In-season forecast of soil water-nitrogen and corn-soybean yields

The corn and soybean crops are halfway through their life cycle and are currently in the most critical phase of growth. Final yields will be determined by a combination of soil-plant processes that are highly affected by the prevailing climatic conditions in August, existing status of the soil water and nitrogen reserves, biomass production and N uptake to date, and biotic factors such as insect and disease pressure.

This article is an update of the status of the soil water-nitrogen and the expected corn-soybean yields at the end of the season for eight cropping systems in Iowa. The data provided here and the analysis are part of the pilot Yield Forecast project that is coordinated by the ISU. Briefly, this project combines the use of the Agricultural Production Systems sIMulator (APSIM) cropping systems model, the Weather Research and Forecast Model (WRF), and parallel in-field data collection to provide in-season systems-level forecasts. This pilot project focuses on two locations (central and northwest IA), two crops (corn and soybean), and two planting dates (early and late planting). Additional information can be found in a June 17, 2015 ICM News article.

Results from the July 28 forecast show that soil water reserves have substantially benefited from the rainfall events (~3 inches of water) on July 27 and 28 (Table 1). The soil nitrogen status for central Iowa is low but reasonably low given that the crops have taken up significant amounts of N to date. The model analysis currently shows no N stress. In contrast, there is a risk of water stress at the Northwest Iowa location.

Over the next 10 days, corn will accumulate dry mass at a rate of 330 lbs/acre/day and soybean at a rate of 180 lbs/acre/day (Table 2). The lower crop growth rate for soybean compared to corn is due to the C3 versus C4 photosynthetic pathway and the cost of energy-rich proteins and oils in soybean. Water and N needs by the crops are currently at peak usage rates because the crops have to build new biomass (mostly grain) and at the same time sustain existing biomass (mostly vegetative). Overall, soil water and nitrogen reserves together with N supply from organic matter mineralization will satisfy crop needs without major stress over the next 10 days.

Compared to the first forecast (June 12), the range of uncertainty (difference between 10% and 90% probability) has decreased. In this particular year, the 50% probability (median) is close to the 10% probability (best case scenario) for the corn in central Iowa (Fig. 1). This reflects excellent growing conditions to date. The 90% probability reflects the minimum yield that can be achieved (worst case scenario). The 90% probability for corn in Sutherland is lower because of the water stress conditions that occurred in the mid to late vegetative stages. Soybean yields are similar between locations and planting dates, with levels above 55 bu/acre.

The corn and soybean crops are in at a good status to date (July 28) and have the potential to reach high yield levels, about 4 to 17% higher compared to the historical yields (Fig. 2). Much of the yield increase can be attributed to recent rainfall events that occurred during critical periods of development for both crops. To highlight the importance of this particular precipitation event, a model analysis was conducted to predict yields without the rainfall on July 26-28 (Fig. 2). Corn yields in Sutherland benefited the most from nearly 3-inches of rainfall with a change in predicted yield from –28% to +4% compared to the historical average.

The forecast provided by this pilot project is valid at the time the forecast was completed (July 28, 2015). Additional forecasts will be made throughout the growing season to update the information provided in this ICM Newsletter article.