I still am not sure about that cows having magnetic sense thing. Birds I can accept, but the very first thoughts to cross my mind when I heard about cows:
1: Grazing lands are usually lands people don't want to live in
2: The southern face is more livable and hospitable.
Those two facts suggest to me that it's possible on a global scale cows are mostly on a certain face of a hill. Supposing most cows are on the southern slope, could it not be possible they simply elect to face uphill?
I'm not saying my speculations are any kind of proof, that they are well supported, or anything else like that. Only reasons the scientists may not have found the true cause. As they say, correlation does not imply causation.