When the government reduces the support price.
Not before.
***
I know....I was kidding around. I don't really expect prices to ever come down. It's sort of like the notion of a "temporary" tax. There is no such thing. Once you have a tax, it almost never goes away. Once a price goes up, it rarely comes down, and if it does, it's only temporary. Usually, when it goes up again, it goes beyond the original price...to offset the losses from the temporary price reduction.
Most would rather be free to make their own planting decisions and rely on their productivity to compete in a free global market.
The government's support prices are designed to protect the most inefficient among them. And whenever there is a vote, the farmer who plants 2000 A of corn has only one vote, no more than the farmer who plants 20 A.