Weather conditions over the last three weeks were far from average (Fig. 1). Across our Forecasting and Assessment of Cropping sysTemS (FACTS) locations, precipitation was 48% below average, while heat stress (defined as maximum temperature above 86oF), growth degree days (GDD), and radiation were 53%, 25%, and 19% above the long-term average. However, our northeast Iowa was the exception; it received 7 inches of rain from June 5 to June 25, while all the other sites have received less than 1 inch of rain (Fig. 1).
Figure 1. 2016 weather deviations from climatic average for the period June 5 to June 25, 2016.
Will this unusual weather impact end-of-season crop yields? Are crops sensitive during vegetative phases? Can Iowa soils offset the lack of June rain? The answers depend on the current stage of the crop and soil moisture conditions. To explore the impact of high temperature and moisture stress from June 5 to 25 on corn and soybean yields we used a mechanistic model (APSIM) and 20 combinations of sites, crop and management treatments that are currently monitored in the FACTS project.
FACTS corn yield predictions for June 23 showed (on average) an 8% yield decline from the historical average across six sites (Fig 2). The largest yield reductions were observed in southeast Iowa (~24%), followed by central Iowa (~11%), and northwest Iowa (6%). Southwest Iowa showed no yield reduction, while northeast Iowa showed a 15% yield increase from the historical average. These results are reflected in the current U.S. Drought Monitor, where drought or abnormally dry areas have been predicted to have yield reductions in our analysis.
Figure 2. Simulated corn yield predictions and scenario analysis for different sites across Iowa. Dates on x-axis refer to planting date. Asterisks next to location name indicate fields with subsurface drainage.
To separate moisture stress from high temperatures and quantify the magnitude of each on crop yield we performed a what-if scenario analysis. We found that the impact of temperature and moisture stress is site, crop, and management specific (Figs. 1–2). On average, the lack of rain had a two-fold larger impact than the high temperatures on crop yields.
What are the underling mechanisms for these responses?
Water and temperature affect many crop processes simultaneously. This includes crop phenology (crop staging and life-cycle duration), morphology (leaf area production, elongation, and senescence), canopy architecture (leaf angle and rolling), biological processes (photosynthesis and respiration), grain physiology (kernel number and grain filling rate), water balance (soil water evaporation and plant transpiration), nitrogen (N) cycling, and plant tissue N concentration. The mechanistic APSIM model accounts for the majority of the above effects.
Why was corn negatively impacted?
Our model analysis based on actual weather information until June 24 indicated that corn yield reductions were mainly caused by:
- Increased crop development rate which means less days available for growth. That was mainly caused by higher than normal temperatures.
- Reduced leaf area index which means less green area available for photosynthesis and light interception leading to low crop growth rates. That was mainly caused by water limitation.
Our model analysis did not show substantial high temperature or water limitation effects on the rate of photosynthesis or respiration. Grain number at this stage of growth (June 25) is indirectly negatively affected by the low crop growth rate.
Why were soybean yields not affected?
Soybean yields were not negatively affected by the June 2 to 25 moisture and high temperature conditions (Fig. 3). However, more favorable conditions such as more rain and less heat would have further increased soybean yields. The difference between corn and soybean lies on the growth habit. Corn is a determinate plant whereas soybean is indeterminate. The indeterminate growth habit of soybeans allows formation of new leaves during early to mid-reproductive stages that compensates for any loss during vegetative stages. Also, soybeans development rate is strongly mediated by day length (photoperiod), which may reduce the effect of high temperatures on development rate.
Figure 3. Simulated soybean yield predictions and scenario analysis for different sites across Iowa. Dates on x-axis refer to planting date. Asterisks next to location name indicate fields with subsurface drainage.
Can Iowa’s soils offset for the lack of rain?
Iowa soils can mediate but not eliminate abnormally dry or drought conditions. On average Iowa soils have a water holding capacity of about 10 inches to a six feet depth. Measurements from the FACTS project have shown that root depths are currently (June 25) at 2.5 to 3.5 feet deep and keep growing, while the groundwater table has declined in June from about four feet to six feet for central and southeast Iowa. In these sites, the surface soil is dry (0–1 ft), while the subsoil (1–2 ft) is wet and roots can make use of the moisture.
Can FACTS’ corn and soybean yield forecasts change? Yes, but we have to wait to see how the weather will develop over the next few weeks. June 26 rainfall brought relief in some areas, but more rainfall is needed to get back on track according to our scenario analysis, especially in central and southeast Iowa. For more updates and comments see the FACTS website. This analysis is valid as of June 25, 2016.
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