Corn Emergence And Heat Unit Accumulation

 

Warm, dry weather promoted significant corn planting last week, especially in western Ohio. According to USDA/NASS estimates (https://www.nass.usda.gov/Statistics_by_State/Ohio/Publications/Crop_Progress_&_Condition/2017/cw1817oh.pdf)

as of April 30, 42 percent of the corn crop in Ohio has been planted. However, much cooler temperatures forecast this week may slow germination and emergence of these late April plantings.

 

Corn typically requires 100 to 120 growing degree days (GDDs) to emerge (but emergence requirements can vary from 90 to150 GDDs). To determine daily GDD accumulation, calculate the average daily air 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, but if the low temperature is less than 50 degrees, use 50 degrees as the low in the formula. Similarly, if the high 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 so it does not continue to slow as temperatures drop further.

 

If it takes a corn hybrid 110 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 11 days (110 GDDs to emerge/10 GDDs per day = 11 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 more than 3 weeks (110 GDDs to emerge/5 GDDs per day = 22 days) for corn to emerge. In past years, corn planted in mid to late April has taken as much as 3 to 4 weeks to emerge in many fields.

 

Temperatures at or below 50 °F may also impact final plant stands, especially when there is protracted period of low temperatures following planting. When such conditions occur, stand loss is usually greater on heavier and poorly drained soils.

 

Given the relationship between GDD accumulation and emergence, we should not be too surprised that it sometimes takes early planted corn up to 3 or more weeks to emerge. Seedling emergence is dependent on soil temperature and air temperature. Also, keep in mind that estimates of emergence based on GDDs are approximate and can be influenced by various factors including residue cover, tillage, planting depth, hybrid differences, and soil organic matter (soil "color") and moisture content.