Why did growing oats allow farmers to use horses for plowing?

The transition from using oxen to horses for plowing on the European Plain was made possible by the adoption of the three-field system of crop rotation. Under this system, farmers began planting oats in the spring on a portion of their land that was previously left fallow.1

The cultivation of a surplus of oats provided a reliable, high-energy feed source necessary to sustain horses, which require more concentrated nutrition than oxen to perform heavy labor like plowing. Because oats are well-balanced and provide the energy needed for medium to heavy work, this surplus allowed farmers to substitute the swifter-gaited horse for the slower ox. This shift in agricultural practice, combined with the invention of the horse collar—which allowed horses to exert their full tractive power without restricting their breathing—significantly increased agricultural efficiency.213