# Heat Unit Accumulation and Corn Emergence

By Peter Thomison OSU Extension

There have been reports of slow corn emergence in some areas and that corn planted more than two weeks ago is not yet emerging. Is this cause for concern? Not necessarily. Corn requires about 100 growing degrees days (GDDs) to emerge (emergence requirements can vary from 90 to 150 GDDs). To determine daily GDD accumulation, calculate the average daily temperature (high + low)/2 and subtract the base temperature which is 50 degrees F for corn. If the daily low temperature is above 50 degrees, and the high is 86 or less, then this calculation is performed using actual temperatures. If the low temperature is less than 50 degrees, use 50 degrees as the low in the formula. Similarly, if the high temperature is above 86 degrees, use 86 degrees in the formula. The high cutoff temperature (86 degrees F) is used because growth rates of corn do not increase above 86 degrees F. Growth at the low temperature cutoff (50 degrees F) is already near zero, so it does not continue to slow as temperatures drop further.

If it takes a corn hybrid 100 GDDs to emerge, and daily high and low temperatures average 70 and 50 degrees following planting, 10 GDDs accumulate per day, and corn should emerge in about 10 days (100 GDDs to emerge/10 GDDs per day = 10 days). However, if daily high and low temperatures are cooler, averaging 60 and 45 degrees after planting, 5 GDDs accumulate per day, and it may take nearly 3 weeks (100 GDDs to emerge/5 GDDs per day = 20 days) for corn to emerge.

Given this relationship between GDD accumulation and emergence, growers should not be too surprised if their mid to late April planted corn will require more time to emerge than later planted corn. While there have been periods since mid- April when air temperatures have been warmer than average, with maximum high temperatures above 70 degrees, soil temperatures (as measured at the 2-inch soil depth) may be considerably cooler, in the 50 to 60 degree F range. Seedling emergence is dependent on soil temperature and air temperature. Also, keep in mind that these estimates of emergence based on GDDs are approximate and can be influenced by various factors including residue cover, tillage, and soil organic matter (soil “color”) and moisture content.