Tractors had taken over by the 1950's
Just some personal notes...
Around 1930, my great-grandfather Marcus Eaton, closed his small livery stable in Gordon, Nebraska.
There are family pictures and a story about my dad helping some of his Sprague cousins make hay with horses in the late 1940's.
Johnny Seier at Duff, Nebraska (in southern Rock County) still had enough workhorses that he used a team of horses to feed his cattle during the Blizzard of 1949. Lester Miles at Chambers, Nebraska, also took hay to his cattle with workhorses during the winter of 1949.
I was born in 1951, and I don't remember seeing anyone farming or making hay with horses when I was little. (They may have been doing it, but I don't remember it.) I do remember a lot of old horse-drawn hay-making and farming equipment sitting around. It was easy to identify a piece of equipment as horse-drawn because there was a seat for the driver built into it. Such a seat was unnecessary for tractor-drawn equipment. The driver sat on the tractor.
My neighbors here in Christian County, KY, talk about mules used on small farms here, through the 1950's and even into the 1960's. Of course, it is quite rare to see anyone, other than Amish or Mennonite farmers, using mules in the field today. I doubt if any U.S. mines still use mules.
My husband grew up in Independence, MO. They had a little donkey right in town to plow the garden and help with yard work. In the mid-1960's, one of the neighbors got a small tractor and plow. Grandpa Netz sold the donkey then, because the neighbor could plow the garden.