That's a big cow!
So the clue was : since 1970 annual production of this per cow has grown from around 10,000 pounds to around 20,000.
She guessed "meat". The answer is "milk". And I get it: who thinks of milk in terms of pounds instead of gallons?
But really, meat? That would mean that cows were more than twice as big now as they were in 1970. That would have been pretty noticeable, I'd have thought...