The decrease over this time span was primarily due to the advent of the automobile for personal and commercial use (urban) and tractors for agricultural use on the farm (rural).
This period is also known as a golden age for agriculture and productivity down on the farm.
I would like to put a little different spin on this productivity miracle.
|Source: Humane Society|
|Source: Fifteenth US Census|
This means from 1915 to 1960, 112,020,00 (that is 112 Million!) FEWER acres of land were required to be used for horse and mule fuel (22.404 million fewer horses and mule X 5 acres).
Put in perspective, this is equivalent to the combined TOTAL acreage of the agricultural States of Nebraska, Iowa and Indiana, that did not go to the feeding of horses and mules but to people.
Note: I am assuming no change in technology and the horse and mule population would have stayed pre-1915 constant. Maybe, maybe not.
Physical land resources, especially in urban areas, were freed up for alternative uses and allowed those areas to progress at a different pace and on a different trajectory.
The untold story of advancement in technology is also reflected in the demise of the horse and mule powered economy.
Just thought that was interesting. Hope you do too.