Animal productivity refers to the quantities of food, fibre and other products that are generated on farms or by other enterprises that produce those products. Productivity is usually measured as kilograms of meat per animal, litres of milk per cow, number of eggs per hen, and so on. Increases in animal productivity have been made by improving our scientific understanding of the body processes that lead to body growth, milk production, egg laying and the like. At the same time animal welfare standards have generally improved, although some other problems have been created. For instance, we have learned that special care must be given to high producing and closely stocked animals because they have a smaller safety margin for their welfare than do other animals. Their welfare can be more easily harmed. This makes them more vulnerable to animal welfare problems.