Guten Morgen Steffi,
so wie ich die Infos auf der offiziellen Seite des U.S. Customs verstehe, geht es eher um Erdreste, etc., nicht dass Du die Maul- und Klauenseuche einschleppst:
Also check “Yes” if you have been on a farm or in close proximity of livestock, as an agriculture specialist may need to check your shoes or luggage for traces of soil that could harbor foreign animal diseases such as foot-and-mouth.
Wenn Du also nicht gerade die Gummistiefel trägst, mit denen Du am Tag vorher noch auf dem Traktor beim Pflügen warst, sollte das kein Problem sein.
Also ich denke in Deinem Fall kannst Du beruhigt "nein" ankreuzen. Aber vielleicht hat der eine oder andere hier auch noch eigene Erfahrungswerte, mal sehen.
Bei Tripadvisor habe ich das hier gefunden - im schlimmsten Falle gibt es wohl einen kleinen Schuhputz:
We live in the Dallas Fort Worth area, and returned home from Buenos Aires this morning. At U.S. Customs at DFW, we were asked if we had visited a farm or ranch. We replied that we had taken a day trip to an estancia. We were asked if we would soon be visiting a farm or ranch here, and our answer was no.
We were routed to an area for agricultural issues, and we were asked to present the shoes we had worn to the estancia. Mine had a very small amount of dry "manure" on the bottom, and I had packed them in a bag for cleaning at home.
A customs officer examined our shoes, and actually cleaned mine in a sink with special soaps, and my husband's as well since he was already having to clean my shoes.