Extension Update by Jenny Rees

Emergence and Growing Degree Days: For this week’s column, I’m going to share information my colleague, Nathan Mueller, Extension Educator in Saline, Gage, and Jefferson counties wrote in his recent blog post. “Many factors affect corn growth and development, especially early in the growing season. A common question this time of year after corn is planted and some fields have emerged whiles other have not is “How many Growing Degree Days (GDD) does it take for my corn to emerge?” Since corn emergence is directly related to soil temperature (and of course soil moisture), the days to emergence vary especially when one compares early planting dates to later planting dates. The general assumption is 120 Growing Degree Days abbreviated GDD for corn to emerge under favorable conditions. However, we know that some planting practices and environmental conditions can decrease or increase the amount of GDD needed for corn to emerge. We use the GDD calculation for air temperature to estimate how long it will take corn to emerge even though soil temperature is the driving factor.

Growing Degree Days (GDD) or Growing Degree Units (GDU) calculation is determined from air temperature. The corn equation for GDD or GDU = (Daily Maximum Air Temperature + Daily Minimum Temperature)/2 – 50. When the maximum air temperature is greater than 86 degrees, we set the value at 86 in the equation, as the growth rate of corn does not increase much beyond 86. Likewise, when minimum air temperature is less than 50 degrees, we set the value equal to 50 in the equation. The sum of daily GDD or cumulative GDD for corn emergence is approximately 90 to 120 under favorable conditions. As a base line for GDD required for corn emergence, colleagues at the University of Wisconsin report that 125 GDD are required for emergence. Based on research in Iowa, corn typically required 90 to 120 GDD from planting to emergence. This range assumes adequate soil moisture and will vary with planting depth, tillage system, and residue cover.

Research shows some adjustments are needed to help fine tune expected emergence dates based on GDD determined from air temperature. Planting practices that change the amount of GDD for corn to emerge include planting date, depth, and residue cover (view full table of variables at https://cropwatch.unl.edu/growing-degree-units-and-corn-emergence). It takes about 10-25 more accumulated GDD for emergence with early planting dates. Planting deeper than 2 inches will increase the number of GDD to emergence by about 15. More than 75% residue cover increases the accumulated GDD needed for emergence, ranging from 30 to 60 GDD more. Additionally, the soil moisture, soil condition, and soil texture change the needed GDD for corn to emerge. Dry seedbed conditions will require more GDD. Crusted or cloddy soils can increase GDD by 30 more. Heavy textured soils require more GDD than do coarse textured soils. Corn genetics also can affect GDD needed for emergence. Therefore, the amount of accumulated GDD from planting to corn emergence can range easily range from 90 to 200 GDD.

In Nebraska, the U2U tool (https://hprcc.unl.edu/gdd.php) can be used to determine local accumulated GDD based on your planting date. For example, at the tri-county corner of Saline, Jefferson, and Gage counties from May 1 to May 13, we accumulated 114 GDD and the 30-year average is 139. In summary, remember that numerous factors drive corn emergence and assuming a standard 120 Growing Degree Days (GDD) for corn to emerge will not always hold true.”

Tree seeds and leafing out: I’ve been watching silver/red maples and ash trees noticing that some, including one of mine, is very heavy in seed production like what we experienced in 2019. There’s also quite a range in oaks with some leafing out normally and others leafing out rather slowly. I think the seed production possibly is due to the warm March. Information from Ohio State shared that, “Every spring, maple trees produce small flowers that turn into seeds. Normally, a cold frost kills some blossoms, but this year the usual chill didn’t arrive at the right time. More blossoms than usual turned to seed.” Oaks leafing out at different rates could be due to the fact we’ve had a cool April/May and it’s also a survival mechanism to not all leaf out at once. We’ve also been experiencing some oak decline (which is also observed in August when leaves prematurely turn brown), and this can also result in slower leafing out. These are just some thoughts; I really don’t know the answers, just sharing that for those who are asking, I’m also observing this.