How old does a cow or heifer have to be until they their periods?

Cows and heifers do not have periods. They experience estrus cycles, also known as heat cycles. The estrus cycle is the reproductive cycle of female cattle, which includes periods of fertility and infertility. On average, a heifer will reach puberty and start showing signs of estrus between 6 and 18 months of age, depending on the breed and individual heifer. The duration of the estrus cycle in cattle is approximately 21 days. During this period, the cow or heifer will display behavioural, physical, and hormonal changes indicating that they are ready to mate.