Estrus is monitored in dairy cattle so that artificial insemination (A.I.) is performed at the appropriate time relative to ovulation. The emphasis that has been historically placed on estrous detection has diminished because of the development of systems for timed A.I.

Timed A.I. evolved as a method for inseminating dairy cows because it reduced the need for estrous detection. Timed A.I. also became a necessity perhaps because modern high-producing cows are more difficult to detect in estrus. Consolidation within the dairy industry certainly played a role as well. Dairy cows in large confinement operations (more than 1,000 cows) may be observed less on an individual basis than their counterparts in smaller traditional dairies (60 to 100 cows).

Timed A.I. is effectively used for first insemination and can be used for second and additional inseminations as well. Nonetheless, many dairy cows are inseminated based on signs of estrus, even when placed on timed A.I. programs. There is a need, therefore, to understand the basic biology of estrus and methods for its detection.

Hormonal control of estrous behavior and ovulation
Reviews of estrous behavior in cattle state that the behavior originates from the hypothalamus. The hypothalamus is a region at the base of the brain that controls the luteinizing hormone (LH) surge and a variety of other pituitary hormones. Regression of the corpus luteum and development of the preovulatory follicle lead to low blood progesterone and high blood estradiol concentrations, a hormonal milieu that can cause estrus.

There is general agreement that progesterone must be low for cattle to express estrus. Small amounts of circulating progesterone effectively suppress estrus and the LH surge in cattle. The increase in estradiol from the preovulatory follicle triggers behavioral centers in the hypothalamus that control estrous behavior (mounting, standing, etc.) and also endocrine centers in the hypothalamus that control the release of gonadotropin-releasing hormone (GnRH), which in turn releases LH.

The LH surge is initiated after the beginning of standing estrus (period in which the cow will be receptive to the mount from another cow). The LH surge initiates the mechanisms that lead to ovulation. The timing of estrus relative to the LH surge makes sense because the LH surge causes follicular cell differentiation and a decrease in blood estradiol. If the LH surge occurred before estrus then estradiol from the follicle might decrease before the expression of estrus.

Cows continue to express estrus after the LH surge and are in estrus for a period of 30 minutes to more than 30 hours. Ovulation occurs approximately 30 hours after the LH surge. The time of ovulation is more closely tied to the time of the LH surge than it is to the time of estrous behavior.

Estrous behavior
Estrus has been traditionally described as an all or none response. This means that once blood estradiol reaches a critical threshold then the cow expresses estrus. Additional estradiol in the system will not necessarily change or amplify the behavior. The threshold level of estradiol is different for individual cows. Behavior of individual cows in estrus is highly variable with mounts received ranging from 1 to more than 100 and time in estrus ranging from 30 minutes to more than 30 hours. The highly variable nature of the behavior makes estrus difficult to study scientifically because the high coefficient of variation for estrous traits (number of mounts, etc.) makes it difficult to detect statistical differences when treatments are imposed.

Estrus involves a variety of behaviors that are expressed by the individual cow either alone or when interacting with other cows in the herd. Cows that are approaching estrus (prereceptive phase), in estrus (receptive phase, standing to be mounted) and going out of estrus (postreceptive phase) cluster together and form sexually active groups (SAGS). The classical and definitive behavior of the cow in estrus is standing still when mounted from behind by another cow. A variety of other behaviors precede this behavior including mounting other cows, licking and being licked, chin pressing and having chin pressed upon, and sniffing and being sniffed in the urogenital area.

Cows are most active at the beginning of estrus. Mounting and standing activity decreases in a near-linear fashion thereafter. Cows in estrus are more active because they are restless (walking more) and interact physically with other cows.

Factors that affect estrous behavior
There are a large number of housing and environmental factors that can affect the expression of estrus. Most notable among these include housing. Classic studies demonstrated that cows prefer a dirt surface over a concrete floor for the expression of estrus. Sore feet and legs diminish activity and estrous expression. The number of mounts received by lame cows was decreased by 70 percent in one study.

Other important factors include stress-induced hormone release (inhibitory to estrous expression) and environmental temperature (high environmental temperatures are inhibitory to estrous expression). A variety of other inhibitory factors have been suggested including rain and snow storms, dominant cows, poor conditions (deep mud and low ceilings), crowding and feeding.

Conditions that distract or frighten cattle or that inhibit normal social interactions will decrease estrous expression. It goes without saying that a cow must be standing up to show the standing behavior. This may explain why estrus is sometimes detected when cows are moved to the milking parlor (a time when cows lying in freestalls are forced to stand up and interact with other cows).

Non-behavioral signs of estrus
Estrus is initiated by hormonal changes in the cow. These hormonal changes trigger estrus and also impinge upon the reproductive tract and the biology of the animal as a whole. Increasing concentrations of estradiol during the prereceptive phase lead to reddening and swelling of the vulva and changes in the amount and consistency of vaginal mucous.

Cows in estrus may have a decrease in milk yield on the day of estrus (perhaps caused by reduced feed intake and greater activity) and an increase in body and milk temperature. Hormonal changes for cows in estrus include low blood progesterone that can be detected in milk samples.

Considerations for monitoring estrus and ovulation in dairy cows
The scale of the dairy industry is changing in the United States. The old recommendation for twice-daily sessions of estrous detection is certainly outdated. Most of the available literature suggests that the modern high-producing dairy cow is in estrus for less than eight hours with about one to two standing events per hour. Thrice or four-times daily sessions of estrous detection would, therefore, be necessary to visually detect cows in estrus.

Large farms have large pens for holding cattle. One person cannot observe an entire pen at the same time. The inherent difficulties in detecting cows in estrus visually in large herds have pushed the industry in one of two directions.

The first direction is toward the intensive use of timed A.I. The second direction is toward methods of estrous detection that work for large herds. The latter direction is served by either simple and efficient methods with potentially high error rates (once-daily tail chalking and insemination based on rubbed tailheads) or automated systems of estrous detection that are more expensive, but may be more accurate.

Systems for estrous detection
The ideal system for estrous detection would include:

1. continuous surveillance of the cow
2. accurate and automatic cow identification
3. be operational for the lifetime of the cow
4. require minimal labor
5. be accurate

A variety of systems are commented upon (below) with respect to these requirements.

Simple systems to identify cows in standing estrus
Tail chalk, tail paint, Kamar heatmount detectors, estrus alerts, and other pressure-sensitive non-electronic devices all work on the same principle. Cows that stand when mounted by another cow receive pressure and rubbing on the tailhead. Removal of the marking (chalk or paint) or activation of the device is an indication of estrus. Whereas these devices were traditionally viewed as estrous detection aides (i.e., supplementing visual observation), they can replace visual observation in some systems.

These devices allow for continuous surveillance and require minimal labor. They are not operational for the lifetime of the cow and their accuracy may be low if used alone (no additional information on the cow). A camera and software system has been developed to read a non-electronic device mounted on the tailhead.

Electronic systems that identify cows in standing estrus
The HeatWatch system and related systems, that electronically record the number and time of standing events, have been in use in the dairy industry for several years. These systems go beyond the simpler systems listed above because the number and time of mounts is known.

These systems allow for continuous surveillance of the cow, can accurately identify the cow in estrus (ID transmitted along with the mount data) and are accurate. The devices that are applied to cows must be monitored and managed to ensure that they remain in place. Devices that do not transmit to a central computer must be examined daily to determine which cows are in estrus. The most commonly cited limitations of these devices are their expense and the labor required to manage the system.

Electronic systems that measure activity
Activity monitors (either pedometers or neck chain monitors) can be used to detect estrus on dairy farms. A threshold is set and cows that have activity above the threshold are considered in estrus. False positive rates are increased when the threshold is set too low. False negative rates are increased when the threshold is set too high. The primary issue that needs to be resolved for activity is the high number of false positives.

Activity monitors offer continuous surveillance of the cow, can accurately identify the cow in estrus, are operational for the lifetime of the cow and require minimal labor. As stated previously, the high rate of false positive is the primary detraction. Computer algorithms may resolve some of the problems in the accuracy of these systems.

Comparisons with visual observations
Two studies demonstrated that either the HeatWatch system or activity monitoring were superior to simple visual observation of cows in estrus. Both studies suggested that HeatWatch and activity were about equal in terms of efficiency of estrous detection. The efficiency of estrous detection was the greatest when data from HeatWatch, activity and visual observation were combined. An approach that integrates a variety of information on the individual cow may be the best method to predict estrus.

Biochemical measurements to predict the time of ovulation
High-producing cows may have a less-intense estrous behavior. Thus, biochemical methods of detecting approaching ovulation or detecting the LH surge would circumvent problems of estrous detection in high-producing dairy cows. Milk progesterone testing can be used to confirm estrous signs, but has little utility in terms of detecting cows in estrus when nothing else is known.

Systems to measure vaginal mucous conductivity are too labor-intensive for large farms. Changes in milk production, milk temperature or body temperature do not appear to have the sensitivity that is necessary for detecting cows in estrus.

The possibility of using a biosensor to measure milk progesterone concentrations in real-time has been proposed for a number of years, but to our knowledge this technology has not been commercialized. There are changes in estradiol in milk around estrus, but there appears to be considerable cow-to-cow variation that would need to be overcome.

The LH surge can be measured in blood, but a routine and noninvasive method to measure or predict the time of the LH surge is not available. Thus, there is a need to continue work on biochemical methods to detect an approaching ovulation in dairy cattle. The evolution of such a method would change dramatically the manner in which cows are managed reproductively on farms.

Conclusions
Estrus is a behavior that is used to time the insemination of dairy cows. The intensity of the behavior may have diminished in recent years because of high production in dairy cows. Most systems used to monitor estrus do so by measuring behavioral changes in the cow. These systems become less efficient under high production because estrous behavior is diminished. The popularity of timed A.I. may be explained by difficulties in estrous detection in high-producing cows.

Future systems that predict the time of ovulation based on biochemical changes in the cow could revolutionize cow breeding. Unfortunately these systems are technically challenging and have not evolved for on-farm use. PD

References omitted but are available upon request at editor@progressivedairy.com

—Excerpts from 2006 Dairy Cattle Reproductive Council Annual Meeting and Convention Proceedings