Why Btonze Age?

[abaris]

The Germans also used both German and Cossack horse mounted cavalry for anti-partisan operations in yogoslavia and on the eastern front.  Throughout WWII, much of the supply of the German army from the railhead to frontline troop was pulled by horses despite German reputation for mechanization.

It was only about 10 percent, if I'm not mistaken. At least at the start of the war. And with fuel supplies getting shorter and shorter, I doubt the number rose significantly later on.