Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu
The last two years (2021 and 2022) have been relatively dry over the growing season, with 2018 being the wettest recent year (Figure 1). Across the state we accumulated similar amounts of rainfall (Figure 2), but very dry periods in Georgetown were evident across August, when most grain fill is taking place.
Figure 1. Average Delaware cumulative rainfall over the last five years.
Figure 2. Average cumulative rainfall at three 2022 locations.
Grain fill is not the only likely time when stress may have occurred, as pollination may have also occurred during times of high temperatures and low soil moisture. Corn becomes stressed when daytime temperatures are above 86°F, which occurred several times between May and September (Figure 3). It should be noted that Figure 3 represents maximum and minimum temperatures, and that day-time temperatures did not necessarily sustain temperatures above 90°F all day long.
Figure 3. Average min and max statewide temperatures in 2022.
If you planted corn in Newark on April 15th, pollination would have occurred around July 11th versus planting on May 15th, when pollination would have occurred around July 16th. Due to cooler weather in April, growing degree days accumulate slower leading to a week’s difference in growth stages, even when they were planted a month apart. There was a smaller window of cooler weather around July 11th, and soil moisture was still adequate (Figure 4) in northern Delaware. However corn pollinating at the end of the week may have seen higher temperatures.
Figure 4. Soil water content at three DEOS stations in 2022.
In the southern end of the state, planting April 15th should have led to pollination around July 5th, while planting mid-May pushed back pollination to July 11th. While this was also a warm period, it was not as hot as late July when June planted corn may have been pollinating.
The problem for southern Delaware was the spotty rainfall and lack of soil moisture, which is evident in Figure 4. As measured by our DEOS station in Georgetown, volumetric water content was very low the entire summer, so that any fields without irrigation should have been extremely stressed (due to finer texture, Newark soils will show higher water content than Kent and Sussex counties). Additionally, during July, nighttime temperatures were often above 70°F, increasing corn respiration and using up any energy stored from the sun. While the cooler nights in early April would have helped with grain fill, it is likely drought conditions and heat had already limited any yield potential. These graphs cannot cover all the conditions observed across the state but can give you a general idea of where stress may have reduced yields compared to the past few years.