The guardian of the flock ..

great bw tones

You've captured the decisive moment - the sheep are scattering, the dog is rushing in.  The composition works well with the slightly blurry dog way off to the side.  The overcast day makes for a luminous quality that causes those sheep to glow beautifully.  The vignetting all around is obvious, but seems appropriate, as if to mimic the intense focus of the dog.  The vignetting, plus the monochromatic treatment, add to the timeless, nostalgic quality.  A well done photo.