I don’t think the threshing had anything to do with it, but whether the farmers grew grain for his own livestock or grain to be sold, and thus transported. Even if he grew it for his own use, he’d affect the market because he wouldn’t be buying grain for his own use, which would have to be transported to his farm.
Or something like that ....
Wickard v. Filburn, 317 U.S. 111 (1942)
the penalty is incurred and becomes due on threshing. [Footnote 38] Thus, the penalty was contingent upon an act which appellee committed not before, but after, the enactment of the statute, and, had he chosen to cut his excess and cure it or feed it as hay, or to reap and feed it with the head and straw together, no penalty would have been demanded. Such manner of consumption is not uncommon. Only when he threshed, and thereby made it a part of the bulk of wheat overhanging the market, did he become subject to penalty.