Sept. 12 Yield Forecasts

• Western Corn Belt

• Central/Eastern Corn Belt

Interpreting the Yield Forecasts

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 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 the 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, dry weather or early killing frost) from Sept 12 until crop maturity, whereas the 25% scenario could occur if weather is more favorable (adequate rainfall, no late-season frost) than is typical for this period. 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 Aug. 30 forecast is shown to understand how weather has affected Yp during the last two weeks (second column from right).

Finally, the probability of 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 the 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. 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, experience indicates that Hybrid-Maize yield potential estimates are robust. In fields with poor establishment, high disease or pest pressure, or affected by hail or flooding, as well as in fields replanted late due to devastating storm damage (as is the case for many fields in south central and eastern Nebraska this year), we expect Hybrid-Maize yield forecasts to be considerably higher than actual yields. Likewise, the model will tend to overestimate yields in crops that suffered large kernel abortion due to severe heat and water stress during pollination.

Sept. 18, 2014

Grain filling continues in Nebraska and most of the Corn Belt, except for southern locations where black layer has already been reached. There were good rains the last two weeks across the entire Corn Belt and relatively lower temperatures in Nebraska and Iowa and warmer temperatures in the eastern Corn Belt, resulting in a respective increase and decrease in the risk of early-killing frost. 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 Sept. 12 for 25 locations using the Hybrid-Maize model (http://hybridmaize.unl.edu). Details about Hybrid-Maize and underpinning methodology to forecast end-of-season yields can be found in a previous 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 Sept 12 are shown in Table 2 and Table 3. Crops have already reached black layer at the sites in Kansas, with final simulated yield 7% to 29% above the long-term mean. In the other sites, median forecasted yield has changed little since the Aug 30 forecast and the range of forecasted yields (i.e., the difference between 75% and 25% scenarios) has narrowed because crops are approaching black layer. Therefore, we expected final simulated yields to be very close to the median forecasted value shown in Tables 2-3, excepted at sites where a high risk of early-killing frost can reduce significantly the grain filling duration.

Due to relatively low temperatures during the last two weeks, risk of early-killing frost has slightly increased in Nebraska and Iowa whereas warmer temperatures have decreased frost risk in Illinois and Ohio. If frost occurs, its timing will ultimately determine the magnitude of the yield impact. For example, little yield reduction is expected if frost occurs just a few days before the predicted black layer date and this may be the case for most locations across the Corn Belt. It should be noted, however, that there are other negative impacts on an early-killing frost besides yield reduction such as low test weight, high moisture content, increasing drying cost, and combine losses due to stalk breakage and diseases. (See CW: Frost/Freeze Effects on Corn and Soybean Plants; Temperature Summary for Sept. 12-15).

Compared with the previous Aug. 30 forecast, probability of above-average irrigated yield in central and west Nebraska has increased. Thus, above-average irrigated yields are now expected at all sites in Nebraska, except for Concord where the probability of below-average irrigated yield due to early-killing frost is relatively high. It should be noted, however, that median irrigated yield forecasts will be within ±10% of long-term average at all sites, except for Clay Center (+16%).

The median dryland yield forecast has slightly improved since the Aug. 30 forecast at three locations in the eastern Corn Belt (DeKalb, Ill., Custar, Ohio, and South Charleston, Ohio) due to a combination of good rains that broke a dry spell and relatively warmer temperatures that have reduced the risk of an early-killing frost. Above-average dryland yields are expected at all simulated sites across the Corn Belt, except for Sutherland, Iowa, and the two sites in Wisconsin where yields are likely to be near or below-average due to high probability of early-killing frost. It is remarkable that median dryland yield forecast is well above (>10%) the long-term average in 12 of the 17 sites.

Conclusions

Irrigated and, especially, dryland yields are forecasted to be above average at a majority of sites. The probability of an early-killing continues to be high at northern sites in the Corn Belt, with a slight increase in Nebraska and Iowa and a decrease in Illinois and Ohio during the last two weeks. It should be noted that these forecasts do not take into consideration problems with stand emergence due to residue, hail/flooding damage, replanting situations, disease, or nitrogen leaching. We will follow-up with further forecasts in late 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