I smell an Ig Nobel in the making.
Apparently Margo Lillie, Ph.D. research associate of zoology at UBC (the same department that gave us a study of Herring Farts, which won the 2004 Ig Nobel), and her student Tracy Boechler took on the calculation of the physics of cow-tipping.
Initial results by Ms Boechler suggests a cow of 1.45-metre height pushed slightly upwards would require 2,910 Newtons, which she calculated to be equivalent to the force exerted by 4.5 people. A refinement by Dr Lillie says 2 people would suffice under the right conditions.
The problem is apparently that human beings cannot push hard and fast enough:
...the faster the [human] muscles have to contract, the lower the force they can produce.
Of course, although they didn't assume a spherical cow, they did assume the cow is static.
A cow that is capable of waking up, apparently, is a lot harder to push over.