Wheat Whole wheat is often used in some of the better brands. While there is nothing inherently wrong with wheat, some animals are allergic to it, or develop allergies to it after prolonged feeding, so it's probably a good idea to avoid it altogether. Soy Soybeans are difficult to digest even when cooked. You'll note that most human soy-based products are fermented in some form or another, and our systems are supposed to be able to digest food from plants. Soy is the number one allergy cause in pet foods.
Wheat is second.
Other grains Rice, oatmeal, and barley are fairly common in the better foods. These are excellent sources of carbohydrates. Be sure these are in whole form, especially the rice portion. Look for whole brown rice listed as such. Most of us have learned to avoid BHT, BHA, etc.