When the range of possible (i.e., simulated) end-of-season yield potential for the current year is compared to the long-term average yield potential (Long-term Yp, third column from left in Tables 2 and 3), it is possible to estimate the likelihood for below-average (75%), average (median, 50%), or above-average (25%) yields.
Comparing estimated 2014 median Yp versus the long-term Yp gives the yield difference from the mean. Minus (‘-‘) and plus (‘+') signs next to the median Yp forecast (fourth column from right) indicate that median Yp forecast is well below (i.e., <-10%) the long-term Yp (minus sign) or well above (>10%) the long-term Yp (plus sign). At a given site, the 75% scenario is most likely if weather conditions are harsher than normal (for example, hot and dry weather or early killing frost) from Aug 15 until crop maturity, whereas the 25% scenario could occur if weather is more favorable (cool, adequate rainfall, no late-season frost) than is typical for this period. Therefore, for a given location, there is a 50% probability that end-of-season yield potential will fall between the 75% and 25% scenarios. Another way to interpret these odds is to note that end-of-season yield is likely to be above-average when the 75% scenario is higher than the long-term Yp. In contrast, below-average end-of-season yield is likely when the 25% yield scenario is lower than the long-term Yp.
As the season progresses, the range of yield outcomes will shrink as the estimated yield levels for the 75% and 25% scenarios converge toward the median value. The change in median Yp since the last Aug 1 forecast is shown to understand how weather has affected Yp during the last two weeks (second column from right). Finally, the probability of an early killing-frost during grain filling in the current 2014 season is given in the last column based on the approximate date at which 50% of the corn area was planted and dominant hybrid maturity at each location (see Table 1). Frost probability will be higher in fields planted later or with hybrids with longer maturity. Note that yield loss from an early-killing frost depends on how late during the grain-filling period the frost occurs. If the frost occurs a day or two before the crop reaches black layer there would be relatively little yield loss, whereas if frost occurs a week or more before black layer the yield loss would be much larger.
So, how reliable are Hybrid-Maize forecasts? In well-managed, timely-planted fields, with good crop establishment that have not been damaged by hail, flooding, diseases, weeds, and insect pests, past experience indicates that estimates of yield potential using Hybrid-Maize are robust. In fields with poor establishment, high disease or pest pressure, or those affected by hail or flooding, as well as fields replanted late due to devastating storm damage (as it is the case of many fields in south central and eastern Nebraska this year), we expect Hybrid-Maize yield forecasts to be considerably higher than actual yields from such fields. Likewise, the model will tend to overestimate yields in crops that suffered large kernel abortion due to severe heat and water stress during pollination.
Grain filling is underway for most corn throughout Nebraska and the Corn Belt. The past two weeks there has been below-average temperatures with relatively good rainfall across Nebraska and the Corn Belt. To evaluate, in "real-time" fashion, the impact of this season's weather on corn yield potential, and its spatial variability across the Corn Belt, simulations of 2014 end-of-season corn yield potential were performed Aug. 15 for 25 locations (Figure 1) using the Hybrid-Maize model. Details about Hybrid-Maize and the underpinning methodology to forecast end-of-season yields can be found in an Aug. 6 CropWatch article.
The Hybrid-Maize model simulates daily corn growth and development and final grain yield under irrigated and dryland conditions. The model estimates "yield potential," which is the yield obtained when the crop is not limited by nutrient supply, diseases, insect pressure, or weed competition-conditions that represent an "optimal management" scenario. It also assumes a uniform plant stand at the specified plant population, and no problems from flooding or hail. Because weather and management factors are "location-specific," Hybrid-Maize simulations are based on actual weather data and typical management practices at the location being simulated as provided by extension educators in each state. (See contributors and site information, Table 1).
Irrigated and dryland yield forecasts as of Aug 15 are shown in Tables 2 and 3). For irrigated corn there has been little change (≤8%) in yield potential forecasts since Aug 1. Median irrigated yield forecast is within ±10% of long-term average in Nebraska and Kansas, except for Clay Center (+11%) and two locations in Kansas where the forecast median irrigated yield potential is +12 to +25% higher than the long-term median. Moreover, there is a 75% probability of above-average irrigated yields at all sites in Kansas and at two sites in Nebraska (Clay Center and Mead).
For dryland yields, there has been little change in yield forecasts since Aug 1, except for a remarkable +35% increase at Clay Center, a +13% increase at Hutchinson, Kan., and +11% increase at Lewis, Iowa due to good rains the past two weeks that broke a dry spell at these locations (2.9, 1.7, and 2.6 inches, respectively). Nevertheless, at all dryland sites but one, median dryland yield forecast is near (17% of the sites) or above the long-term average (78% of the sites), with a high probability of above-average yield at 78% of the dryland sites.
Late planting in 2014 has resulted in a high probability of early-killing frost during the grain filling in the northern edge of the Corn Belt (Wisconsin and northern Nebraska, Iowa, Illinois, and Ohio), partly offsetting the positive impact of above-average rainfall and cooler temperatures on dryland and irrigated yields in the current season. However, the frost would most likely occur very late in the grain-filling period with a relatively small impact on yield, except for Arlington, Wisc., where yields are likely to be well below the long-term mean because frost is likely to occur much earlier in the grain-filling period.
Conclusions
Median dryland and irrigated yields are near or above average at all sites but one, with a high probability of above-average irrigated yields at some locations in Kansas and Nebraska and above-average dryland yields at most sites across the Corn Belt. The risk of an early-killing frost is high at northern locations in the Corn Belt. We will follow-up with further forecasts in early- and mid-September.
Patricio Grassini, UNL Assistant Professor of Agronomy and Horticulture, Extension Cropping System Specialist and Robert B. Daugherty Water for Food Institute Fellow Francisco Morell, UNL Post Doctoral Research Associate Haishun Yang, UNL Associate Professor of Agronomy and Horticulture and Robert B. Daugherty Water for Food Institute Fellow Roger Elmore, UNL Professor of Agronomy and Horticulture, Extension Cropping System Specialist and Robert B. Daugherty Water for Food Institute Fellow Ken Cassman, UNL Professor of Agronomy and Horticulture and Robert B. Daugherty Water for Food Institute Fellow Jenny Rees, UNL Extension Educator Charles Shapiro, UNL Extension Soils Specialist and Professor of Agronomy and Horticulture Keith Glewen, UNL Extension Educator Greg Kruger, UNL Assistant Professor of Agronomy and Horticulture and UNL Extension Cropping System Specialist Mark Licht, Extension Cropping System Agronomist, Iowa State University Ignacio Ciampitti, Crop Production and Cropping System Specialist and Assistant Professor of Agronomy, Kansas State University Peter Thomison, Extension Specialist and Professor, Ohio State University Joe Lauer, Professor, University of Wisconsin-Madison
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