The Jerome County sheriff's office says the farmer found the plot of pot growing between his tall, green stalks of unripened corn early Monday and called authorities.
The sheriff's office says the 314 low-grade marijuana plants are valued at $628,000 and would have been ready for plucking in the next month or so - just before the corn harvest.
A detective says the pot was started from seed and relocated to the field, a common way marijuana growers hide their plants.
Authorities have ruled out the farmer as a suspect, saying there have been at least two other similar reports in the past year.
