That number is complete rubbish.
Take out the owners, herd managers and the farm support people who aren't on the farm every day: Milk Inspectors, DHIA Technicians, Holstein Classifiers, Milk Haulers and vets. And then the number (unless you're on an Amish Farm) is 99.9%.
That is correct. The daily hired labor on dairy farms in the US not part of management in almost 100%. NONE of them are "believed" by anyone to be undocumented. ALL are known to be illegals.
That may be true where you live, but not here. In my area there are farmers that hire only illegals, some that hire both legal and illegal workers, and some that never hire illegals. The sad thing is the farmers that don’t hire illegals have to compete with those that do.