Back in the dark ages when I was going to college, we made a field trip to the Dubois Experimental Field Station, a research facility in Idaho studying various aspects of the sheep ranching industry. One big project they had going at that time - this would have been in 1979 to 1980 - was the training and evaluation of guard dogs. Don is right in that Pyrenees were and are used. The Station at that time was also studying a few other breeds, mostly old world european breeds. The couple I remember were Komondors (mentioned them in an earlier post but had the spelling wrong) and Kuvasz's. These dogs were weaned and then immediately placed with a contained sheep herd with minimal human contact.
These dogs were and are breed and used as guard dogs, not herding dogs, and as such are extremely territorial and protective of their charges (the sheep in the band, the hearder and the hred dogs - typically border collies). At a distance they look like big white hugable pets. Up close there is absolutely no "hugable" in their attitude. That's not a criticism of the breed but rather a comment on specific training goals and requirements. The same breed from a different breeding line and with different training and socialization make great family dogs. I new a contractor that was tired of people sneeking onto construction sites and steeling materials so he purposely trained a black lag to be a guard dog. Meanest S O B I've ever dealt with and I'm a confirmed Lab lover.