"carne de res" is more common, though you can get away with just "carne". In most spanish speaking countries, default "carne" (which is simply "meat", though there are many kinds of meat) will almost always mean meat from a cow since coincidentally, any other meat is not as common; perhaps even due to geographical considerations (I cannot think of a spanish speaking language where it'd be more common to eat, say, goat meat, than beef).