According to wikipedia, the popular saying "[it will happen] when pigs fly" (or when pigs have wings) is traditionally used to mean that the specified event will never occur. Well, pigs do have wings.
Today, the American Meat Institute, the National Meat Association and the National Milk Producers Federation announced that they have petitioned the Agriculture Department to enact a total ban on the slaughter of downer cattle. The American Meat Institute and the National Meat Association said they would encourage companies to enact a voluntary moratorium until the Agriculture Department changes the rule.
Downer cows also might have contracted such infections as mad cow disease. The animals are also more prone to infections such as Salmonella and E. coli partly because they wallow in feces before slaughter.