Which type of royal canin? I tend not to think much of them because their marketing of food for specific breeds of dogs is kind of laughable. Not in theory, but in practice. They make a big deal over some ingredients in their boxer formulation, for example, that are supposed to prevent cardiac disease, but the fact is that the percentage of dogs who get that form of cardiac disease based on a deficiency of the ingredient they add is almost statistically insignificant compared to those for whom it's a totally different disease process, so to me they're marketing to people's ignorance of science. Plus some of their other information is wrong, so they haven't done their homework in my breed, at any rate.
Here are the first 19 ingredients from the Puppy small breed food on their website:
Chicken meal, brown rice, chicken, corn gluten meal, chicken fat, brewers rice, dried egg powder, natural chicken flavor, dried beet pulp (sugar removed), anchovy oil, brewer's yeast, potassium chloride, sodium silico aluminate, flaxseed, salt, fructo-oligosaccharides, rice hulls,
Lots of grains in there.
Here are the first 19 ingredients for the food I feed (Artemis)
Chicken, Turkey, Chicken Meal, Turkey Meal, Brown Rice, Pearled Barley, Lamb Meal, Oatmeal, Chicken Fat (Preserved with Mixed Tocopherois), Egg Product, Salmon, Natural Chicken Flavor, Canola Oil, Flaxseed, Potatoes, Carrots, Peas, Apples, Cranberries,
I think there are far worse foods than Royal Canin, but you might be able to do better. I don't have a small breed, so their nutritional needs may be different, but >30% protein would have my dogs running off the wall (I understand terriers are different -- I just don't know if they NEED more protein or whether protein makes them worse). I'd check out breeder websites and the Wheaten Terrier club webpage and see if there's breed-specific nutrition recommendations out there. There may be good info on levels of protein, etc. that are ideal.