Corn Development and September Yield Forecast, 2010

September 28, 2010
ICM News

By Roger Elmore, Department of Agronomy

The September 2010 USDA—NASS corn yield forecast remains at 179 bushels per acre (bu/acre) for Iowa — the same as the August report. If realized, 2010 will boast the third highest yield in Iowa's history behind 2004 (181 bu/acre) and 2009 (182 bu/acre).

As we think about this possibility, remember that the USDA-NASS September forecast is mainly calculated from ear counts and ear dimensions. These measurements are then combined with ear weights from the last five years, depending on the maturity of the individual plot samples obtained in late August. If mature, ears were weighed which improves the accuracy of the forecast. The next forecast, Oct. 8, will include more ear weights providing an even more accurate representation of harvestable yields. USDA-NASS uses a complex set of statistical models to forecast grain yields. This is the best source for reliable yield forecasts.

This year's Growing Degree Day (GDD) accumulation to date outpaced seasonal averages by 6 percent (Table 1). However, GDD accumulations between silk and dent averaged 116 percent of normal. By mid-August, 70 percent of Iowa's corn was in or past the dough stage (R4) compared to 47 percent normal and only 25 percent for 2009 (USDA-NASS, 16 August). Accumulations since denting this year were 98 percent of normal (Table 1).

Cool temperatures since mid-August (dent stage R5) certainly resulted in slow GDD accumulations and slower crop development but with the fast pace of crop development earlier in the season, this will have minimal effect on yield. Charts on Iowa crop conditions - as well as those of other states — from the USDA-NASS Crop Progress and Condition report clearly shows the advanced pace of crop development this year.

Crop development racing from silking to dent is not necessarily positive. The rapid pace of kernel fill could reduce seed weights and yields (see Sept. 3 ICM article).  A silver lining is that the early maturity will result in faster drydown and thus lower grain drying costs (see Sept. 28 article).

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Elmore can be contacted by email at or (515) 294-6655.


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