Construction machinery.
The use of bulldozers & skipshovels would crush many zombies to destruction or immobility.
As would agricultural combines.
After 90 days, decomposition would begin to take its toll, as would carrion birds.
This is a point I’ve never seen addressed in zombie fiction: Ok, you whack them out in scores with your trusty mini gun from the roof of your house. You’re stacking up zombies like cordwood. No problem.
However, they simply revert to what they were before, namely corpses. Hundreds or thousands of corpses rotting away in the warm spring breeze. There would be a real health threat to the living from such a thing, no? In such a senario, disease would possibly take out a good part of the remaindered population.
This is addressed in Day of the Dead.