To ensure a steady supply of milk, farmers arrange for their cows to calve every 12 months. Attention to breeding has led to steady improvements in yields. A good cow may produce up to 2,000 gallons during the lactation period.
Cows are normally milked twice a day, morning and evening. Except for the smallest herds, milking in advanced countries is usually done by machines which suck the milk from the cows' udders and transfer it to covered containers. But there are still many countries where milking is largely done by hand, in the traditional way.