One of the principle reasons that cow tipping is no longer considered to
be possible is that cows, unlike horses, don't sleep standing up. They
'doze'. Dozing is evidently not the same as sleeping. Dozing puts the cows
in a sort of dreamlike trance where they're senses are still alert enough
to detect an approaching tipper.
The other reason that tipping is not possible is that cows are HEAVY!
They're cows for crying out loud. It's pretty difficult to push over a two
thousand pound animal, even if the animal is asleep.