Appelbaum: What replaces deported immigrant workers? Not Americans.

Most American dairy cows are milked by immigrants. On Dale Hemminger’s farm in upstate New York, the cows are milked by robots. When a cow wants to be milked, it walks up to a machine that cleans its udder, attaches cups to its teats, draws the milk and dispenses a treat.

In a barn that Hemminger plans to open this year, other robots will roam the floor like little automated pooper scoopers, picking up manure.

President Donald Trump should pay a visit. He might learn something about...