She analyzed the company statistics between 2004 and 2007of 150 pork pig farms and 150 breeding pig farms. She concluded that factors such as the living conditions of the pigs, their feed and the company results do not have any influence on the amount of antibiotics used. The growth and the mortality rates of the pigs are also not related to antibiotics use in the various farming concerns.
Factors which do influence the use of antibiotics are: the type of farm (mixed or specialized), the number of animals on the farm, and the pig density in the region. 'More pigs in an area means more risks from diseases', Van der Fels explains, referring to the results. And yet, these 'technical' factors cannot account for the big differences in antibiotics use among the farms. 'We have come across pig farmers who use almost no antibiotics, and others who use antibiotics every day to keep their pigs disease-free. Social economic factors, including the wish to use less antibiotics, greatly affect the number of daily doses of antibiotics given on the farms.'