You have to admit it's a damn good excuse. :)
In addition, the land is sprayed with weed killer before planting and once during the growing season. The weed killer will prevent the marijuana from growing, and will kill any marijuana that emerges.
Also, corn grows to 7-8 ft; marijuana 10-12 feet. I think you'd spot it if you were looking.
The bottom line? You'd know it.
And it seems like everyone else knew there was marijuana except the owner. His father knew, Bill Fancher knew, his son Jesse knew, his wife Kathy knew, but poor Ronnie Puckett didn't know he had 10 acres of 12 foot marijuana plants growing in an area that was not sprayed for weeds and which produced 250 pounds sitting in his barn.