Standing estrus, also referred to as heat, is a brief period within a female’s reproductive (estrous) cycle during which she will allow herself to be mounted. In cattle, the estrous cycle is approximately 21 days in length but can range from 17 to 24 days in length. The period of standing estrus, however, is extremely short, lasting only about 16 hours in beef cattle, although this too is variable and can range anywhere from 6 to 24 hours.
Inaccurate detection of standing estrus in breeding females is often the culprit behind less successful artificial insemination programs. Heat-detection aids can significantly improve the likelihood of identifying females in standing estrus.
Females not in standing estrus near the time of insemination are unlikely to conceive. Cows ovulate 24 to 32 hours after the onset of estrus, and the fertile lifespan of the oocyte (egg) after ovulation is approximately 8 hours. Bull spermatozoa can survive in the female tract for 20 to 24 hours, but they are not capable of fertilizing an oocyte until they have spent approximately 8 hours in the female tract. These various timepoints are why detection of standing estrus is critical to the success of artificial insemination and embryo transfer programs. Should a female be bred too early, the spermatozoa will have died before ovulation occurred. If a female is bred too late, either the oocyte is no longer capable of becoming fertilized or the spermatozoa were not able to undergo the changes necessary for oocyte penetration before the oocyte degenerated. Knowing when a female has entered into her period of standing estrus allows the proper timing of insemination so that these timepoints overlap and increase the likelihood fertilization (Figure 1).
Figure 1. Proper timing of insemination.
Visual observation is the most common method of heat detection. The only definitive sign that a heifer or cow is sexually receptive and at the optimal time for insemination is standing still while another animal attempts to mount her. Beef females are mounted an average of 50 times per estrus period, whereas dairy females are mounted an average of 14 times per estrus period. With each mounting event lasting eight seconds at most, beef and dairy females in estrus can only be observed standing to be mounted for 6.5 and 1.9 minutes per estrus period, respectively. For this reason, it is extremely important that cows be monitored as frequently as possible for anywhere from 30 minutes to 2 hours per observation period depending on herd number and land expanse. In fact, when observed for estrus four times per day (6 a.m., 12 p.m., 6 p.m. and 12 a.m.), nearly 20% more females were detected in estrus than when observed only two times per day (at 6 a.m. and 6 p.m.). Females are also more likely to exhibit estrus behaviors during periods of darkness, so if heat detection can only be performed twice per day it is recommended to do so early in the morning and as late in the evening as is practical.
Fortunately, the onset of standing estrus occurs gradually, coincident with an increased concentration of estradiol. Various behaviors and other physical changes can be observed that indicate that a female is coming into standing heat and should be monitored closely for standing estrus. These behaviors include an increase in vocalization and restlessness, a decrease in appetite, smelling the vulva of other cows, resting her chin on another cow’s rump and attempting to mount other cows. However, she will not allow herself to be mounted. The increase in estradiol also induces changes to the reproductive tract, some of which can be visualized. These changes include a reddened, moist and slightly swollen vulva (Figure 2), as well as a clear mucus discharge, either present on the vulva (Figure 3) or smeared onto her tail or flank. There are also several physical indications that mounting behavior may have taken place. These include hair on the tailhead being roughed up (standing up instead of lying flat) (Figure 4), patches of missing hair on the tailhead or flanks that have been rubbed off (Figure 5) and, if pastures are a little muddy, you may notice mud on both the left and right of an animal’s flanks (Figure 6). Unfortunately, some of these behaviors and physical indicators can also be observed in females that have just finished their period of standing estrus (Figure 7). All females exhibiting secondary signs of estrus should be closely monitored over the following 48 hours for standing estrus. A record should be made of any female that did not exhibit standing estrus within that 48-hour period, and those females should be closely monitored for their subsequent heat beginning 17 days from their recorded secondary signs.
Figure 2. Swollen vulva.
Figure 3. Clear vaginal mucus.
Figure 4. A comparison of (a) smooth hair on a tailhead and (b) hair on a tailhead that has been roughed up, an indication that she has been mounted.
Figure 5. Missing hair on flanks.
Figure 6. Mud on flanks.
Figure 7. Timeline of expression of secondary signs of estrus.
For many producers, performing heat detection more than twice daily is not feasible. There are several estrus detection aids on the market that can be used in conjunction with visual observations to increase the number of females detected in estrus. These aids create (or remove) a mark on an animal only when a standing mount has been performed. This mark can be visualized at the subsequent heat check and identifies animals that exhibited standing estrus between observation periods. These heat detection aids vary in their method of application, method and ease of detection, and cost.
Marker animals, also called teaser or gomer bulls, are animals that have undergone an alteration to prevent pregnancy (Figure 9). These animals are extremely effective at detecting standing estrus and can be especially useful as they are more adept at detecting females that exhibit short or weakened heats or when very few females are in estrus concurrently, compared to other heat detection aids. Bulls selected to be altered should have a mild temperament, be of moderate adult size and possess a strong libido. Additionally, the selection of a virgin bull decreases the risk of venereal disease transmission. Alteration should be performed several months prior to the start of breeding season to allow for an adequate recovery period. Marker animals are usually equipped with a chin-ball marker, which leaves marks on the back and rump of mounted females (Figure 8).
Figure 8. Teaser bull marking a female with a chinball marker (image used with permission from Teaserbull Company: teaserbulls.co.nz).
Mounting-activity detectors are applied to the female’s rump and undergo a visual change when mounting occurs. There are several commercially available aids that differ in application, change upon activation, cost and accuracy of detection (Figure 10). Most of these devices require minimal time to apply, and, with the exception of the chinball marker, do not require a marker animal. It is worth mentioning that a few of these aids require more skill in interpretation than others. The electronic monitoring systems are the only system that can entirely replace visual observations.
In order to maximize detection, cows must be easily identified and visualized. Animal identification (ear tag or brand) should be legible, and observers should be able to get close enough to accurately interpret the visual aid or have binoculars. There should be adequate space to allow for cow-cow interaction. Crowded areas cause a reduction in estrus behaviors, so checking for heat in confined spaces, such as a holding pen, is not recommended. Presence of feed can also minimize the expression of estrus behaviors, so it is not recommended to perform estrus detection at feed bunks.
A large group of females should be housed together (not crowded) for maximal estrus detection. Mounting activity increases when multiple females are in heat simultaneously, which increases the accuracy of estrus detection. Additionally, it is important that your females are structurally sound. Lame animals will not mount or stand to be mounted due to the discomfort.
Observers should be trained and not pressed for time. Animal identification, all behaviors witnessed, and time of observation should be recorded. These records can be used to predict future heats. This prediction allows observers to know when to begin paying closer attention to females near their next estrus period. Females that do not enter back into estrus are good candidates for pregnancy tests.
Labor toward estrus detection can also be reduced with the implementation of a synchronization protocol. Synchronization protocols can either concentrate estrus detection into a period of five to seven days or remove the need for estrus detection entirely.
Accurate detection of estrus is critical to a successful breeding season. Visual observation of estrus behavior is the most effective method of heat detection, and the more frequent the observations the better. However, it is often not feasible to observe the herd more than twice daily and, in those instances, there are a variety of commercially available heat detection aids that can be used to identify animals that stood to be mounted in between visual observations.
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Morgan, G. L. and L. J. Dawson. 2008. Development of teaser bulls under field conditions. Vet. Clin. Food Anim. 24:443-453.
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