It is quite possible to have a high yielding variety that isn't GM. In the absence of insects, weeds, drought or some other environmental challenge, GM varieties don't necessarily yield any better, and if overall genetics are better in a non-GM variety, it will produce a higher yield.
One therefore needs to know how the test was done. For example if there were no corn borers, no weeds, and no root worms there would be no need for GM corn. If there was a drought and they didn't water then a non-GM drought resistant type would easily win. As is the case with all research, the devil is in the details.
In this case, the question refers to the results of what is called our Variety Testing Program where seed companies pay to enter their varieties in common field trials conducted on anywhere from 3 to 12 farmers’ fields throughout the state of Illinois, to see how they compare with other varieties. The results from any given year are certainly appropriate to use in a marketing sense, but would not be considered scientifically robust unless trends are viewed over multiple years.
The data presented would indicate that the 17 non-GM hybrids entered into the study show the same range of yields as the 86 GM hybrids entered into three locations in the North Illinois region. What isn’t included in the report is the costs to the farmer to produce that yield, or the quality of the grain produced, as GM corn shows clear reductions in contamination by fungal pathogens that lead to moldy ears.
The North Illinois location in 2013 was a “high yield” environment, where the tillage, soil insecticide, fertilizer, and herbicide treatments, in combination with good rainfall throughout the season, resulted in relatively little stress on the plants. Thus, the trial conditions made it more difficult to observe the advantages that GM traits are designed to provide.
So when you see research like this, it’s wise to ask questions. This research provides part of a large puzzle, but ultimately farmers make the call when it comes to what seed is right for their fields and their business. Thanks for posting this question – curiosity like yours gives us a chance to elaborate and clarify.