If you have driven around Putnam County this past week, you have likely noticed some significant changes in the fields. Corn that was planted nearly three weeks ago can finally be seen poking out of the ground. Have you ever wondered why corn emerges shortly after planting in some years while in other years it may take up to 3 or 4 weeks to emerge? This difference in timing is due to differences in temperature from year to year following planting. Corn seeds generally need to accumulate 100-120 growing degree day units (GGD) of heat before they will germinate. This number depends upon the variety of corn grown and can vary from 90 to 150 GDD.
Science has shown that corn will grow when the temperature is between 50 and 86°F and little growth occurs when temperatures are above or below this range. By knowing the outdoor daily low and high temperature, we can calculate GDD for that day. Adding or accumulating GDD over time provides information that is useful in predicting crop growth and development, as well as insect and disease activity.
So how does one calculate GDD? Farmers and gardeners alike can calculate the daily GDD by taking the average daily air temperature (high temperature + low temperature)/2 and subtracting the base temperature of 50°F for corn. When the daily low temperature is more than 50°F, and the high temperature is 86°F or less, then use the actual temperatures to calculate GDD. However, if the low temperature is less than 50°F, then 50°F will be used as the low temperature. Likewise, if the high temperature is above 86°F, then 86°F will be used as the high temperature.
Using two examples from this past week: On May 12, the high and low temperatures were 63°F/46°F. Because 46°F is lower than corn’s base temperature of 50°F, we use 50°F in the calculation. GDD for May 12 = (63-50)/2 = 3.5 GDD. On May 13, the high and low temperatures were 71°F/43°F. The GDD for May 13 were (71-50)/2=10.5 GDD. Over these two days, corn accumulated 14 GDD. By using this calculation, we can estimate when corn varieties will reach various stages of development, including tasseling and maturity.
To keep up with the many issues and developments related to agronomic crops across Ohio, subscribe to the OSU Crop Observation and Recommendation Network (C.O.R.N.) newsletter published weekly during the growing season. Go to https://agcrops.osu.edu/newsletter/corn-newsletter and enter your email to subscribe, or contact the extension office and we can subscribe you as well. For more information, contact the Putnam County Extenstion office at 419-523-6294, by email at firstname.lastname@example.org, or stop in at 1206 East Second Street in Ottawa. You can also find us on Facebook by searching for OSU Extension Putnam County.