Despite the fact that heifers are more fertile and have a longer estrous period than cows, everything changes when summer heat rolls in. During the summer, the number of standing events during estrus drops from an average of 16.9 to 5.7 for Holstein heifers, says Carlos Risco, professor of food animal reproduction and medicine at the University of Florida. That makes it more difficult to detect heats visually and increases the need for detection aids or synchronization programs in order to get heifers bred in a timely fashion.