Forecasted corn yield potential and attainable yields
At this point in the season, a larger proportion of Kansas corn is at the reproductive stages than at this time last year. The most recent Kansas Agricultural Statistics Service crop progress report (July 21) projected that almost 75% of the Kansas’ corn crop is at the silking stage and 22% of the crop is already at the dough stage, well ahead from last year. Overall, close to 50% of the corn crop in Kansas was classified by the USDA as good and 15% as excellent. Pollination conditions around the state were very good for most of the corn planted before May 1 and the potential for high yields (close to 200 bu/acre) seemed attainable a couple of weeks ago. However, weather conditions have changed over the past week, with less rain and heat -- exerting a certain degree of stress and imposing uncertainty about the conditions until harvest time.
Potential corn yield estimation
Estimation of potential corn yields can help to understand the maximum yield attainable if management is optimal and in absence of unmanageable adversities, such as hail or flooding. A research team based at the University of Nebraska and The Water for Food Institute (professors Patricio Grassini, Roger Elmore, Haishun Yang, and Ken Cassman) is leading a project for forecasting corn yield using historical and current weather and management information.
The corn simulation model -- Hybrid-Maize Model (http://hybridmaize.unl.edu) -- was developed by researchers in the Agronomy and Horticulture Department at UNL and takes into consideration several factors such as weather, plant population, hybrid relative maturity, planting date, and soil type, among other factors. The model assumes optimal management, with no limitation imposed by nutrients or biotic factors (weeds, insect pests, pathogens) and no adversities such as flooding and hail. Therefore, the values depicted by the model provide an overall guideline of the maximum yield attainable if management is near optimal. Likewise, the model does not account for yield losses due to large kernel abortion that results from severe heat and water stress during pollination. The “yield gap” between the value predicted by the model and the harvested yield will increase if management was sub-optimal or there were other adverse factors not accounted by the model that may reduce corn yield.
Simulations can be performed to forecast current-season corn yields. Factors such as site-specific weather conditions from planting until the simulation date and historical weather information to simulate the rest of the 2014 growing season are used for the simulation. Myriad yield scenarios could be produced depending on the growing conditions from the simulation date until harvesting time, but forecasts are more accurate and reliable as the simulation time approaches corn maturity.
Simulation results for Kansas
For Kansas, the estimation of corn yields for the current growing season was performed at five different locations around the state (Fig. 1). Sites include Garden City, Hutchinson, Silver Lake, Manhattan, and Scandia. A separate yield forecast was performed for irrigated and dryland corn for Scandia, while only irrigated crops were simulated at Garden City and Silver Lake. Only rainfed corn was simulated for Manhattan and Hutchinson.
Daily weather data used for simulating these locations were retrieved from the High Plains Regional Climate Center (HPRCC http://www.hprcc.unl.edu/). For each location, local agronomists provided information about soil properties and crop management (hybrid maturity, plant populations, and historical and 2014 planting dates) required for the simulations. The following agronomists should be properly acknowledged for investing their time and providing their expertise: Eric Adee, Agronomist-in charge, Kansas River Valley Experimental Research Field, Topeka; Gary Cramer, Agronomist-in charge, South Central Kansas Experimental Field, Hutchinson; and John Holman, Southwest Research-Extension Center Cropping Systems Agronomist, Garden City.
Forecasted corn yield potential (“Yp” in Table 1) was calculated first as long-term yield potential, based on 25+ years of weather data. The model then calculated 2014 forecasted yield potential, utilizing current-season weather. The 2014 forecasted yield potential is presented under favorable (25%), average (50%), and unfavorable (75%) weather scenarios from now until crop maturity.
At almost all sites simulated in Kansas, there is a 75% probability of achieving above-normal irrigated and rainfed corn yields this year.
Under irrigated conditions, the median estimated yield for 2014 is forecast to be 20+ bushels per acre higher than the long-term average from 25+ years of weather data. An exception is at Garden City where the forecasted 2014 yields under median conditions is only 10 bu/acre higher than the yield using long-term averages.
Under rainfed conditions, a similar benefit of 20+ bushels per acre is forecast at all locations compared to the long-term average. However, if the conditions until harvesting worsen, the forecasted yield advantage will narrow to 10+ bushel per acre for 2014 as compared with the long-term average. Still, it should be emphasized that forecasted yield for corn regardless of the weather scenario is showing some promising yield expectation for this growing season.
Figure 1. Locations utilized for simulation purposes for Kansas.
Table 1. 2014 In-season Yield Potential Forecasts for Kansas.
Attainable yield and yield-limiting factors
The final attainable yield will be ultimately defined by the growing conditions from silking until harvesting. Thus, the forecasted yield potential can differ from the final attainable yield if the following “yield-limiting factors” occur in the coming weeks:
- Abiotic stress (e.g., heat and drought conditions): Stress during pollination can increase asynchrony between pollen shed and silk extrusion; increasing the probability of poor fertilization and a reduction in final kernel number. A visible symptom for the lack of effective pollination is when silks are green and keep elongating (“long silks issue”). This occurs when silks have not encountered pollen and ovules are not fertilized, resulting in a lack of kernel formation (Fig. 2).
Figure 2. Lack of effective ovule fertilization in corn due to combined heat + drought stresses at pollination. Photos by Ignacio Ciampitti, K-State Research and Extension.
Post-silking kernel abortion can be related to insufficient water supply and heat conditions. Corn is actually more affected at this stage of development by a lack of variation between day and night temperatures than high heat alone. Also, corn has a high demand for water during pollination and grain-filling processes. An example of kernel abortion occurring at different stages in corn can be seen in Figure 3.
Figure 3. Kernel abortion process in corn affected by combined heat + drought stresses, before and after pollination. Photos by Ignacio Ciampitti, K-State Research and Extension.
- Biotic stresses (e.g., insect and diseases): Insect damage and foliar diseases can severely impact attainable yield. In Saline Co., the presence of stinkbugs early during the ear elongation process affected the final number of kernels due to a restriction in cob growth and elongation on the side that those insects were feeding from. The consequence of this infestation was fewer number of kernels in banana-shaped and exposed ears (the ear is outgrowing the husk). In addition, the ears affected by this insect damage are more susceptible to weather and pests in general (Fig. 4).
Figure 4. Kernel abortion process in corn affected by insect damage early during ear development. Photos by Garrett Kennedy, Pioneer Hi-Bred International.
For Kansas, yield forecasts from 5 locations indicate above-average yield potential for well-managed dryland and irrigated corn. Yield forecasts can go down (represented by the 75% scenario in Table 1) if stress conditions are evident during the coming weeks until maturity. However, if adequate rainfall and moderate temperatures resume through the end of July and early to mid-August, we the forecasted yield can go up (represented by the 25% scenario in Table 1).
Related to the growth stages, past experience shows that when corn is in the reproductive stages, biotic or abiotic stress conditions (e.g. high temperature, drought, pests, hailstorm, etc.) can exert high impact on yields due to the effect on final kernel number and kernel weight -- reducing the final number of grains and/or shortening the dry matter accumulation period. Thus, there is still a portion of the yield that remains to be determined in the coming weeks in many fields. Information from research experiments showed that short periods of drought stress (4-7 days) during early reproductive stages could reduce yields close to 50%. Later in the reproductive stage, stress has less impact on yields.
The most important task from this point to the end of the season is to scout the fields for the presence of biotic and abiotic stress conditions for deciding, and determine what steps can be taken to protect the potential yield expected for this corn season.
You can find the full paper related to forecasted yields in 25 locations around the Corn Belt (prepared by UNL faculty), at:
Ignacio Ciampitti, Cropping Systems and Crop Production Specialist