He could always state that ethanol must stand or fall on merits w/o subsidies. One can support a market, and not support government manipulation of that market. If Iowans are stupid enough to waste water and land resources on growing corn for fuel (and - believe you me - Iowans are stoooopid as Nebraskans when it comes to growing food for fuel)then let them drown in the garbage. In no case am I in favor of subsidies for any sector; And if Mr. Trump does favor such interference in the market, then that is where he and I diverge.
They grow feed corn, they get both products from one crop, as they feed the spent corn-mash to livestock. It is a better feed than the original corn. Now who's stoopit?
If you mean Cruz, he's already done that, in essence. From the lefty DesMoines Register, he makes it fairly clear, and in other places as well.
Cruz has long been opposed to government subsidies for renewable energy industries. All of 'em. Across the board. It's why he won't budge on ethanol. Cruz has the same political compass that I do.
Trump is a hell of a lot more iffy, it looks to me when I go online and search out where he stands on the issue.