I have received several calls on this in the past couple days. We are seeing a lot of fields around the country with random pale green to yellow plants scattered across the field. Of course we could think of micronutrients, but those generally show a show as striped corn leaves or portions of leaves being yellow. Some corn varieties also have an occasional genetic mutation, but those are generally white and less broad spread. The cause for this yellow corn is called rapid growth syndrome.
Note the picture above (taken just north of Estelline, SD this morning) where the upper leaves are yellow and the leaves just below are twisted. The upper leaves were "trapped" until recently. The condition results when growing conditions change and corn shoots out a burst of growth. New leaves try to expand faster than the outer whorl can open up. Because the leaves could not develop normally and exit the whorl, when they finally popped loose they were chlorotic green to yellow. Late applications of certain herbicides can also mimic this effect by tightening the whorl.
Nothing in particular can be done, and it certainly does not affect all fields or hybrids. These leaves will generally green up in a short amount of time with exposure to sun. Twisted leaves will remain wrinkled and misshapen throughout the season. Maintain good plant nutrition and be watching for the development of diseases that can be controlled by fungicides later in the season.