War horses interbred with draft horses. Thus, more horses were available for farmers, displacing oxen as draft animals. And increasing farm yields because of their superior speed.

graph TD
HS[Horse strength] --> |More power| DH[Draft Horses]
H[Horses] --> |More available| DH
DH --> |Faster than oxen| FY[Farm Yields]

style Inventions fill:yellow,stroke:#333,stroke-width:4px
Inventions[Heavy plow<br/>Horse collars<br/>Horseshoes] --> DH

Source: Medieval Technology and Social Change by Lynn White