You can deduct business mileage if you have a home office that qualifies as your regular workplace, and drive between your home and another work location in the same trade or business. If your home isn't your regular workplace, driving between home and work is considered a commute, and therefore isn't deductible

If you do not have an actual home office for the farm but can a separate structure such as a barn used exclusively and regularly for farming or ranching count as the principal place of business and so then be your main job and then could you deduct the mileage to go to a second job of day labor?

I don't think you need a home office.  You need to work at two places in the same day.  Then, the mileage between them is deductible.  So, at least one-way mileage would count if the cows get milked every day.  Have you checked Pub 225 to see if that situation is covered?  

When he does day labor elsewhere, is he an employee there? If so, that would be commuting.

No he is not an employee he has his farm/ranch and then day labor of his own other business.

Is his own other business also a farm?

"and another work location in the same trade or business"

No the other job is not farm, a different business

Not that it makes any difference whether the second workplace is a farm, or a job, or other self-employment.  

He arises at his home in the morning, goes outside to the farm and feeds the cows, then drives from the farm to his off-farm job in town, works 8 hours in town, drives back to the farm, feeds the cows, then goes into the house.  It appears that his mileage to and from the job sites is all business mileage.

So he works 8 hours a day for someone else but is not their employee?

@sjrcpa  Answer is not from the original poster.