I have read the previous thoughts on this and I see someone has mentioned several different breeds. I would like to add for those considering a dog to also consider Great Pyrenees. These dogs have the very thick double coats that protect them from attacks, are well known for their ability to take down bears and mountain lions when kept in packs, and are some of the best herd dogs on Earth. If you are wanting to breed them you will have to get the dogs about two years before you are expecting to have pups as generally giant breeds do not get pregnant before that time frame. I have a small amount of each chicken, rabbit, cattle, swine, goat, lamb, and cats and these dogs allow the “herd” to literally crawl all over them. Now these dogs will be much more inclined to bark. I happen to prefer the barking because it alerts me. Although this breed is large and intimidating, my dogs have never bitten anything. This breed is very independent and takes no training to teach them to guard. If you prefer a more obedient dog, Great Pyrenees may not be for you. I prefer them because they require no training, and work nonstop. There is little to no worry that they will get me sued. Mine do well in one of the warmest states in the country and are built for extreme colds also. The double coats allow them to thrive in either environment. They can be from 150 to 200 lbs full grown and do eat quite a bit. I keep my cost of feeding them down by giving them all the scraps from the household, and a gallon or more of fresh dairy milk each day. You will need to either teach them their boundaries by a regular schedule of walking the perimeter of the area you want them to guard or by putting up some sort of boundary markers, unless you want them to feel free to guard everything in sight. Since I see my herd of animals as key to my survival, I did not think rottweilers, doberman pinchers, or American bulldogs were good choices.
As a side note, I did want to mention that if you are on well water and hooked into the grid, you can have a hand pump attached on your existing electric pump to pump water by hand if the grid goes down. Some of these new hand pumps claim to be able to pump water as far as 300 feet in an emergency and regularly from a depth of up to 200 ft. And a note about fences……in very flat treeless remote locations, a fence can be as much a view block as it can be a trespasser hindrance. I think everyone should consider their location and circumstances when choosing what to do. I find in my situation a fence would actually give a possible intruder a way to get close to my house without being seen. Whereas without the fence there is not a house or tree or anything else to block my view for miles around, so it really is not worth the expense. One of the things I keep is a DVR full of our favorite movies because I suspect either cable would not be available or I would not be able to afford it. If you have scrap metal around from things you have done to improve your SHTF plan make sure to not throw them away. Scrap metal is at an all time high right now. I keep a huge trailer to keep trash metal on and scrap it every so often. In a TEOTWAWKI  situation, everything is worth something. – Pat P.