A sexually mature cow that is not pregnant or diseased comes into heat every 17 to 24 days. The average heat cycle begins every 21 days. Often the reactions of other animals in the herd are used to determine if an individual cow is in heat.
When cows are ready to breed, other herd members attempt to mount them. Normally, a cow would reject these advances, but during heat, the animal stands still and allow them. Intact and castrated bulls as well as freemartin cows (infertile females) can be used to test if a cow is ready to breed. A cow in heat also attempts to mount other animals and is unusually interested in sniffing and licking their genitals. If a cow is not allowed to breed naturally or has no herd mates, owners can detect heat by watching for vaginal discharge, a swollen and red vulva, restlessness, and loss of appetite.