Horses in Agriculture (gallery)

Threshing Grain by Horse Power, US patent,1834.
Steam engine-powered thresher on the Silas Imbler farm just northeast of Moscow in the 1890s.
Advertising poster for ‘The Modern Hero 2 and 4 Horse Power and the Grinding Mill,” Produced by the Appleton Manufacturing Company of Appleton, Wisconsin, 1892.
Production model of the Idaho National Harvester of 1911.
“Luard Gilmore outfit, plowing and harrowing in a field about one-and-one-half miles northwest of the Potlatch ‘Y’ in 1915.”
Combine outfit on the Randall farm southeast of Moscow towards Lenville. Picture by Eggan Studio in 1917.
This Idaho National Harvester was converted to pull operation using eight horses for combining hilly ground, 1915.
Thirty two horses pulling gasoline engine powered combine harvesting oats northeast of Moscow early 1940s.

Horses played a pivotal role in the agricultural transformation of the United States in the nineteenth and early twentieth centuries. This gallery illustrates how new mechanical systems in agriculture evolved around horses and deployed them for tasks such as reaping and harvesting. The gallery demonstrates how horses remained an enduring feature in American agriculture during a period of rapid technological change. 

Why do you think horses remained pivotal for agriculture for such a long period of time? What advantages did horses offer that might explain their persistence, and even growth, as a source of agricultural power?