Oyster shell should never be given to young hens before egg production since calcium overdose may cause kidney damage. Only hens in production should be provided a calcium supplement such as oyster shell, and even then there is some debate on how much. If you feel small ‘bumps’ on the surface of your hens’ eggs, then they are probably getting too much. If they free-range, they probably don’t need it since they’ll pick up what they need naturally. However, if they are pen or cage raised, then providing a pinch of oyster shell once a week would be advised. Too little calcium in a producing hen’s diet will cause her to steal calcium from her own bones to make up for the deficiency. Free-feed oyster shell should be fine for adult hens under any circumstances, provided that you don’t have a ‘guzzler’.
For more information, view my Poultry Health Article on Toxins.