When estrus occurs, the cow will show an increased tendency to stand and permit herself to be mounted by other animals as hereinabove stated. When the mounted animal allows the mounting animal to remain for about five to eight seconds, a "standing heat" has occurred. The most frequent mountings occur by bulls or "teaser" animals. Accordingly, the repeated mountings of a cow by an animal of either sex and for a time interval regarding each mount of about five to eight seconds is a good indication that the frequently mounted cow is in peak estrus or in "standing heat". The normal estrous period usually lasts from about twelve to eighteen hours, with peak estrus being about six hours.
The above-identified visual inspection and observation techniques of the herd require constant observation of the herd to determine whether an animal is in heat. Such visual observation must be conducted frequently and must be accompanied by immediate identification and/or segregation of the animal in heat if it is to be artificially inseminated. Also, unless the herd is maintained under constant visual observation, estrus may go undetected in many animals that never "peak" but maintain a low but constant level of estrus for a number of hours, and such low level estrus can only be detected by a bull or "teaser" animal. Visual observation of the herd is further made difficult, since many animals are nocturnal, particularly cattle, and their increased night sexual activity goes undetected. Most experts agree that approximately 68% of estrus in cattle occur after 6:00 p.m. and before 6:00 a.m. with 43% occurring between midnight and 6:00 a.m. Therefore, unless 24-hour visual observation of the herd is maintained by dedicated personnel, heat detection is inaccurate, inefficient, and depends largely on the ability and experience of the herdsmen, which usually is lacking. If the herdsman is not skilled in observing these visual signs, he can easily miss the occurrence of heat. Further, inclement weather may deter his observation regardless of his dedication and ability. These problems make it difficult to optimize breeding of the herd annually so that the offspring are born within the same "time window", mature at approximately the same time, and are ready for market or to be bred themselves at a common time.
Various devices and systems have been used or described for improving the efficiency of the detection of estrus. A commonly used device is a marking device such as identified in U.S. Pat. No. 1,237,983 issued to Werner on Aug. 21, 1917, and entitled "Marking Device." Other such marking devices utilize a tube or capsule mounted on the tail head of the cow. The tube contains a suitably colored dye. When the cow is ridden, the tube or capsule breaks and the dye is spread over the tail head of the cow. Such devices are passive in nature and are activated by pressure from the brisket of the mounting animal. While such devices have been somewhat satisfactory, these devices still require observation by the herdsmen. Moreover, careful visual observation is necessary in order to determine if the dye mark is fresh or old. Further, the presence of the dye mark merely indicates that a mounting has taken place. The mere fact that a mounting has taken place does not provide a true indication of whether the animal is in estrus, because standing heat requires that the mounting has occurred for about five to eight seconds and mountings have generally been repeated many times. Some cows in intense heat permit themselves to be ridden up to fifty times in a twelve to eighteen hour period. Other cows permit rides only a few times and of short duration.
