If I understand what you are saying.... I set of soil tests would confirm this - at least in my experience.  Pull a set of soil cores in the area where the old pasture is, and a second set of cores in a similar area on the other side of the side, and compare.  An old pasture would show up with higher soil OM level, and most likely higher P & K levels as well.   This would be easy enough to do to make this confirmation.

edit:  to the OP....looking at the pics more.... if you have not done so, I would pull leaf tissue samples. Try to pull some of the healthier ear leaf from the damaged part of the field; and pull some ear leaf samples from the field section that looks green.  I would do this just to rule out nutrition issue as a possible cause.  I would do plant tissue sample in conjunction with soil samples.  

I was wondering if you had taken plant tissue samples earlier in the season in this field.  

We should not rule out the idea that multiple issues might be going on this field.