Time to get Forceful!
As we know the weak point on a Zombie is the head. How much force does it take to break that open? A good 500 pounds of force.
M is the mass of the object, A is the acceleration, because the baseball bat weight is a set number we will have to find how fast you swing. F in this equation is Force, which will be your 500 pounds mentioned earlier.
500 pounds of force must be converted into Newtons, which is a lovely 2,224.11 Newtons. Now we need the mass of the baseball bats! Zombie face
baseball bats

The weight of each bat is set, and their averages are:

Wooden Bat: Can range between 30-50 Ounces or 0.85 to 1.41 kilograms
Plastic Bat: 2.6 ounces which is 0.07 kilograms
Metal (aluminum) Bat: 24 ounces or 0.68 kilograms.

So plug and chug your information in by dividing your Force by Mass.

So after you plug in chug for each bat you will have to swing pretty fast. About 2235.2m/s^2. So instead of crushing the skull you might be able to decapitate the zombie or knock him down with a good swing.