Cows mate by insemination from bulls during their monthly estrus, which lasts from four to 30 hours. Cows only accept bulls during this time. A cow's visible signs of heat include redness and swelling of the vulva, restlessness, and the riding of other cattle, both male and female.
Depending on their breed and rate of growth, cows typically reach puberty at eight to 18 months of age. A cow's estrous cycle lasts an average of 21 days, with ovulation occurring about 12 hours after the end of estrus. Successful mating usually occurs from insemination in the last half of the estrus period.