Given an understanding of corn leaf stage and heat unit calculation, a grower can estimate what leaf stage of development a particular field is at, says Ohio State University Extension.

Being able to predict when a field of corn will reach particular leaf stages can be useful for scheduling post-emergence applications of certain herbicides and sidedress N fertilizer, especially if your farming operation is so large that regular field inspections are difficult to work into your busy schedule. Leaf stage can be predicted on the basis of heat unit accumulation (aka growing degree days) from planting to the date in question.