Q How do cows make milk?

— Piper Hirsch, Madison, Wis.

A Laura Hernandez, assistant professor of lactation biology at the University of Wisconsin-Madison:

Well, the first thing that has to happen is that the cow has to get pregnant. She has to be able to have a baby before she can make milk.

Once the cow gets pregnant, there are these things called hormones, specifically estrogen and progesterone, that increase as the cow is pregnant. And 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 they grow a lot. By the time the cow finishes her pregnancy, which is about nine months like in a human, her cells are completely ready to go, and they’ve been primed to make lots and lots of milk.

So once the cow gives birth, she has her baby calf, and then she is actively able to secrete the milk out of her gland.

The first time you milk a cow, she makes what’s called colostrum. We call that liquid gold because it has lots and lots of fat and proteins.

After the first five days, the milk becomes what we call mature milk, and that’s when we can take her milk and put it into storage so that it can go off and be pasteurized and homogenized. And then it can go out onto your shelf, or it can be used to make cheese or yogurt or butter.

So for the first five days, we give some to the calf, and we dump the rest out because it’s changing in its composition because the hormones are changing in the cow. And then when she gets to that point, she starts making milk that has lots of good amounts of protein, fat, lots of minerals like calcium and phosphorus that are all really important initially in evolution for the baby to grow. But that is making a good, complete, nutritious food that we can go out and buy.

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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