According to the "Deutsche Bauernkalender" (translation: German farm calendar) website, the theme for the 2013 installment was "Back To Nature." The site also notes that the models in the calendar, all girls who work on farms, "show up as naturally as they really are" — which I assume means little makeup or retouching is going into these photos.
The calendar feels unique (there is also a Swiss version) in that it celebrates women at work, without excessively dolling them up and and retouching them to oblivion. Famous calendars from companies like Pirelli and RyanAir also get their name out to the public with calendars of hot, scantily clad babes — but those models aren't necessarily making those companies' tires or the airplanes. Calendars of people "at work" tend to be of men — like the annual FDNY one, which is full of shirtless firefighters on the job.
Of course none of these calendars are really natural at all (you wouldn't farm with your shirt tied to reveal your white lace bra), and they often feel awkward, objectifying, and overly staged. With regards to objectification, though, Deutsche Bauernkalender makes a male version too.
But anyway, as these things go, it's pretty cool that these ladies got to pose with their chainsaws.