It was illegal for farmers to grow wheat above a certain amount... on their own land... to feed their own animals... The wheat limits already assumed that farm

It was illegal for farmers to grow wheat above a certain amount... on their own land... to feed their own animals... The wheat limits already assumed that farmers would feed their own animals first and then sell the remainder on the open market. The goal was to prevent the price of wheat from dropping too low by limiting the amount available for non-feed purposes. However, by doing the opposite, selling up to the limit and then growing "extra" to feed his own animals, Filburn was deliberately contravening the wheat restrictions. If he had succeeded in challenging the law, everyone would have done the same, the supply of wheat would have skyrocketed, and the price of wheat would have dropped through the floor, financially ruining Filburn and thousands of other farmers. The wheat restrictions worked for the purpose they were intended: they eliminated the swings in wheat prices during a time of economic crisis, allowing the price (and the market) to stabilize. It worked so well that the these restrictions were included in successor laws and are still part of U.S. agricultural law today.