, however, the mucus does not appear externally until the cow is palpated during insemination and the mucus is expelled. Mucus also may be smeared on the tail, thighs, flanks, or perineal region.

Mucus discharge
String of clear mucus being discharged.

Swelling and Reddening of the Vulva

During heat the vulva swells and the interior becomes moist and red. Generally these symptoms appear before heat and remain for a short period after heat. Thus they are not precise indicators of estrus. During midcycle the lips of the vulva are pale and more difficult to separate.

Bellowing, Restlessness, and Trailing

Cows in heat are more restless and alert to their surroundings. When allowed to interact with other cattle, cows coming into heat “proestrus” and cows in heat persistently trail behind to try to mount other cows. Research shows that cows in heat spend less time resting than nonestrous herdmates. Prior to and during heat, they remain standing and alert while their herdmates are lying down and resting. This is more noticeable for stanchioned cows. Cattle may bellow more frequently during estrus. Although these are not definitive signs of heat, cows exhibiting such behavior should be watched closely for standing behavior.

Rubbed Tailhead Hair and Dirty Flanks

As a consequence of being ridden, the hair on the tailhead and rump is fluffed-up, rubbed, or matted, and the skin may be exposed. The legs and flanks may be smeared with mud or manure.

Chin Resting and Back Rubbing

Prior to mounting, cows often rest or rub their chin on the rump or back of the cow to be mounted. This application of pressure may be considered a test for receptivity to being mounted. Both cows should be observed closely for mounting and standing behavior.

Chin resting in dairy cows
Chin resting and trailing activity.

Sniffing Genitalia

Sniffing the genitalia and licking the vulva of other cows occur much more frequently with cows in proestrus and estrus.

Head Raising and Lip Curling

Generally this activity follows sniffing of the genitalia and occurs more frequently if the cow being investigated is in heat and urinates.

Decreased Feed Intake and Milk Yield

Estrous cows spend less time feeding. Some studies also have reported decreased milk yield during estrus, and a Canadian study reported a slight increase in milk yield toward the end of heat. But since many factors other than estrus can affect milk yield on a specific day, this is not a reliable indicator of estrus.

Metestrous Bleeding

Some cows and most heifers have a bloody mucus discharge one to three days after estrus, but onset of this symptom, called metestrous bleeding, is quite variable. High estrogen levels during estrus cause blood to leak from vessels near the surface of the uterus. This discharge indicates that the cow was in heat and does not mean that she failed to conceive. However, such animals should be watched closely for a return to heat in 18 or 19 days.