Another invention that increased farm yield at the time was the three plot farming system. By planting summer crops on one plot, winter crops on another, leaving the third field fallow, and then rotating the fields every year, farmers gained several things. First, instead of 1/2 of the land being used, now 2/3 of the land was used. Second, the three-field rotation led to more fertile soil, since legumes fix nitrogen in the soil. Third, one of the most popular winter crops was oats, which turned out to be a far more effective food for horses than grass.
graph TD TFS[Popularity of Three Field System] --> |Better land use| FY[Farm Yields] TFS --> |Increased soil fertility| FY TFS --> |Popular winter crop| O[Oat Yields] O --> |Horses ♥ Oats| DH[Draft Horses] DH --> FY
Source: Medieval Technology and Social Change by Lynn White