Stage 3 — from the move outside, onward
This is the most common mix-up in the whole category. One grinds and does not dissolve; the other dissolves and is calcium.
By Scooter M.·Published September 1, 2026
The short answer
Grit is insoluble crushed granite that stays in the gizzard and grinds food. Oyster shell is a soluble calcium supplement at 33 to 38 percent calcium for laying hens forming shells. They do different jobs, they are needed at different times, and both go in separate free-choice dishes rather than mixed into feed.
A chicken has no teeth. Food goes to the gizzard, a muscular organ that grinds it against small stones the bird has swallowed. Without those stones the gizzard cannot process anything that is not already ground.
Manna Pro publishes its poultry grit as insoluble crushed granite, and insoluble is the operative word. It stays in the gizzard and does its job for weeks before wearing down. Oyster shell dissolves in acid within hours and grinds nothing.
When grit is needed: the moment a bird eats anything that is not complete crumble or pellets. Scratch, treats, kitchen scraps, grass, insects — all of it needs grinding. Birds ranging over stony ground find their own; confined birds on complete feed plus treats do not.
Manna Pro publishes 33 to 38 percent calcium, from oyster shell and coral calcium, in a coarse crushed form. A laying hen forms a shell overnight and needs a supply of calcium available while she does it. The coarse form matters: it sits in the gizzard and dissolves slowly, releasing calcium over hours rather than all at once.
When oyster shell is needed: from the first egg, and not before. A growing pullet has no use for it and the excess is a burden on her kidneys.
| Grit | Oyster shell | |
|---|---|---|
| What it is | Insoluble crushed granite | Crushed oyster shell and coral calcium |
| Published composition | Insoluble granite (Manna Pro) | 33.00% to 38.00% calcium (Manna Pro) |
| What it does | Grinds food in the gizzard | Supplies calcium for eggshells |
| Dissolves? | No | Yes |
| Needed by | Any bird eating anything but complete crumble | Laying hens only |
| When | From the first treat onward | From the first egg onward |
| How offered | Free-choice, separate dish | Free-choice, separate dish |
| Mixed into feed? | Never | Never |
Both products go in their own small dish where the birds can help themselves. There are two reasons and they are different for each.
Grit: birds regulate their own intake and take more when they are eating more coarse material. Mixing it into feed means they eat it in proportion to feed rather than to need, and it displaces nutrition.
Oyster shell: this is the important one. In any flock some birds are laying and some are not — pullets coming up to lay, hens in moult, a rooster. Free-choice means the laying birds take calcium and the others leave it alone. Mixed into feed, every bird gets it whether they can use it or not.
A small amount, topped up when it goes down. These are supplements consumed in grams, not pounds; a bag of either lasts a backyard flock a very long time, which is why the sensible purchase is the smallest bag you can find rather than the best value per pound.
Both keep indefinitely if dry. Both turn into an unusable lump if they get wet, so a covered dish or a spot under shelter is worth arranging — the same storage discipline as feed, scaled down.
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Grit and oyster shell go in separate free-choice dishes, never mixed into feed, and both come in bags that outlast a small flock by a long way.
Manna Pro
Insoluble crushed granite
Insoluble crushed granite, which is the actual definition of grit and the reason it is not oyster shell — a bird with no access to stones cannot digest whole grain, scratch or grass.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Manna Pro
33-38% calcium
Thirty-three to thirty-eight percent calcium in a crushed form, offered free-choice — the supplement that fixes thin shells and the one most first-time keepers confuse with grit.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Gamma2
20-25 lb, 12 x 12 x 9.75 in
The small version, and the honest use for it is not feed at all — it is the second and third container you need once you are also storing grit, oyster shell and scratch.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
No. Grit is insoluble crushed granite that stays in the gizzard and grinds food. Oyster shell is a soluble calcium supplement at 33 to 38 percent calcium, for laying hens forming eggshells. Oyster shell dissolves within hours and grinds nothing, so it cannot substitute for grit.
Any bird eating anything other than complete crumble or pellets does, because whole grain, treats, grass and scraps all have to be ground in the gizzard against swallowed stones. Birds ranging over stony ground find their own; confined birds getting treats need it offered free-choice in a separate dish.
From the first egg, and not before. A growing pullet cannot use the calcium and the excess is a burden on her kidneys. Offer it free-choice in a separate dish rather than mixing it into feed, so the birds that are laying take it and the ones that are not leave it alone.
Yes — dried, crushed eggshell is calcium carbonate and hens will take it. Crush it thoroughly so it does not resemble an egg, because a hen that makes the connection can start eating eggs in the nest and that habit is close to incurable. Dry it properly before offering it.
Every figure cited on this page came from one of these documents. If one of them has changed since we read it, the number here is out of date and we would like to know.
Feeders, waterers, feed storage and supplements, with published capacities, birds-per-nipple figures and winter power draw.
Starter, grower, layer, all-flock and scratch — what the guaranteed analysis says and why calcium is the dividing line.
The 18-to-22-week range, the physical signs that precede the first egg, and what delays it.
Sealed feed containers on published capacity, seal mechanism and material — the half of the rodent answer people skip.
Nest boxes on published dimensions, with the one-per-four-to-five-birds ratio that decides how many you need.