薄利多销: demand and supply curve
The lower price will increase demand effectively, so more supply realized finally.
谷贱伤农: Maybe because price lower than the basic cost.
There is no rational supply curve to match the demand curve.
If you insist farmer to do so, then
the farmer will not recover all the investment.
The ability to stay on business is so jeopardized that he may apply chapter 7 bankrupt shortly.