Corn ethanol is a value added product of feed production which uses a waste stream from it called the carbohydrates which normally are wasted onto the ground undigested in livestock manure.  It leaves the feed(i.e. the proteins, fats, and minerals) alone, actually enhanced to be healthier, more digestible, and more productive. 

As such we will grow the same amount of corn acres with or without ethanol.  No destruction of farmland.  

Corn ethanol does not even show up as a blip on this corn chart over time:

https://uploads.disquscdn.com/images/91d7502e043c7c976bdbac4d3953fb82813e0ab4ef4723a5d70e734514dcac9c.jpg

In fact, we had more acres of corn in the the 1920s, 1930s, and 1940s than we do today with 1932 the greatest at >113 million acres.  Compare that to 2015, the year corn ethanol mandate maxed out, at 88 million acres of corn.

https://quickstats.nass.usda.gov/results/08C6BF8A-850C-3EDB-A929-C39BD2395217