Across most of Iowa, corn is near V6 stage and much of Iowa is under some degree of drought stress. The exception being extreme southeast Iowa. Being at V6 stage means corn ear initiation is occurring and in the coming week the number of kernel rows on the ear will be determined. This is the second most critical time of yield determination with the first most important timeframe being the 2 weeks before and after pollination.
Using central Iowa to illustrate the impact of this drought stress, we used the APSIM model for a central Iowa field to provide some insights into the impact of these conditions on yield potential and subsoil moisture status. Roots are estimated to have exceeded 1.5 feet depth and are growing fast (~1.1 in/day). Soil moisture is low compared to historical years. Major findings include:
- There is enough moisture in the subsoil (below 1 feet) for the roots to take up water, but the root system is not yet well developed to extract the full amount of soil moisture needed. Some stress is expected with leaf rolling becoming apparent in many fields.
- The deeper the soil layer, the more soil moisture. In most cases soil moisture is below field capacity. In a normal year, the soil moisture at a 3 feet soil depth is above field capacity. Check the FACTS soil condition for more info on soil moisture by depth.
- The most troubling finding is the depth to water table (Figure 1) that has dropped substantially compared to the past years. This implies central Iowa soils have lost their “drought” buffering capacity this year.
- Scenario based yield prediction as of June 12, indicate a wide range of possibilities (from 100 to 260 bu/ac, Figure 2). The weather over the next couple weeks, as kernel development continues, will dictate how severe the early season drought conditions will be.
Figure 1. Simulated depth to the water table with actual weather data until June 12, 2021 (black line) and using historical weather years (blue, green and red lines) after that. Gray line is typical water table depth for mid-June.
Figure 2. Predicted 2021 corn yields for four weather scenarios. The horizontal broken line indicates the 2021 yield potential for a 111 RM maize hybrids planted May 1st in central Iowa. The scenarios are based on weather conditions up to June 12, 2021 and the corresponding year’s weather after June 12.