As this snake was in a cornfield, it was probably looking for food and will not have been a threat to you at all.
The gray rat snake usually preys on vermin or birds for its food, so looking at the fact that you saw it in a corn field, it was looking in a good place for the kind of food it would like.
The reason that is it likely to have been looking for food is that the gray rat snake tends to spend its time in trees or forests and can get quite high up in a tree so that it can stay away from any of its predators.
The appearance of the gray rat snake is a great way to identify it as, unlike many other snakes, it does not change its appearance from when it is born and the gray colour and spots will stay with it for its entire life. The gray rat snake is usually between three and six foot long, which is quite big for a snake.
As the gray rat snake will cause you no harm, there is no need to approach it or attack it. It is acceptable by law in all states apart from Georgia to kill a snake but as it will cause you no harm, you should let it get on with its day and get on with yours too, after the initial shock of course.