Depending on how remote your farm is and how strong is the WiFi signal outside your premises (from a nearby public road, ..).

Average case you are in the middle of bumfuck nowhere with a huge private property surrounding your WiFi and nothing's gonna happen.

Worse case some script kiddie attacks your vulnerable router (if using default password) or a smart gadget (if you have some and it's a couple of years old) to join a botnet and get your IP address on a blacklist limiting your internet usability (blacklisted IPs may not be able to send e-mails, may get captchas on major websites like Google, ...)

Worst case someone driving around noticing you have an open WiFi may drop a battery/solar powered raspberry pi with a 4G modem nearby your WiFi and use your WiFi as an untraceable VPN/proxy to perform some illegal stuff (e.g. upload child pornography or perform some serious hacking) and getting you in trouble with the law.