Oyster shell is oyster shell. Mana Pro has a couple sizes that are great for the backyard flock or larger flocks. Its affordable and a good source of Ca for your birds. It is clean, has no smell, and makes egg shells STRONG. Offer free choice or mixed in, depending on your bird management practices, and you'll notice a difference. It doesn't go bad so long as you store in a dry place free of mold. There is little to no difference between oyster shell products though, so I am not bond to Manna Pro if another product pops up cheaper. It is widely available and lasts a long time. It isn't meant as a Ca replacer for poor feeds, but is pretty good filling that roll for home-made feeds. Since this is something that stores very well, the largest size is the best bargain. For comparison of effectiveness; I drop my own eggs all the time. None of them have broken since using Oyster Shell. Ever. My friend's flock who I just maintained for a few days, use no Oyster shell. I dropped an egg from 12in into grass and it cracked instantly like a cheap store bought egg. We have the same breeds of chicken as well, of the same age and very close background breeding. The product works- and that is what we want!