They would definitely notice. Modern farming equipment is way more technologically advanced than most people think it is. I used to work for a company that manufactured the electronics for farming equipment; it was usually some of the most high tech stuff we built and we also made the boards for medical equipment. So even if the farmer didn’t notice, their equipment definitely would. Specifically, most new crop sprayers and fertilizer spreaders will actively monitor soil nutrient levels and crop health to determine exactly how much of whatever they’re spreading needs to be spread in each location. Patches of crops hit with weed killer would definitely start throwing up some red flags in that software. You’re far better off just making old school crop circles with a plank on a rope.

Now as far as morality/legality goes, this would definitely be vandalism and depending on how many crops you destroy likely even a felony. Crops are insured so you really wouldn’t hurt the farmer too much. However you would incur the wrath of an insurance company. So if you value your financial well being and staying out of prison, I would not mess with a farmers crops.