Iowa 2012 Corn Yield Potential Continues To Slip

Iowa 2012 Corn Yield Potential Continues To Slip

Continued drought has already cut state's corn yield potential, and the forecast isn't good for the rest of the summer.

Farmers and market analysts saw history made this past week as drought conditions continue to blister the Corn Belt, sending corn prices above $8 per bushel and soybeans over $17, prices never seen before.

Corn and soybean prices have shot up 40% in the last six weeks as the magnitude of this summer's hot, dry weather has developed into the worst drought in a quarter-century. Although Iowa hasn't been hit as hard as states such as Indiana, Illinois and Missouri, the toll the searing weather is taking on the Iowa crop is becoming more apparent each day.

Searing heat and dry conditions the last few weeks have significantly reduced Iowa's corn yield. What kind of yield can you expect on your farm this year?

The U.S. Drought Monitor map released by the National Weather Service on July 19 shows the entire eastern half of Iowa now designated as in "severe drought." The rest of the state is classified as "moderate drought." The forecast for the first five days of the week beginning July 22 is for temperatures to top the 100-degree mark in central and southern Iowa.

Searing heat the last few weeks has significantly reduced Iowa's corn yield

In a July 7 ICM News article, Iowa State University Extension agronomist Roger Elmore addressed two questions about corn yield potential through July 8 using Hybrid-Maize, a crop model. Yield potential through July 2 was not affected at any of the three locations modeled. However, yield potentials fell when the high forecast temperatures with no rain through July 8 were put into the computer model. Last week Elmore ran the computer model again and updated his look at Iowa's corn yield potential.


What impact has the last few weeks of stress had on Iowa's corn yield potential? "It all depends on what kind of year we have from now through the end of the growing season," says Elmore. He has come up with three possible scenarios.

* Best-case scenario: Yields are reduced by 11% to 43%, depending on location. That means that if the best possible weather year (as recorded in the weather database at each location) occurs starting Monday, July 16, yields could be reduced by 11% to 43% from maximum yield potential modeled so far in 2012.

* Median-case scenario: If we have a year like that of the median year--that is, half of the years yield more and half yield less--yields at the four stations with the longest weather database (northwest, northeast, central and southeast Iowa research farms) are reduced by 14% to 21% percent from the maximum to date for the year.

* Worst-case scenario: Yields are as good as, or better than, any previous year at two of the six locations modeled (northwest and southwest farms) and lower than the worst previous year at the other four farms (northwest, northeast, central and southwest farms). Neither the north-central farm at Kanawha nor the southwest farm at Lewis have had years with severe drought pressure in the weather database, which only goes back to 1997 at each of these farms.

What does this mean? Reasonable yields still possible, if we get rain soon

"Although we have lost the top-end of yield at this point," says Elmore. "The wide range of potential yields possible at each of these locations in Iowa provides hope that reasonable yields are still attainable in 2012-- at least at these locations and with the assumptions I used in the model."


Watch the survey to keep updated on the current Iowa corn situation


Last Monday's Crops & Weather weekly survey results from USDA's National Ag Statistics Service was discouraging but not unexpected. Iowa's corn rates only 36% "good to excellent" and, on the other end 27% "poor to very poor." Three-quarters of the Iowa crop had silked by July 16. The short- and long-term weather forecast is for more heat with only isolated, pop-up thunderstorms, which may not provide much, if any, measureable precipitation. But, as many have said, "We'll take what we can get!"

"Although the crop condition erodes with every day of extra-high day and night temperatures with limited precipitation, a sizeable portion of the Iowa corn crop remains in surprisingly good condition," notes Elmore. Reports from ISU Extension field agronomists around the state mention that in most lesser-stressed fields, pollination and ovule fertilization went well and, thus, kernel numbers are good.

In fields with less weather stress, pollination is good and so are kernel numbers

"We know that kernel number correlates positively with yield; more kernels, more yield," says Elmore. "However, we aren't out of the woods yet. Kernel abortion can and will occur if we have continued stress conditions--through R3 growth stage, which is the blister stage. And of course, kernel weight reduction can occur if stress occurs all the way up to maturity."

* Model runs and assumptions. Hybrid-Maize, the computer model that Elmore uses to look at the possible outcomes for the Iowa corn crop, uses historic weather data from automated weather stations. "In this case, I used weather data from six of Iowa State University's Research and Demonstration Farms," he explains. "The model allows users to compare yield potentials given the weather actually recorded up through the simulation date. In this case, it included 2012 weather data through Sunday, July 15."

The model generates real-time yield predictions for the current season. "What that means is that actual weather conditions up to the date of the simulation are, in a sense, considered the base from which to start," he notes. "That is what we have to work with; unfortunately, we can't change what has happened so far in 2012. The model assumes that there are no other limiting factors such as diseases, insects, low N availability, soil compaction, poor root development, etc. Only 2012 weather up to July 15 and historical weather plus the assumptions I made affect yield predictions."


* It asks a series of "what if" questions. The computer model 'asks' a series of 'what if' questions. For example: What is yield potential if, from this day forward, we have weather conditions like those we had in the best possible year in the weather database for that location? What if the worst historical weather occurred? Other scenarios are:

  • 75 percentile, the year that yields more than three-fourths of the years in the database
  • Median, the year in which half the years had higher yields and half the years had lower yields
  • 25 percentile, the year that yields more than only one-fourth of the years in the data base

The weather record begins in 1986, 1988 or 1997 for the individual Research and Demonstration Farms Elmore is working with. Weather data used here comes directly from their automated weather stations. The limited years of data available at the Kanawha and Lewis locations are an obvious limitation to this type of analysis and interpretation.

Common inputs for all six sites modeled are provided. Factors that varied across locations, such as soil textures, are accounted for. Crop residue levels at planting, corn suitability ratings and other field-specific information are not factored into the analysis. However, some of the variability, especially in the early-season factors, is removed by using emergence date rather than planting date in the model. Soil moisture at planting was set at 75% field capacity for the topsoil and 100% for the subsoil. This was based on Hybrid-Maize runs conducted earlier in the year, which showed that differences in soil moisture status at planting had minimal impact on outcomes (see ICM News Jan. 31, 2012) . These are similar assumptions reported in the July 7 ICM News mentioned above and referenced below. For further information on Hybrid Maize and applications, see the following articles:

ICM News, Sept. 4, 2009

ICM News, July 7, 2012

University of Nebraska Crop Watch, July 10, 2012

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