We had 2 English mastiffs when we first brought our chickens home. I kept them completely separated for almost a year because the younger dog would rush at the coop, scare the hens & I was afraid of what might happen. Unfortunately, we had a death in the family and had to hire a dog/chicken sitter while we left home in a rush. The sitter knew how gentle the dogs were & never imagined there would be a problem, so let them all out together. Luckily, she was right and nothing bad happened. She sent me pictures of them all together & I nearly had a stroke! I did learn that the younger dog only ran at the coop if the hens were in it & completely ignored them while they were in the yard. Now that she’s 8, she lets them drink out of her water bowl at the same time.
The older dog died last year, so we got a 15 week old Great Dane puppy & she thought chasing the hens was the most fun thing in the world. We didn’t want to relegate the hens back to their coop so often, so the puppy required CONSTANT watching. I bought a shock collar, turned it on it’s lowest setting (tried it on myself first to be sure it wouldn’t hurt her), and spent A LOT of time in the yard with her. It took about a month to train her to leave the hens alone, and I honestly only had to use the shock function about 5 times. The collar also has a beep & vibrate function, so the minute I saw her even looking at the hens, I’d give her a beep to change her focus. If she didn’t turn, I’d follow by vibrating it.
That being said, if the dog has a strong prey drive, it’s going to be rough. My husband dog sat for a friend (a pit/lab mix) and he had to leash the dog on every outside trip because that dog would stand at the door & cry to get at the hens. It was a rough weekend!