The primary and only sure sign of heat is standing to be mounted by another animal. The importance of this standing behavior is that it is related to the time of ovulation. Reduced milk production is not a reliable indicator of estrus. Many factors other than estrus can affect milk production. In some cases milk production may actually increase during heat.
Secondary signs vary in duration and intensity.
These signs may occur before, during, or after standing heat and are not related to the time of ovulation. Dairymen should use these signs as clues to watch the specific cow more closely for standing behavior. One may want to test the cows exhibiting secondary signs, by isolating them with an aggressive cow or a surgically-altered bull to determine if these cows will stand to be mounted. Some cows and many heifers will have a bloody discharge 2 to 3 days after heat. This is termed metestrus bleeding.