lol. You have that backwards.
Trump peeled loose from Cruz in Iowa mere hours after Cruz made the statement that he would seek to have the Renewable Fuels Mandate repealed. Governor Branstad went ballistic (his son is an Ethanol Lobbyist), and said 'Cruz must be defeated, whatever it takes'.
I believe Trump decided to capitalize on what he perceived as a political opportunity and announced shortly afterward that he would not only keep the mandate, but seek to have it increased, and use the EPA to enforce it to the fullest extent of the law, perhaps in an attempt to win favor with Iowa's farmers, ethanol industry, the Governor, and the Iowa GOP.
That failed, and the lies about Cruz started, in earnest.