The maiden heifers will calve when they are around 30 months old and then give milk for about about 300 days. They then have a rest for 60 days before calving again. Cows – like wild horses, deer, wildebeest etc. – have evolved to have one pregnancy a year. Traditionally, they would have their young in the spring, when the grass is most abundant, and they would be able to produce lots of milk to give the calves the best start in life.