Corn made its debut in dog foods during
the 1950's when the kibbling process was
first developed by the Ralston Purina
Company. They borrowed an extruder
from their Chex cereal facility, put it to use
making dog food, and gave rise to new era
in pet food.
The kibbling process they developed requires the dough to contain a large quantity of starch, otherwise the mixture won't gelanitize. The starchy carbohydrates (potatoes, corn, and other grains) were prime candidates. Corn received first billing in kibble because it was relative cheap and there was a lot of it to be had.
Today, most of the cheaper dog foods and many of the more expensive ones feature corn as an ingredient--a fact that has caused innumerable debates between dog food companies, dog trainers/breeders, and veterinarians.
The lines are clearly drawn. The pro-corn side of this controversy will tell you corn is a wonderful ingredient with a low glycemic index, oodles of wonderful minerals and vitamins, and also serves as an exceptional source of protein. The anti-corn side argues corn is a cheap and useless filler that causes multitudes of health and behavior issues.
So, is corn a super food? Or is it one step from toxicity?
Corn is no better or worse than other carbohydrates in terms of its glycemic index, mineral content, or nutrient balance.
The most obvious problem with corn is easily measured by counting the number of piles you clean up from your lawn. Corn is high in fiber and is also very difficult for dogs to digest unless it's been thoroughly processed (which unfortunately increases the glycemic index). What is not digested properly will promptly be deposited in your backyard.
While corn possesses a relatively high level of protein for a carbohydrate, it has nowhere near enough to replace meat as the primary ingredient. It also lacks certain necessary amino acids.
For instance, when compared to other sources of protein such as chicken or salmon, corn has particularly low levels of tryptophan, which is a precursor to seratonin. Therefore, when corn serves as the primary protein source (i.e. in the top three ingredients), it can have a distinctly negative impact on animals sensitive to serotonergic under activity. Seratonin levels impact mood regulation, appetite, sleep, muscle contraction, and even cognitive functions such as memory and learning.
One last thing to take into consideration: fats are the best source of energy for your dog. As a carbohydrate, corn is mostly fat-free. Carbohydrates provide intense bursts of energy (think 'sugar buzz') whereas fats provide a more moderate steady stream of energy throughout the day.
In Summary. . .
Corn is not a 'bad' ingredient, but it certainly should not be the main ingredient. Feeding a corn-based diet can potentially lead to a moody dog with an irregular appetite, large bowel movements, insomnia, aggression issues, sporadic energy bursts, and a short memory. This results in a pet who can be very challenging to live with!