Scientists at the University of British Columbia in Canada have investigated the physics of cow tipping and concluded that the force required to actually tip a cow means it's pretty unlikely that anyone could ever really do this.
Here's the maths...
m = mass of the cow = 682 kgs
g = gravitational force = 9.81 m/s^2
θ = angle of the lever = 66 degrees
a = length of the lever above the centre of mass = 0.79m
b = length of the lever below the centre of mass = 0.79m
1360 newtons of force requires 2.07 people to exert it, if you assume that each person weighs 67kgs and can push their own bodyweight.