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August 2011 Corn Yield Forecast Too Optimistic?

August 2011 Corn Yield Forecast Too Optimistic?

USDA's first official corn yield forecast for 2011 was released August 11 and indicates a U.S. average yield of 153 bushels per acre and an Iowa average yield of 177 bushels per acre. The Iowa estimate may be a little high.

USDA released its first corn yield forecast for 2011 on August 11. The previous USDA yield numbers for 2011, such as those USDA released in July, were just projections based on certain assumptions. But the August Crop Report is the first official yield forecast for this year, since it is based on actual in the field estimates of yield which were made the last few days of July and around August 1.

Figure 1: Corn yields over 30 years for Iowa and U.S. Trendline (tl) yields are shown in the open diamond (Iowa) and square with diagonal lines (U.S.). The August 2011 forecast (fc) yield for Iowa is the same as the trendline yield, which is 177 bu/acre. The forecast (fc) yield for the U.S. is shown in the square with vertical lines. Data is from USDA-NASS.

Trained crop enumerators, personnel from USDA's National Agricultural Statistics Service (NASS), actually go into farmers' fields in late July and the first couple days of August and measure actual ear size and count the number of ears per acre, to come up with the data to make USDA's August yield estimate.

The August 2011 forecast is for the U.S. to average 153 bushels per acre this year, similar to last years' average and six bushels below the 30-year trend line (Figure 1) for the nation's corn yield average. "This 153 bushel per acre forecast is not surprising," says Iowa State University Extension corn agronomist Roger Elmore. His colleague, ISU Extension climatologist Elwynn Taylor, agrees. They base their observation on the fact that adverse weather this growing season has occurred in a number of states, not just in Iowa. In fact, the adverse weather has hit Corn Belt states outside of Iowa harder, in a number of cases.


Iowa's estimated 177 bu. corn yield average for 2011 seems a little high

On the other hand, the USDA forecast for Iowa in 2011, which is 177 bushels per acre, matches that of the 30-year trend line and is 12 bushels above last year's final corn yield estimate for the state. Thus, if realized, an average yield of 177 bushels per acre for Iowa this year would rank third highest behind the years 2004 (181 bu. per acre) and 2009 (182 bu. per acre) for Iowa. "This 177 bushels per acre forecast seems unlikely this year," says Taylor. Elmore agrees.


Pollination issues like those shown in the recent ICM article, Weather Impact on Midwest Corn 2011, are just part of the problem. The other issue is kernel weight reduction that will likely occur because of the high night-time temperatures that took place during pollination this year, which resulted in rapid development of the corn crop. This sounds like a replay of 2010, at least for Iowa corn, notes Elmore.


"Remember these forecasts arise from the meticulous late-July counts and measurements made in the fields by the team at USDA-NASS. And keep in mind that the forecast is actually a forecast and not simply an update of a previous projection," points out Taylor. "USDA-NASS uses new and expanded observations from a large number of sample locations taken from farmers' fields to formulate expected yields based on field observations for every subsequent USDA monthly Crop Report."

USDA September yield forecast will likely be lower than August forecast

Also, keep in mind that in making the forecast USDA assumes the weather during the remainder of the grain filling season is going to be normal. The September yield forecast is likely to be lower than the one just issued in August because ear size in 2011 will be less than the average ear size you usually see, say Elmore and Taylor. Later, in the September and October USDA monthly crop reports, when the yield forecast includes actual kernel counts taken from the sampled areas of the fields, the yield forecasts will again change. 


Of course, it is possible that the weather could be better than average for crop yield and actually increase the yield forecast in subsequent months. This is usually associated with temperatures that extend the grain filling period beyond normal. Also, don't forget that when acres are dropped from the harvested area due to flooding, etc., the yield increases simply because yield means yield per harvested acre.


Earlier this week USDA-NASS reported that about two-thirds of Iowa's corn was already in or past the milk stage of kernel development. At dent stage, the crop needs about a month of growing season to finish well. Kernels contain less than half of their final weight at dent stage. "So, you need to keep in mind that much can change in a month," says Taylor, "as you look forward to USDA's next 2011 yield estimate update, which will be in the September 12 Crop Report."

TAGS: Extension
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