The corn is worth the same if you sell it or feed it.  If you can buy it from somewhere else for less than what it costs you to grow it, it doesn't make sense to grow it even if you feed it to cattle.  You would be better off buying it from someone else to feed to the cattle.  Livestock and crops are separate enterprises.  That said, if you already have the corn in your bin, you can estimate what you will make or lose feeding cattle, but it won't magically make corn worth more.  Any profit you make on cattle is a return to your investment in calves and facilities and a return on your labor.