Factors Contributing to Slower Drydown

Several growers are noticing their crops are behind from previous years and the drydown is slower than normal. There are a few factors playing into this, but the main driving factor in plant growth and development is temperature, which is commonly measured in growing degree units (GDU). GDUs, also called growing degree days (GDD) and heat units (HU), are the calculated accumulation of heat on a daily basis. The corn plant is very dependent on temperature to move through its growth stages. The warmer the temperature (to a max of 86°F) the faster a corn plant develops. When we experience cooler temperatures, the growth slows and plant development ceases below 50°F.

Figure 1 is an example of DS-5250AM™ planted in Wabash, IN on May 15. The purple line shows the average GDUs and the green line shows actual GDUs for 2023. Although actual GDUs are running slightly below the average GDU line for a May 15 planting date, it is tracking pretty close. Another factor to keep in mind is plant growth and development is limited under stress conditions. During drought or heat stress the plant is not responding to all of the accumulated heat units.

For many areas this year, drought stress showed up from about May through June. Figure 2 shows the amount of rainfall from April through June across the Dairyland footprint. Several areas received less than 9 inches total in three months which created issues for crop emergence. It was not uncommon this year to have varied emergence timings across a field or to have crops emerge two weeks or more after planting.

Factoring in the early drought conditions and varied emergence timing, we can re-run the information in Figure 1. If we delay the start date for GDUs by about 7 days and make it May 22 instead of May 15 we find that the black layer date changes from October 25 to a predicted date of November 3-10. Drydown passed that point also requires heat units which gets harder to accumulate with late fall temperatures. The drought stress coupled with varied emergence timings and slightly below average temperatures this year seemed to be the major factors pushing back plant maturity and drydown. Some have questioned if the smoke was to blame for delayed plant development and the answer is it’s not directly responsible. The smoke likely reduced photosynthesis, but plant development is mainly driven on temperature thus the main driving factor comes back to temperature.

Figure 1 was created using the Useful to Usable (U2U) tool, which is a multi-state tool that using climate data to help predict when your corn will reach certain growth stages. This tool can be found at https://mygeohub.org/groups/u2u/purdue_gdd. Start by entering your location then enter the planting date of interest. Next you can manually enter the GDUs to silking and to maturity which can be found on the product tech sheets. If you have any questions, feel free to reach out to your local DSM or agronomist.