Blue Sky Science: How do cows make milk?

Piper Hirsch


How do cows make milk?

First the cow has to get pregnant, so she has to be able to have a baby before she can make milk. Once the cow gets pregnant, hormones, specifically estrogen and progesterone, begin to increase. As they increase in the blood, the cells inside the mammary gland, or the big udder that you’re used to seeing, start to grow and grow a lot. By the time the cow finishes her pregnancy, about nine months long, her cells are completely ready to go. They’ve been primed to make lots and lots of milk.

Once the cow gives birth, she is actively able to secrete the milk out of her gland or udder.

The first time you milk cows they make what’s called colostrum. We often call it liquid gold because it has lots of fat and proteins. For the first five days, we give some to the calf and dump the rest out because the hormones are changing in the cow and the composition of the milk is also changing.

After those first five days, the milk becomes what we call mature milk. This is when we can take the milk and put it into storage to be pasteurized and homogenized and made safe for us to drink. It can then go onto grocery store shelves or be used to make cheese or yogurt or butter.

The milk has good amounts of protein, fat, minerals like calcium and phosphorus, things that are all really important for the baby calf to grow, but are also part of a complete, nutritious food that we can go out and buy.