Old-timers said on hot July nights you could hear the corn grow. Later, farmers said they swore corn grew an inch a day in July. Modern technology has moved those time periods into late June in most cases. The fact is that corn does grow quickly once it reaches a certain growth stage. And there is basis for it. Scientists have determined that corn produces leaves after the 10th lead stage, or V10, with about half the heat unit requirements needed to go from one leaf stage to another from emergence until the 10th leaf stage.
Heat units are measured through a system called Growing Degree Days. For corn, 50 is the minimum and 86 the maximum. The formula assumes that corn doesn't grow at temperatures below 50 or above 86 degrees. In other words, readings lower than 50 don't detract, but readings above 86 don't add to corn growth. Heat units are cumulative, adding day to day. In most parts of the Corn Belt they started accumulating faster than usual this year, with a warm March, and have continued to accumulate at a fast pace. What matters to the corn plant is how many growing degree days accumulate after plan ting.
According to Purdue University's Corn & Soybean Field Guide, published annually by the Purdue Diagnostic Training Center, from emergence to V10, it takes 82 GDD heat units to produce a leaf. However, from V10 forward, it only takes about 50 GDDs per leaf.
The formula is so accurate that sometimes crop insurance agents or agronomists use the requirements per leaf to go backwards or forwards, and figure out where corn was in size on a certain date, or will it should be on a future date, assuming average GDD accumulation. For example, if the crop emerged May 5, and 535 GDDs accumulated, you know it would be at the six-to-seven leaf stage without ever visiting the field. The growing point goes above the ground at about the fifth to sixth leaf stage.