At first they actually had some sort of migrant worker system (forget what it was called), but that changed maybe 50 years ago(??). Sano actually knows more about that than I do.

I think you're referring to the BRACERO programs that started during WW2 to facilitate Mexican laborers crossing over to do necessary manual labor while Americans were overseas fighting the Axis.

In 1952 The Immigration and Nationality Act INA) of 1952 and H2 programs were passed to deal with problems faced by immigrant labor. Big employers did not, and still object to formal programs because it makes it more likely they will be required to be less abusive of labor.

1poorguy is correct. The big employers know 'the wall' is just a culture warrior rallying cry. They know walls will never work because walls have never has worked unless they have manned machine gun emplacements with overlapping fields of fire and instructions to fire at will. (Berlin wall, Korean DMZ, etc). Big ag knows how their bread is buttered.