Yes they have full run of the pasture, used to close it off at night, but after having them out for a few nights, I discovered they kept the deer from eating the corn out back, which is a huge plus.As far as big brands that are readily available, I have fed the Diamond naturals for years with good success. Purina One also has good results for most dogs.
You mentioned that the dogs work pretty hard, and if that is the case you might consider a higher fat and protein. I have been feeding Kinetic 30K to my Malinois for his whole life, and he does really well on it. It is not cheap however.
They run in shifts, one will run the front fence line, the other one will run the back 40 and they will often meet in the middle, and to come up front to get water.
it’s pretty amazing to watch them work as a team.