In reality, American agriculture does well because of machines, Mexican agriculture where labor costs less than $3 a day can't compete with us because of machines --- it's the machines we have that allows the same guy who would make $3 a day in Mexico to make ten times that here.
That's a good point. For Mexico's leaders, it's the best of all worlds - they export their poor and in return get billions in greenbacks. Such a deal! And we're stupid enough to let it happen.
Agriculture keeps getting more efficient so there is less of a need for workers. In fact, there are too many farmworkers here now for the number of farm jobs available.