The purpose of estrus detection in cows is to identify cows in heat in a timely manner, determine the correct breeding time, perform insemination at the right moment, and prevent mistimed or missed matings. Practice has proven that correctly understanding the estrus and ovulation patterns of cows, and seizing the optimal insemination period, are among the most important measures to improve conception rates. Below is a detailed overview: the estrous cycle of cows and the four main methods for estrus identification.

I. Estrous Cycle of Cows

Estrus characteristics.
Once a cow reaches sexual maturity, it will periodically show a series of sexual activity phenomena known as estrus. During estrus, the reproductive tract mucosa becomes congested and swollen, sticky secretions are discharged, behavior becomes abnormally excited, and the cow may mount other cattle. Follicles in the ovaries develop and mature, and ovulation begins. In production, the stage when cows show concentrated estrus characteristics is called the estrus phase, and the interval from the beginning of one estrus to the beginning of the next is defined as one estrous cycle.

Duration of estrus.
In production, estrus duration is measured from the time a cow begins to accept mounting until she refuses mounting. Estrus duration varies depending on breed, reproductive performance, and individual differences, generally lasting from 6 to 36 hours, occasionally up to 48 hours, with an average of 18 hours. Because estrus is relatively short, close observation is essential to avoid missing the estrus period and the optimal breeding time, which would reduce conception rates.

Length of the estrous cycle.
The estrous cycle is influenced by environmental factors (temperature, light), management, nutrition, and individual conditions. It generally ranges from 17 to 25 days, with an average of 21 days. During estrus, cows show different signs, which are divided into four stages:

• Proestrus (4–7 days): preparation for estrus, vaginal congestion, increased secretion that is watery, but the cow does not accept mounting.

• Estrus (about 18 hours): peak sexual desire, decreased appetite, excitement, vulvar congestion and swelling, secretion of large amounts of transparent mucus, shiny vaginal mucosa, and willingness to accept mounting.

• Metestrus (5–7 days): signs of estrus gradually disappear, sexual desire decreases until it vanishes, secretions reduce, swelling subsides, and cows begin to refuse mounting.

• Diestrus (also called interestrus): the corpus luteum gradually regresses, follicles begin developing again, preparing for the next cycle.

II. Methods of Estrus Detection in Cows

Artificial insemination is the most widely used breeding method in cattle. To inseminate at the optimal estrus period and improve conception and reproductive rates, accurate estrus detection is essential. Cows usually ovulate at night, so understanding this helps ensure timely insemination and a higher success rate.

The commonly used methods for estrus identification include: external observation, teaser bull method, vaginal examination, and rectal examination. In practice, these methods are often combined.

1. External Observation

Estrus signs observed externally include restlessness, frequent bellowing, reduced appetite, vulvar redness and swelling, and thin vaginal secretions.

• Early estrus: cows are restless, bellow, lose appetite, vulva is red and swollen with scant mucus.

• Mid-estrus: vulva is swollen and shiny, cows accept mounting, hind legs spread. However, insemination at this stage is premature, as ovulation has not yet occurred.

• Best insemination time: about one day after mid-estrus, cows become calmer, raise their tails, show standing reflex, vulvar swelling subsides with wrinkles appearing, secretions turn from yellowish to whitish, and the cervix changes from soft to firm.

2. Teaser Bull Method

This involves using a vasectomized bull introduced into the herd during the day and separated at night. Estrus is judged based on the bull’s mounting attempts and the cow’s willingness to be mounted. If the cow shows interest in the bull and arches her back, she is likely in estrus.

3. Vaginal Examination

A vaginal speculum is used to observe changes in the vaginal mucosa, secretions, and cervix.

• Estrus cows show reddened, moist, and shiny mucosa, with the cervix congested, relaxed, and open, discharging large amounts of clear, stringy mucus.

• Early mucus is thin, becoming thicker as estrus progresses. In late estrus, mucus decreases, becomes less sticky, opaque, and may contain traces of blood.

• Non-estrus cows have pale, dry vaginal mucosa, and a tightly closed cervix with no mucus discharge.

4. Rectal Examination

Rectal palpation is useful for cows with poor nutrition or irregular ovulation, where external signs are insufficient. Follicle development can be divided into four stages:

1. Follicle appearance stage: follicles slightly enlarge (about 0.5 cm), not clearly fluctuating; cows already show estrus signs.

2. Follicle growth stage: follicles enlarge to 1–1.5 cm, spherical with obvious fluctuation.

3. Follicle maturation stage: follicles stop growing, wall becomes thin and tense, ready to rupture.

4. Ovulation stage: follicular fluid disappears, wall collapses, depression appears, and 6–8 hours after ovulation, the corpus luteum begins to form.

Summary

By combining four key indicators—estrus duration, degree of vulvar swelling, characteristics of vaginal mucus, and cervical changes—farmers can accurately identify cows in heat and determine the optimal time for insemination.

Estrus cows often show restlessness, reduced feed intake, decreased milk yield, tail raising, chasing and mounting other cows, and standing still when mounted. The clearest sign is the standing reflex (accepting mounting without moving), combined with vulvar swelling and mucus discharge.

Correct estrus detection is therefore essential for improving conception rates and ensuring efficient cattle reproduction management.