Where I'm from, MANY people who grow outdoors grow in corn fields. Regarding the chances of the farmer walking the fields and stumbling upon your plants, 15-20 acres is VERY VERY SMALL when it comes to a corn field and that is a possibility. What I'm surprised nobody has touched on in a thread about growing MJ in cornfields, is that there is a high probability of this being a sweetcorn field, especially since it's small. If it's sweetcorn, you will never harvest your plants since sweetcorn is harvested (in my area) from late July through September. Just starting to bud and then..... poof, nothing left but heartache. You may never eat sweetcorn again out of spite and bitter reminder, lol.
That being said, even if law enforcement were to find the plants, they're well aware that the vast majority of people who grow outdoors do so on someone else's land or state land. So they wouldn't do much of anything about it aside from ask the fella if he seen anything suspicious or if he knew who it could be.... which is still not very cool.....but it is what it is and nobody here has to like it. I do have to agree with others that if you have your own land, clear a little and do your thing.
As for the guy who sees " no negative consequences" to asking the farmer if you can grow an illegal drug on his property ? Probly one of the stupider things I've heard someone mention in a while. I'm sure you value your security more than that guy does. With an astronomical high chance he'd say no, he could A) call the authorities B) WILL guaranteed tell people about your proposal. Not to mention that him and the never-ending chain of people who hear about it will all know you as a marijuana grower. If there's corn nearby and people own acres of land, you likely live in a small rural area where people love to gossip. By noon, the next day it's all over town that you're running a king kong size MJ growing empire, lol. That has the potential to not be productive when it comes to the task of growing Cannabis, lol. But then again... maybe he's right and there's no downside, lol. Best of luck to you