By Clarke McGrath
Corn typically requires 90 to 120 Growing Degree Days (GDD) from planting to emergence. Of course this GDD range assumes adequate soil moisture and varies with planting depth, tillage system and residue cover. As a rule of thumb, if 120 GDD have accumulated since planting and seedlings haven’t emerged, check the condition of planted seed ASAP. For example, in west central Iowa we have had around 120 GDD’s since April 11th. Some acres were planted the week prior and had just a few more GDD’s, and then a lot of corn went in from the 10th on. According to GDD’s much of this corn should be spiking soon, so in between showers this week would be a good time to take a look and see how it is progressing.
FYI: As soil temperatures get to around 60* (which we just touched over the weekend in my part of Iowa finally), corn comes up in about 10 days, which is hopefully where the corn we plant in the next week or two will end up.
You can track GDD accumulations for the Corn Belt location of your choice by hitting your fields of choice on the map at this interactive site-
https://mygeohub.org/groups/u2u/gdd
There are also good free apps to track GDD’s, check your respective app store to see what you like.
You need to remember that GDD’s are calculated based on air temperatures using the 86/50 method typical for corn production. Using that method, if air temperatures remain at or below 50°F, emergence will not occur. We hope this isn’t a continuing issue now that we are heading into late April, although our forecast for this week is pretty hideous; starting Wednesday we are projected to get six days in a row of highs in the mid 50’s to low 60’s and lows in the mid to upper 40’s. It will be especially critical to check corn emergence and seedling health after we get through that stretch and corn starts to have significant growth again.
Since GDD calculations are based on air temperatures, four-inch soil temperatures may actually better predict seedling emergence than accumulated GDD’s. The Mesonet provides a daily update of both the Iowa soil temperature and GDD, you can find the info here:
https://mesonet.agron.iastate.edu/agclimate/#tmpf
Lab studies have shown that for most corn hybrids grown in the Midwest, seedling emergence is about three weeks if the soil temperature is 51°F and is about one week if the daily soil temperature holds near 70°F.
If the soil temperature is averaging 50 to 55°F (10-12.8 C) at the time of planting, corn may take three weeks to emerge. Temperatures averaging 60°F (15.6 C), may have emergence in 10 days to 12 days. Soybean emergence usually requires that soils be about 10 degrees warmer than for corn although soybean does begin to respond at 50°F.