The Dog Channel says that broccoli is an excellent vegetable to feed to dogs. It should be steamed or microwaved before feeding because dogs have difficulty digesting raw vegetables. One or two bite-sized portions is the appropriate serving size for a dog.
Trupanion suggests cutting vegetables such as steamed broccoli into pea-sized pieces to use as low-calorie dog treats or snacks. Most vegetables are fine to feed to dogs, but onions and garlic must be avoided because they are toxic to dogs. Trupanion also recommends not feeding grapes, rhubarb, mushrooms, corn or citrus fruits to dogs. Grapes, mushrooms and rhubarb can be toxic, and citrus fruits can cause stomach upset. Corn is a common allergen among dogs.