CHAPTER 36 Estrus Detection

Faulty estrus detection is the most important problem in reproductive management confronting the herd manager and the veterinarian. For example, Barr ¹ determined that the correlation between days lost because of missed heats and total days open was 0.92. Research using computerized simulation analysis clearly illustrated the economic advantage to improving the efficiency of estrus detection in dairy herds.² In this study, moderate improvement in the rate of estrus detection from 40% to 50%, coupled with a conception rate of 50%, increased income by $34 per cow per year. Further improvement to 60% estrus detection returned a total of $56 per cow per year. Although the costs to achieve such improvements were not considered in the analysis, the expenses associated with additional labor to more intense estrus detection, possible use of estrus detection aids, or an estrus synchronization program should result in a favorable net return per dollar invested. Earlier research reported a 4 to 1 return on investment to improve the rate of detection from 35% to 55%.³ Failure to detect estrus or inaccurate heat detection results in an estimated annual loss of over $300 million to the dairy industry in the United States.⁴ Poor estrus detection can be costly for a beef cattle cow-calf unit using artificial insemination for several reasons: (1) more conceptions occur late in the breeding season, reducing the potential value of calves because they are born late in the calving season; (2) cows bred late are likely to be bred late in subsequent years; and (3) more cows probably will be culled because they are not pregnant.

This chapter describes the characteristics of estrus, factors affecting estrous behavior, and methods to enhance estrus detection to include detection aids.

ESTROUS BEHAVIOR

Standing to be mounted is the most definitive sign of estrus in cows. During the period of standing heat, cows stand to be mounted by other cows. Cows that move away quickly when a mount is attempted are not in estrus. The average duration of a mount is approximately 2.5 seconds.⁵ Considerable variation has been reported in the number of mounts received by an estrous cow. Using electronic pressure-sensitive mount detectors, Dransfield and co-workers determined the average (±SD) number of standing events for Holstein cows to be 8.5 ±6.6 per cow.⁶ The average duration of standing estrus, defined as the time from first mount to last standing event, ranged from 33 minutes to 35.8 hours, and the overall average for cows monitored in 17 commercial herds was 7.1 ±5.4 hours. Standing behavior is critical because the onset of standing heat (estrus) is closely related to the time of ovulation. During the onset of estrus, estradiol (E₂) causes release of luteinizing hormone, initiating the process of ovulation. Using ultrasound examination to determine time of ovulation in relation to the onset of estrus based on electronic detection of estrus, the average interval from onset of estrus to ovulation was 27.6 ±5.4 hours.⁷

Secondary signs of estrus include attempting to mount other cows, mucous discharge from the vulva, swelling and reddening of the vulva, bellowing, restlessness, trailing other cows, chin resting, sniffing the genitalia of the other cows, and lip curling. These signs may occur before, during, or after estrus and are not related to the time of ovulation. Such signs should be used as clues that cows are near estrus so that they can be watched more intensely for standing behavior. Figure 36-1 illustrates the relative changes in standing behavior and certain secondary signs before, during, and after estrus.

Fig. 36-1 Relative changes in standing behavior and secondary signs before, during, and after estrus.

Estrus expression is maximized when the female has been previously exposed to progesterone (P₄), followed by a decrease in P₄ and an increase in E₂. The effect of E₂ for initiating behavioral signs of estrus appears to be “all-or-none.” Once the threshold concentration of E₂ to induce estrus is achieved, additional E₂ does not further enhance the behavioral intensity of estrus. Furthermore, the inhibitory effect of high concentrations of P₄ on estrous behavior also is all-or-none.⁸ Although E₂ and P₄ are considered the primary hormones involved in estrous behavior, the response of hypothalamic neurotransmitters to these steroids probably is the final signal controlling estrous behavior.

Estrus without subsequent ovulation and formation of a corpus luteum (CL) is estimated to occur in 60% of all peripubertal heifers. This is termed nonpubertal estrus and occurs early during the onset of puberty. By contrast, the condition of ovulation without estrus occurs toward the end of the postpartum anestrous period. This is termed silent heat, or silent ovulation, and generally occurs at the first postpartum ovulation in lactating cows. The incidence among dairy cows ranges from 50% to 94%.⁸ The physiologic mechanisms responsible for this syndrome have not been resolved. One theory is that high concentration of estrogens present during late gestation and parturition induce a refractory state to the high concentrations of E₂ present at the first postpartum ovulation. Consequently, estrus is not exhibited. P₄ released from the initial CL formed after the silent ovulation, however, overcomes the refractory state, allowing estrus to be expressed during the next ovulatory cycle.⁸ A second possible explanation for the poor expression of estrus or silent ovulation is the low ratio of E₂ to P₄ concentrations. Vailes and co-workers showed that the estrogen-to-progesterone ratio caused a small but significant variation in mounting activity.⁹ Behavioral traits that require active involvement of the cow (mounting and chin resting) do not appear to be suppressed by P₄ as much as receptive behaviors (mounts received, standing when mounted) are. Standing behavior, therefore, appears to be the behavior most sensitive to inhibition by P₄.¹⁰

Occasionally, pregnant cows exhibit signs of estrus. It is most frequently observed during middle to late gestation. Cows with ovarian follicular cysts have hormonal relationships similar to those in cows in estrus and may express estrus; however, most cows with ovarian cysts are anestrus. Some cows and most heifers have a bloody mucous discharge 1 to 3 days after estrus, but onset of such metestrus bleeding is quite variable. This discharge indicates that the cow was in estrus and does not mean that she failed to conceive.

EVALUATION OF ESTRUS DETECTION

It is important to periodically compare estrus detection efficiency with specific, realistic management goals. Complete and accurate records, including all dates of estrus, services, and pregnancy status information, are needed to calculate estrus detection efficiency. Most dairy record processing centers and herd management computer programs provide an estrus detection index. Other reproductive parameters, such as average days to first service and distribution of interestrual intervals, are useful in evaluating estrus detection. Most calculations of estrus detection efficiency or heat detection rate are based on the voluntary waiting period (VWP), which is the number of days after calving before which a dairy producer will not inseminate a cow, even if she is detected in heat.

Estrus detection efficiency is defined as the percentage of possible estrous periods that are observed during a specified period of time. Several methods have been developed to express the percentage of cows detected in estrus relative to the number of cows assumed or calculated to be in estrus. A review of this topic with equations and tables for determining the efficiency of estrus detection is available.¹¹ Computation of estrus detection efficiency and comparison among the various methods often are difficult because of differences in the time intervals used and in whether pregnant and open cows are included, and because of certain assumptions that may or may not be applicable. These assumptions include the following: (1) all cows are cycling by a certain day post partum; (2) reported estruses are in fact true estrous periods; (3) values used in the equations are accurate; and (4) the duration of all estrous cycles is 21 days, which is not the case with use of prostaglandin F_2α (PGF_2α). The frequency of the use of PGF_2α and other synchronization programs to induce estrus and ovulation should be considered in evaluating any index of estrus detection efficiency.

Characteristics of Herds with Inefficient Estrus Detection

Using summary data from DHIA records or computer management programs or information from herd records such as days to first service and results from veterinary examinations, one can determine if the following characteristics of inefficient estrus detection exist.

• Very few estruses observed or recorded before first service

• Prolonged parturition to first service interval (which should be less than 21 days beyond the VWP)

• Excessive interval between services, especially when many intervals are multiples of a normal cycle (e.g., 42 days)

• Average interestrual interval greater than 35 days

• Greater than 25% of the cows routinely confirmed open at pregnancy examination 35 to 45 days after insemination

• Veterinary examinations confirming cows are cycling normally but estrous periods are not routinely detected

• Herd management computer programs or DHIA data routinely indicating heat detection rate less than 65%

Determining the Accuracy of Estrus Detection

Accuracy is defined as the percentage of estrous periods observed that are indeed true estrus. Frequently, management of herds with suboptimal reproductive performance is characterized by both inefficient and inaccurate estrus detection. Inaccuracies occur when cattle are inseminated at times other than true estrus. Examining the frequency distribution of interestrual intervals has been shown to be helpful in documenting estrus detection errors. Cows determined not to be in or near estrus at their first postpartum insemination on the basis of progesterone levels had abnormal intervals to the second service.¹² Twenty-nine percent and 20% of the intervals were between 11 and 14 days and 30 and 31 days after first service, respectively. Thus, cows incorrectly determined to be in estrus were in the midluteal phase when initially inseminated. Factors that contribute to errors in estrus detection include too much reliance on secondary signs instead of standing behavior to determine estrus status, unfamiliarity with correct signs of estrus by herd personnel, poor cow identification resulting in selection of cows to be inseminated that are not in estrus, and incorrect use or interpretation of estrus detection aids. The impact of all of these factors is magnified as herd size increases.

In addition to evaluating the distribution of interestrual intervals, milk progesterone testing can be a method of evaluating the accuracy of estrus detection. To document the variation in error rates of estrus detection among selected Northeast dairy herds, milk samples were collected on the day of insemination and analyzed for progesterone using radioimmunoassay. Cows with progesterone concentrations greater than 1 ng/mL at the time of insemination were considered not to be in or near estrus. The proportion of herds with various rates of estrus detection errors is presented in Table 36-1. Although overall only 5.1% of the cows were not in or near estrus when inseminated, the percentage of cows classified as not in estrus ranged from 0% to 60% among herds. Greater than 30% of the herds had error rates in excess of 10%.¹² Sturman and co-workers, using thrice-weekly milk progesterone analysis, determined that approximately 19% of inseminations were made when cows were in midcycle or in early pregnancy.¹³

Table 36-1 Error Rates for Estrus Detection in Cows Based on Milk Progesterone Concentration ^*

% of Cows within Herd with 1ng/mL Progesterone Level at Insemination	No. of Herds within Each Category	Error Rate^†
0	283	60.6%
<10	40	8.8%
10–19	102	21.8%
20	42	9.0%

* Data from Northeast dairy herds in which 5.1% of cows overall subsequently were found not to be in estrus.

† Percentage of herds classified as nonestrous.

Modified from Reimers TJ, Smith RD, Newman SK: Management factors affecting reproductive performance of dairy cows in the northeastern United States. J Dairy Sci 1985;68:963.

FACTORS AFFECTING ESTROUS BEHAVIOR

Type of Housing

Any housing arrangement that allows cattle to interact throughout the day provides more opportunity for expression of mounting and standing behavior. In fact, tie stalls and stanchion barn housing are not appropriate for optimizing estrus detection unless cows are released on a regular basis for observation of estrous behavior. If cattle are not allowed to interact, the herd manager must use secondary signs to guess which cows are in estrus.

Using milk progesterone analysis by radioimmunoassay to determine the percentage of cows that probably were not in estrus at the time of insemination, researchers found a slight but significant increase in detection errors in free-stall herds, compared with herds with conventional housing.¹² In addition, the proportion of cows determined to be in or near estrus when inseminated and conception rates were not related to herd size. Although opportunity to observe estrus is greater when cows interact frequently in free-stall barns or loose housing, taking time to observe estrous behavior is essential no matter what type of housing in used.

Footing Surface

The composition of the footing surface where cows interact significantly affects the intensity of estrous behavior. Research conducted in North Carolina compared estrous activity in high-producing Holstein cows that were observed for estrus during 30 minutes on dirt and 30 minutes on grooved concrete. A summary of mounting and standing behavior from this study is presented in Table 36-2. Behavioral expression of estrus lasted longer for cows observed on dirt. Mounting and standing behavior were nearly doubled when cows were observed for estrus on dirt compared with concrete.¹⁴

Table 36-2 Estrual Activities on Dirt and on Concrete ^*

	MEAN NO. OF EVENTS
Estrus Factor	Dirt	Concrete
Duration of estrus (h)	13.8	9.4
Activity during entire period of estrus
Mounts	7.0	3.2
Stands	6.3	2.9
Activity during 30-min observation
Mounts	3.7	2.5
Stands	3.8	2.7