Pig unit ammonia (273 mg NH3/pig.hour) is equivalent to 1 fattening pig, or 78 laying hens (daily removal of droppings), 12 hens (with open slurry storage), 4 laying hens (with deep pit manure system or eq.). The livestock production is considered as an input-output nitrogen model system constituted of 2 parts, the animal and the slurry system. As P present in system doesn’t change much, ratio N:P are considered in the fresh manure and after storage. Ammonia model consider all the nitrogen stages of the system being intake (feed content), yield (meat, eggs, milk, ) direct losses and litter added. As mentioned by other authors, the ammonia emissions are related to the manure surface. A table presenting ammonia production/ animal place-year is also presented for different animals and different production systems.