Chicken Feed Café: The Basics on Grit & Oyster Shell

Grit and oyster shell look alike but play different roles in chicken nutrition and health. They're each necessary, and they're not interchangeable.

by Frank Hyman
PHOTO: Kathy Shea Mormino

As with children, not everything a hen pops in her mouth is food. But unlike children, some of the nonfood items that hens snap up do keep them healthy. I’m talking about grit and crushed oyster shells. These are similar-looking products that play very different roles and aren’t interchangeable.

Nitty Gritty

When we joke about something being “as rare as hen’s teeth,” it’s because chickens have no teeth. They don’t chew their food.

With their tongue, they shove whole leaves, bugs, grains or pellets down their throat. The food fills up their crop, which is a pouch for quickly storing food in case a chicken has to run for her life.

From there, the food bits start a 3- to 4-hour journey through the hen and to the coop bedding (or your pant leg, if you’re holding the chicken). Flowing first to the stomach, the bits get showered with enzymes that begin dissolving them. A second stomach is the gizzard, which is an organ chickens use instead of teeth. This leathery, muscular organ holds the food and small bits of stone or sand, aka grit.

The food tumbles around in the gizzard being shredded by the sharp edges of the grit, which act like our molars. It would be like tossing a bucket of gravel into your washing machine, instead of soap, if you wanted your clothes ripped to pieces instead of getting clean. Fortunately, gizzards have evolved to handle this sort of destructive, dynamic action.

With the food covered in enzymes and broken down into tiny bits by the two stomachs, it flows on through the small and large intestines where the chicken extracts most of the nutrients. Then it’s expelled out through the cloaca, which also serves as the exit for the oviduct. From there, it often falls to the ground.

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Oyster Cockefeller

It would seem reasonable to think that similar-looking crushed oyster shell could play the same role as grit. But being made of alkaline calcium, oyster shells slowly dissolve when subjected to acidic stomach juices. If exposed to the weather, acidic rain also slowly dissolves oyster shells, especially ones that have been broken down into much smaller pieces. On the other hand, grit—most often made from crushed granite—doesn’t dissolve but will eventually pass out of the gizzard and need to be replaced.

Primeval hens gathered grit from their surroundings and didn’t need calcium from oyster shell supplements either, as they produced a limited number of eggs in the spring. But hens bred to lay 200 to 300 eggs year-round need more calcium than they can find when foraging or that comes even in layer feed.

Calcium Deficiencies

A female chicken in her prime needs about four times as much calcium in her diet as a nonlaying hen, chick, pullet, cockerel or rooster. The shell of an egg, like the shell of an oyster, is made mostly of calcium carbonate.

To produce a dozen eggs takes about an ounce of calcium just for the shells. Some calcium comes from the feed, some from foraging (if allowed) and some from being fed spent eggshells. But the rest has to come from crushed oyster shells.

Without enough calcium in her diet, a hen might either lay eggs with soft shells or develop weaker bones by drawing calcium from those bones to make eggshells. Calcium is also a necessary mineral for helping the oviduct contract. On a low-calcium diet, eggs can get egg-bound in the oviduct.

With access to crushed oyster shells, hens snap up as much or as little as they need. Their appetite tells them when they need a calcium boost. Otherwise, they ignore it.

After tossing back a few bits of oyster shell, it works its way through the digestive system, as would a piece of food. It might spend some time tumbling around in the gizzard. But it won’t last long enough to replace the need for harder bits of grit. The calcium dissolves and floods the bird’s body to support new bone cells as well as an almost-daily egg.

Price Points

Over the years my hen population has included between three and six laying birds. And a 50-pound bag of oyster shell or grit seems to last for a year or two easily. At a cost of $10 or so per bag, these will be among your smallest expenses. Some places also sell it in 5-pound bags for about $1 a pound.

DIY Shells

If you have shells left over from an oyster feed, you can make your own supplement. Hose down the shells to rinse off salt, toss them in a burlap sack (called a croaker sack on the coast), and then drive over them many times with your car until the pieces are small enough. You could also wale on the bags with a sledgehammer if you missed a crossfit session.

Whence Grit & Shells?

Granite quarries are the main source of grit. Any stone not worth cutting into slabs is crushed and then screened to sizes appropriate as either gravel or poultry grit. Those bits are then bagged and shipped. Some hen-keepers do well with sand, but use larger-grained construction sand. Play sand is too small to last in the gizzard for very long. Don’t use beach sand, as it’s too salty.

Like beach sand, oyster shells also come from the coasts. Hens in their prime might like crushed oyster shells almost as much as I like oysters. Growing up on the coast, oysters were my favorite type of seafood. This was partly because they don’t run away when you go to catch ’em. Back then, the leftover oyster shells were just a waste material. It was suited for nothing better than dumping into potholes in our sandy driveways.

These days, we coasters know enough to return the spent oyster shells to the shallows. This creates stable conditions for tiny young oysters to anchor themselves, create new oyster beds and reverse the decline in oyster production.

But spent oyster shells that end up inland aren’t worth the expense of shipping back to the coast. They might not be filling potholes, but they are filling a hole in the diets of laying hens everywhere. Factories crush the oyster shells, as well as shells of clams and other bivalves. Then they bag them and ship them to your farm-supply store.


You can put grit in a dispenser that’s exposed to the elements because it won’t rot or dissolve in the rain. In my backyard inventory, I found a 3-foot length of terra cotta pipe with a small part of the bell end busted out. It looks like a kid’s smile with a couple of front baby teeth missing.

To use it, I stood it up in the corner of the hen pen on top of a plant saucer that’s inverted, so it won’t hold water. I fill the pipe with grit, which spills onto the saucer as the hens work their way through it. To keep the saucer high enough that they can’t cover it when they scratch, I put a scrap block of 6-by-6-inch lumber under it. Some bricks or rocks would work, too.

You can also make a dispenser with 3 to 4 feet of PVC pipe that’s 4 or 6 inches wide. Cut a wide, inverted “v” in the bottom with a handsaw or hacksaw. Then secure the top to the fence with some tie wire or a zip tie. Fill and chill.

To keep the crushed oyster shell accessible but out of the rain, I offer it under the coop in the area I call the moat. To keep shells from being covered when chickens scratch, put them in a low flowerpot, a cracked window box or a plastic cement-mixing tub. Heck, even an old feeding trough would work.

This story originally appeared in the November/December 2018 issue of Chickens magazine.

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