This week, Iowa has had beautiful sunny weather with temperatures around 80 degrees—good for this year's delayed crop development—to help crops catch up on growing degree day accumulation. If weather and frost date are near normal this fall, corn would yield about 91% of its potential. That's figuring a killing frost (28 degrees F) date of October 17 in central Iowa.
To maximize yield and reach 100% of the corn potential, the killing frost would have to hold off until October 28.
Using a computer model, Iowa State University Extension agronomist Roger Elmore came up with that analysis earlier this week. "Iowa's corn has experienced an unforgiving growing season," he notes. "Now, with the last breaths of summer in the air many people wonder what it will take to maximize yields from this date forward." Here are his observations and explanation.
Normal or later than normal frost needed
USDA's September 15 weekly crop conditions survey showed Iowa's corn crop is about two weeks behind the 5-year average. Eleven percent is ready for a frost, 71% is in the dent stage. At beginning dent the crop normally needs about 3 weeks to mature. The bottom line - regardless of location in Iowa, the corn crop needs a normal or later than normal frost date to maximize yields.
Planting dates in 2008 were later than farmers would like to maximize corn yields, and that early-season growth was hampered by cold and wet soils. Plant emergence was difficult and slow. Rainfall prevented timely post-emergence operations such as side-dress fertilizer applications and post-emergence herbicide applications.
Cool temperatures after silking are necessary for maximum yields. This happened in 2008. Indeed, slow heat unit accumulation rates characterize 2008 in Iowa. The disadvantage of this is slower crop development coupled with later than desired planting dates necessitates a long growing season, and a late frost.
An example from Central Iowa
In late August, ISU agronomists suggested that to maximize yields in central Iowa, based on the Ames weather data through Aug. 22, large amounts of sunlight and rainfall after silking were necessary to maximize yields from that point forward; plus a late frost on or after Oct. 22. To some extent this has happened. The crop now nervously awaits a freeze – or at least people close to corn production feel that way! Figure 1 shows dates of 28 degree fall frosts for Ames. The average date is Oct. 17.
To test this thinking, earlier this week Elmore used "Hybrid Maize" --a computer model, with 2008 weather data through Sep. 15 for Ames. Table 1 shows a summary of the results.
If the weather after Sep. 16 is like of the median year for Ames, a crop could yield 91% of its normal potential, if there are no other limiting factors. Based on Table 1 a maturity date of Oct. 28 is necessary for the highest yield potential with a frost date later than that. This has happened in four of the last 30 years; 13% of the time (Figure 1). In addition to a late frost, plenty of sunlight is necessary to maximize yields. This week's weather forecast is promising.
Iowa's 2008 corn crop needs a later than normal frost date to maximize yield. Conditions that hasten crop maturity from this point forward or an earlier frost will reduce yield potentials.