At 16 weeks we usually just give them whole wheat in the evening. They get most of their nourishment from the grass. However, with all this dry weather the grass is almost gone so I am still giving them some duck and goose growers pellets as well.
At 20 weeks we change to two thirds whole wheat and one third goose or turkey finisher and continue until Christmas. They also need plenty of water to drink and groom with.
Have you protected them from foxes? They will still be vulnerable, especially at night.