Plowing with mules and horses is so hard, expensive, and inefficient that when the first motorized vehicles came along. America's farmers leaped at the chance to boost production and ease their work load.
Even the smallest tractor from the 1890s outperformed a mule, and unlike a mule, tractors got bigger and better every year, with new attachments made to drive threshers, log-splitters, saws, and conveyor belts. Electric light could even be added!
With an explosion of farm tractors and trucks in the 1930s, 40s, and 50s, a glut of old farm horses and mules appeared on the market. The horse and the mule, once an essential means of production, was now a nearly-useless vestige of another area consuming too much pasture, money, and time.
The solution? Canned dog food
Canned dog food was heavily promoted in newspapers and magazines, and the television sets now cropping up in upper-middle class living rooms.
Horse and mule meat was touted as a pure and obviously good food or dogs -- real meat for real dogs. If people were now eating everything out of cans, surely dogs should too?
Meanwhile, tractors got bigger and too did American farms.
Young adults who had left for war and war production factories never returned to the farm. What might have been a critical shortage of farm labor was in-filled with mechanical labor. Where it used to take one man to work 20 acres, it now took only one man to work 200 acres or more.
Along with bigger tractors came bigger trucks, plows, discs, winnowers, balers, threshers, separators, and cultivators.
Tractors not only spurred an increase in the size of American farms, it also increased our agricultural muscle and made the U.S. an essential trade partner the world over. When Russian wheat harvests failed, America stepped in. When Europe went to war, it was the American farmer on his American tractor that not only fed American troops, but much of the populations of Britain and France.
Today, the transition from mule to machine is complete and there are no longer vast herds of redundant horses and mules waiting to be sold for dog food. Sure, there are scores of thousands of wild horses out west that the U.S. government does not know what to do with and which are degrading public lands, but their numbers are not enough to drive a return to canned dog food, even if slaughtering those horses for that purpose were allowed.
Instead, the protein and fat needs of America's dogs are mostly met with grains, vegetables, chicken, and a wide variety of other scrap meat trimmings left over from beef, hog, and other human food production.