The best dogs to use for guard dogs here are street dogs, they often know instinctively when a 'bad intentioned person' is around, they have lived among people trying to harm them most of their lives. There is little chance you will poison a street dog over the age of 3 year old, most have been lightly poisoned at some point, they know the bad stuff and learn quick. I've sat with dogs who have been poisoned an numerous occasions, the popular poisons here are cheap and don't break the system down fast. An average dog will take about 30 minutes to be taken down so it can not walk, about an hour to become unable to lift its head, about an hour and a half for the internal breakdown and internal bleeding to begin to drown them and then it is just a matter of fight and the size of the lungs and heart. A robber is unlikely to poison your dog in an opportunist situation, if he is going to do that that then he need preparation. So you know if your dog has been poisoned in the yard, keep your eyes open, burglars may be back later.
Street dogs here are wise as they are anywhere. By far the best alarm system and most protective dogs you will come across. You don't need certain breeds that police use etc.These are robots, useless in a relaxed situation.