The Purina Animal Nutrition Oyster Strong System formula provides calcium to hens when they need it most to support strong shells and healthy birds. The larger particle size oyster shell releases calcium slower. This process, combined with high levels of Vitamin D, provide calcium at night, when the hen needs the nutrient most. The Oyster Strong System is included in the full Purina Layena line, including: Purina Layena Pellets and Crumbles and Purina Layena Plus Omega-3.