I see your curiosity here, and I share your interest. We have investigated GM maize, and our studies demonstrated a different observation than you had. The basic design of these studies is a comparison between livestock fed diets containing GM or non-GM maize. We use performance indicators, like overall weight, average daily gain and milk production in the case of dairy cows, to gauge the nutritional equivalence of the GM and non-GM feed. These performance indicators also are a good measure of palatability, because if the animals don't like the feed, they won't eat as well and then wouldn't be as healthy or perform as well. In the numerous controlled feeding studies we've conducted, and in those conducted by others, no health or performance differences have been detected. (Here are two examples of feeding studies in dairy cattle and beef cattle).
We believe following the scientific method is the best way to ask and then answer questions like this one. This method includes making observations, and designing and conducting experiments, in a manner that minimizes the potential for bias. It is important for the experiment to be a fair test, changing only one factor and keeping all other conditions the same. The interpretation of the outcome has to be something that can be supported through carefully crafted experimentation and observation. Knowing that personal and cultural beliefs influence both our perceptions and our interpretations, we must use standard procedures and criteria to help minimize those influences.