In ages past, how many chickens in the interior Canada, US, Europe, etc, had access to oyster shells? The basic mineral is calcium. Calcium carbonate being one of the most common compounds. The element is found in many sources beside oyster shell, although, oyster shell is an excellent, concentrated source. It just isn't the only source. In fact, most garden plants are superior in providing calcium in a digestible form. The chicken litter we use for compost on the gardens is high in calcium, having been excreted by the hens. The plants take up the calcium and the chicken eats the plants. Element cycle. Matter being neither created nor destroyed.
May sound like heresy, but I've never purchased oyster shells. Not once in 50 years. Our shells are hard as rocks and tough to crack and all our customers remark about that fact as well. If I fed Flock Raiser 100%, I'd likely provide them.
Just some rambling thoughts.
