Most herds have one bull and many females. Only females can make milk. They make it to feed their babies, which are called calves. Cows make more milk than their calves need. Dairy farmers collect this extra milk and sell it.
Most people simply call all types of cows 'cows'.
Farmers, however, have different names for male and female cows of different ages. A young cow is called a calf until it is about nine months old. A male is then called a bull, and a female is called a heifer. After a heifer has her first calf. This book, one of a series of four titles describing various types of farming, takes the young reader on a tour of a dairy farm.