First, there is nothing particularly harder about this work than working construction, or janitorial, or in a warehouse. It's just work.
Second, we import foreigners from thousands of miles away when our cities (and countryside) are filled with able bodied unemployed people on welfare. No effort is ever made to connect these people with the agricultural jobs. Something is wrong with this picture.
Do you think you are being accurate here? Think about the manure, the blood, the heat and the backbreaking labor, leaving out whether everyone is suited to slaughter animals.
These are not jobs that can be filled by people who are not used to that kind of work or very motivated.