To this dog lover it's good to see many reasoned and humane suggestions, as stated this problem is the OWNER'S doing. Poor dog doesn't know any better on his own... he may be a helluva' good dog if not running loose and having to revert to his instincts for "survival" as he's able to see it.
I found a sick abandoned Bull Mastiff years back, left out in the woods of my Rod & Gun Club. With veterinary help ($$$$) we nursed him back. Called him "Bronson".
Since we were usually not home working we decided another big dog, female, would be humane for his company... dogs aren't meant to be loners.
Ended up adopting a St. Bernard from a neighboring animal shelter we named "Gretchen". This young dog had been reported for several months running loose around the farms in nearby MA, apparently homeless and living off the land. She was cornered by 5 or 6 men, animal rescue guys who couldn't get anywhere near to collar/leash her... but the female supervisor of the Dept. was able to calm her and bring her in safely.
Always struck me she must have been abused by some jerk who originally owned the dog, who may have dumped her somewhere far from home... or maybe things were bad enough she simply ran off.
She was the finest dog I've ever owned: loyal, sweet, gentle, attentive and SMART! Awesome watch dog, easily taught and learned how to let herself out my back storm door AND the cap on the back of my pickup if some stranger was lurking about. She never hurt anyone but made it clear they'd better get lost if she didn't know them!
I've had more dogs since then, all adopted, three right now. But I do miss those two hulks to this day.
My latest bunch: L to R, Jelly, Dondi, Buie: