U.S. and Japanese scientists reported on Sunday that they had used genetic engineering to produce cattle that resist mad cow disease. They hope the cattle can be the source of herds that can provide dairy products, gelatin and other products free of the brain-destroying disease, also known as bovine spongiform encephalopathy or BSE. Writing in the journal Nature Biotechnology, the researchers said their cattle were healthy at the age of 20 months, and sperm from the males made normal embryos that were used to impregnate cows, although it is not certain yet that they could breed normally.