I stood in front of the feed store wall for 20 minutes my first time: layer feed, grower feed, scratch, grit, oyster shell, crushed eggshell. I bought all of it. I needed about half of it. Do chickens need grit and oyster shell with layer feed? The honest answer is one yes, one no, and one it depends, and the store will happily sell you the wrong combination.
What layer feed already covers
Layer feed is a complete ration: around 16% protein and roughly 3.5 to 4% calcium, ground fine enough that the gizzard handles it without help. A hen eating nothing but layer feed needs neither supplement. The calcium in the bag is calibrated to the roughly 4 grams a day a laying hen uses for shells, and the fine grind means there is nothing to grind. Supplements exist for everything the birds eat besides the feed.
Grit: the gizzard's teeth
Chickens have no teeth. The gizzard, a muscular stomach, grinds food with the help of small hard stones the bird swallows. That is grit: insoluble crushed granite, sold in chick and adult sizes. Pellets and crumbles dissolve on their own, so a pellet-only flock does not need it. The moment the diet includes anything whole, scratch grains, whole corn, forage, garden scraps with texture, grit becomes the difference between digested feed and wasted feed.
Offer it free-choice in a separate dish, not mixed into the feed. Birds self-regulate with eerie precision; a flock on pellets will ignore the dish for months, and the same flock will empty it the week you start throwing scratch. Chicks get chick grit only if they get treats. Starter crumbles alone need nothing.
Oyster shell: the calcium bank
This is the yes. Layer feed's calcium math assumes the hen eats 100% feed, and almost nobody's flock does. Every 10% of the diet that is scratch or kitchen scraps is 10% less calcium than the bag was designed to deliver. Thin shells show up first, then shell-less eggs on the coop floor, which is the flock's way of filing a complaint.
Oyster shell is soluble calcium carbonate. Unlike grit, it is meant to dissolve, slowly, in the digestive tract, giving the hen a calcium reserve to draw on overnight when the shell forms. Put it in its own dish next to the grit. Do not mix it into the feed: that forces calcium on roosters and non-laying pullets that do not need it, and the layers cannot take extra when they do. Crushed eggshells work in a pinch, baked and crushed so they do not look like eggs, but oyster shell dissolves slower and lasts longer, which is why it is the standard.
The two mistakes
Mistake one is buying grit for a pellet-only flock. It is harmless, the dish just sits there, but it is money and shelf space spent on nothing. Mistake two is the expensive one: skipping oyster shell while feeding generous scratch. Scratch is candy, roughly 8 to 10% protein and almost no calcium, and every handful displaces feed that had the calcium built in. The fix is not less scratch, though less scratch would help. The fix is the dish of oyster shell next to the feeder.
What it costs
Almost nothing, which is why this is a strange thing to economize on. A bag of oyster shell or grit costs a few dollars at most feed stores, and one bag lasts a small backyard flock for months. Per hen per month, you are measuring in cents. The thin-shelled egg that breaks in the nest box and teaches the flock to eat eggs costs far more than the supplement ever will.
One thing I still have not settled: whether free-range birds on sandy soil find enough natural grit on their own. Mine are on clay and the grit dish empties steadily, which answers the question for my yard. A friend on sand swears her birds ignore the dish entirely. I have never seen anyone actually measure it, and until someone does, the dish stays out. It is the cheapest insurance in the coop.
Figure the feed side exactly for your flock: Chicken Feed Calculator. Bird type, flock size, season, free-range adjustment, bags per month, and cost per dozen eggs.
Quick answers
Do chicks need grit?
Only if they eat anything besides starter feed. Chicks on crumbles alone do not need it. If you offer treats, use chick-sized grit.
Can chickens eat crushed eggshells instead of oyster shell?
Yes, baked and crushed. Oyster shell dissolves more slowly, so most keepers prefer it as the standing supplement.
Do roosters need oyster shell?
No, which is why it goes in a separate dish. Layers take what they need; the rest of the flock leaves it alone.
Does free-ranging replace grit?
Partly. Birds pick up natural grit while foraging, but on soft or sandy soil they may not find enough. Keep the dish out and let them decide.
Can hens get too much oyster shell?
Healthy layers regulate their own intake and excrete the excess. Trouble comes from forcing it by mixing large amounts into the feed.
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More on feeding: Laying Hen Protein Requirements · Scratch Grains vs Complete Feed · Feed Intake Chart by Age.