First, you have a misconception, I’m glad you asked the question so we can clear it up. Oyster shell is almost pure calcium and is fed to laying hens for egg shell production. A laying hen needs about 4% (more or less) of her diet to be calcium for proper thick egg shells. A growing chick needs about 1% (more or less) of its diet to be calcium for skeleton growth and some other uses. Most chicken feed other than Layer has about 1% calcium, all that they need. Layer has about 4%. The greens you are talking about will provide some calcium too, but that amount is fairly small. The general recommendation is that you only feed them enough treats that they can clean up in 15 to 20 minutes and the rest of their feed is the chicken feed. That way their chicken feed still provides the balance of nutrients they need for good growth, minerals, fats, fiber, protein, and other trace stuff without throwing it out of balance.
The misconception is that oyster shell will work as grit. Chickens do not have teeth and cannot grind up their food by chewing, so they grind up food that needs grinding in their gizzard. To help with that grinding process they eat what we call grit. In the UK it’s called insoluble grit. If you buy it from the store it will be made out of granite, but if your chickens have access to the ground they will find pebbles they can use instead. Oyster shell is soluble grit, something else entirely.
Oyster shell does not work to grind up food for two reasons. Although it seems hard it’s really pretty brittle. It very quickly breaks down to a powder, not much use for grinding food. Also, just like your digestive system the chick’s digestive system is filled with acid. That acid quickly dissolves the oyster shell so it can be used by their body. When it is dissolved in a liquid it is useless for grinding.
One of the first thing a broody hen does with her chicks when she brings them off the nest is take them to peck at the ground. They are picking up bits of dirt to digest for minerals and bits of plant materials for other nutrients, but they are also picking up small bits of gravel to be used as grit. A broody hen does not wait for a few weeks to introduce them to grit, she starts them off with grit.
If green stuff like grass or certain weeds are available, it doesn’t take her long before a broody hen is feeding them green stuff. She might break it up into small bits for them or they might peck and break it up for themselves. A broody hen does not wait weeks to introduce them to greens, she does that real soon after she’s introduced them to grit. They do need grit to grind up those tough greens.
My suggestion is to chop those greens up into fairly small bits and give them a small amount. Since the leaves are not attached to roots in the ground it is harder for them to break off small pieces for them to swallow, at least until they are older. Judge the amount by how much they eat. It’s really possible they will not eat it at first, sometimes it takes them a while to try it, but eventually they will. Eventually could be days. If they don’t clean it all up within 15 to 20 minutes, don’t panic. Just reduce the amount you give them next time. There is some trial and error involved. We’re talking about a balanced diet. They can eat too much of something one day and it won’t affect their overall diet as long as they don’t overeat that treat every day.
One reason to chop those greens up pretty small is that if they eat big chunks of greens they can get twisted up into a ball in their crop or even gizzard and cause a serious medical condition called impacted crop or impacted gizzard. That’s where the twisted ball gets big enough that it blocks the opening for the greens to leave the crop or gizzard. Grit will help a lot to prevent impacted gizzard but doesn’t do anything for an impacted crop. So before you start feeding them those greens it is important they get some insoluble grit, not oyster shell.
Good luck! Yu are lucky to have that source of greens for them, just make sure it is a minor part of their diet.
