乳牛（也称奶牛）是专门培养出来产牛奶的母牛。

一般未曾生育的母牛不产牛奶，更加不会长时间产出人类借以饮用的鲜奶，母牛只有在生产了幼牛后才会产奶。一般一头母牛应该在两岁时产第一头幼牛，随后约每年产一头幼牛。有时两头幼牛之间的间隔也会长一些，但这对产奶不利。

今天大多数母牛是通过人工授精的方法受精的，其优点在于农民不必饲养公牛，而且能够保证生产出来的小牛品质良好。这样对此感兴趣的农民也可以学习繁殖技术。此外人工授精时传播传染病的可能性比自然授精要小。

Dairy cattle (also called dairy cows or milk cows) are cattle cows bred for the ability to produce large quantities of milk, from which dairy products are made. Dairy cows generally are of the species Bos taurus.

Historically, there was little distinction between dairy cattle and beef cattle, with the same stock often being used for both meat and milk production. Today, the bovine industry is more specialized and most dairy cattle have been bred to produce large volumes of milk. The United States dairy herd produced 83.9 billion kg (185 billion lbs) of milk in 2007, up from 52.6 billion kg (116 billion lbs) in 1950, yet there were only about 9 million cows on U.S. dairy farms—about 13 million fewer than there were in 1950.