Yield Potential Slips as of July 15, 2012

By Roger Elmore, Department of Agronomy

In a July 7 ICM News article, I 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 modeled. It's time for an update on corn yield potentials.

What impact has the last few weeks of stress had on corn yield potentials?

It all depends on what kind of year we have from now through the end of the growing season.

• Best-case scenario: Yields are reduced by 11 to 43 percent, depending on location (see Table 1). 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 percent 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 (NW, NE, C and SE) 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 (NW and SW farms) and lower than the worst previous year at the other four farms (N, NE, C and SW farms). Neither the N nor the SW farms 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?

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

Current  Iowa corn situation

Monday's Crops & Weather from USDA-NASS was discouraging but not unexpected. Iowa's corn rates only 36 percent good to excellent and, on the other end, 27 percent poor to very poor. Three-quarters of our crop has silked. The short- and long-term 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 corn crop remains in surprisingly good condition. Reports from ISU Extension and Outreach field agronomists around the state mention that in most lesser-stressed fields, pollination and ovule fertilization went well and, thus, kernel numbers are good. We know that kernel number correlates positively with yield; more kernels, more yield. However, we aren't out of the woods yet. Kernel abortion can and will occur — with stress conditions - through R3, the blister stage and, of course, kernel weight reduction can occur with stress all the way up to maturity. 

Model runs and assumptions

Hybrid-Maize 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 (see the tables). 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 last 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. 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 (Tables 2 and 3) affect yield predictions.

The 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 3/4 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 ¼ of the years in the data base

The weather record begins in 1986, 1988 or 1997 for the individual Research and Demonstration Farms we're working with here (Table 3).  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 in Table 2. Factors that varied across locations, such as soil textures, are shown in Table 3. 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 percent field capacity for the topsoil and 100 percent 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

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by e-mail at relmore@iastate.edu or 515-294-6655.