I give my chickens both. Plus crushed eggshells. But the grit and the oyster shell particles are about the same size. They appear to have the same density. I know the grit is essential to grind down the other food they eat but wouldn't the oyster shells do the same thing? Certainly the shell is not dissolving in their crops very fast... And where does the grit go? I don't see it in their poo. Does it dissolve? Or do they grind it down to nothing? Does it provide its own nutritional minerals? I do it. I just don't understand it.