Before oil, farmers used draft animals to do their heavy work. They powered their draft animals by growing hay and oats on about 30% of their land.
Today a farmer can grow enough oilseed crops on 15-20% of their land to power their tractors, trucks, home and barn heating, crop drying etc...
Farmers can go back to raising enough livestock by pasturing them in the summer and collecting their manure in the barns in the winter for field fertilizer.
Add in crop rotation with crops that fix nitrogen for fertilizer and what you come up with is sustainable farms.
But these sustainable farms will only be producing "for sale" grain crops on about 1/2 of their total tillable acres. So world grain production may get cut in half when this happens. Farmers will continue to get along just fine for food and financially. But there will be mass starvation in the third world and perhaps in some cities in the first world.