Purina® Layena® includes our Oyster Strong® System for strong shells. Oyster shell provides another source of calcium when the hens need it most. Oyster shell is a larger particle size than limestone. This means that oyster shell will stay in the digestive tract longer and will provide a source of calcium for egg shell production over a longer period of time than smaller particle sources of calcium. This is especially important at night when egg shell formation is rapidly occurring and dietary sources of calcium are limited because the bird is not eating.