sudden behavioural change in my Setter

Question
I really hope you can advise. I have had Gordon Setters for over 30 years. I currently have two, a dog Mungo 4 and a bitch Kelpie , 2.  I also have a greyhound I found dumped 18months ago called Toby he is very gentle and submissive and is 6. We had no problems when Toby arrived, the Setters thought he was boring and mostly ignored him. All dogs are neutured. We have had no problems until a couple of months ago when Mungo started becoming more dominant over Toby at mealtimes, (they are fed in the same room but well apart).Now after a meal he will sometimes chase after him and attack him, (although he doesn't actually hurt him other than emotionally). Mungo does not guard his food from me or Kelpie . All the dogs will later happily sleep together on the sofa or ih my bedroom,  and are fine when all walking together on their leads or in the car (although I keep the greyhound in a separate part of the back of the car as he is so long).  Mungo has always been hyperactive and charges about more even than the average Setter. He is super with all other dogs he meets as well as with people and is (mostly) very obedient. The dogs get plenty of exercise, (1-2 hours daily loose running on a massive quiet sandy beach as we live in Northumberland) .

I am really worried things will get worse and Mungo will actually bite Toby as being a  thin skinned Greyhound he would be badly hurt. My bitch, Kelpie tactfully keeps well out of the way in any squabble between the boys. Mungo always defers to her but she in turn if anything is resepctful of Toby as he is so much taller. So our pecking order is circular I think. Thye all listen to my husband and I but Mungo is tending to ignore us when we shout at him if we think he is about to lay into Toby.
Mungo does have some lose of hair around his mouth which I thought could be dietary? maybe his food is making him worse?  They all get complete dog food for working dogs plus a hide chewy daily for their teeth. Mungo and Kelpie also eat seaweed a lot.


I would really apprecaite your help

Answer
It appears that Mungo is attempting to make a strong rank statement to Toby.  "Ignoring" a new addition does not mean the resident dogs consider the new addition "boring", it is normally a behavior that higher ranking individuals (self perceived) demonstrate to make a rank statement to the new addition.  Kelpie is actually most likely running this pack (as bitches do from the inside out, so to speak) and so is not in this mix.  Toby may not actually be as low ranking as you believe him to be (I surmise from Mungo's food related displays of dominance.)  In fact, Toby may actually be offering signs (at this point, after having lived with you for some time) of rank related threat to Mungo.  You have furthered Mungo's endeavor by relegating Toby (and innocently, doing this because of his size) to the "back" of the car and perhaps doing so in other ways (totally unconsciously.)  Toby is not naturally an Omega as the male Greyhound can be quite a handful and can also be quite dominant.  He happens to be apparently (to you) temperamentally way down the line from Mungo; Mungo may not agree.  You may be missing visual signals between these two dogs.  Additionally, you and your husband don't seem to have a clearcut advantage psychologically, or Mungo would definitely immediately defer to your displeasure (expressed vocally without emotion.)  Also, Mungo is definitely showing signs of some possible allergic response or autoimmune problem (hair loss -- alopecia).  Alopecia is often the reaction to something being ingested; a dog food for "working dogs" is totally NOT the proper nutrition, as your dogs are NOT working dogs.  There's far too much protein in this diet; seaweed, although I understand it is supposedly a nutritious supplement, may be introducing other allergens to Mungo that are not affecting Kelpie and raising the protein levels.  A higher than required protein level will exacerbate behavioral problems.

The first thing you must do is bring Mungo to the veterinarian and report this sudden rank related problem, as it (and the alopecia) can also be symptomatic of systemic problems; the second thing you must do is reevaluate the diet, finding a high quality kibble that has a protein percentage much lower than that intended for a dog out in the field all day, every day (and I would definitely eliminate the seaweed.)  You must also immediately STOP Mungo's behavior toward Toby during mealtime.  This will require they be fed in separate rooms and a period of time elapse between the end of mealtime and their renewed interaction.  The third thing you must do is learn to read dog body language; it's quite important for you to be able to "read" what Mungo and Toby are ACTUALLY communicating (eye movement, body movement, tail and ear set, etc.).  Without actual in person observation and evaluation, I cannot determine temperament between Mungo and Toby, but I can encourage you TO LEARN HOW TO DO THIS.  Also, in Britain there is the Association of Pet Behaviour Counselors, originally begun by the late (and great) John  Fisher.  These are credentialed experts fully able to assist you in preventing this problem from developing any further.  This CAN be solved.  I suspect your Greyhound is not quite the retiring prince you consider him to be and I strongly suggest you acquire the assistance of a professional, so that you all can go forward living together in harmony.