It needs to be illegal to enter farms as it has been proven many times over that people, salesmen and others, that travel to many farms transmit diseases between farms by soils and bugs on their shoes. Pigs are more of a problem with this than most farm animals.
I had a sign on the a driveway that no one was to leave a vehicle without plastic booties, which I provided, or face trespass charges. Farmers in the area were able to make these stick after several farmers were wiped out by disease control forcing all livestock to be destroyed, burned and buried. This was in NorthWest Iowa.
Most states will enforce trespass laws for farmers who call in to sheriff's office.