USDA Weather Assumptions Undermining Forecasts

ISU agronomist says to expect a five-year low corn yield.

Published on: Aug 24, 2011

An Iowa State University Agronomist says USDA is underestimating the impact of weather on this year's corn crop and overestimating the yield. Iowa State's Elwynn Taylor says USDA's making an assumption about weather that could undermine its August forecast of a season average yield of 153 bushels per acre. Taylor predicts a five-year low of 149 bushels.

"The USDA estimate is based on observed conditions up to the present time and then the outlook assumes that the weather will be average until maturity," Taylor said. "My estimate is based on weather up to the present time and considers the likely weather up until harvest time."

Based on forecasts that Taylor says show a faster-than-usual accumulation of stress it is important to be measuring growing degree days since silking on July 1.

"The fewer we get after pollination the longer the time until maturity," Taylor said. "Sometimes that can be a disaster because the crop doesn't get mature before the season ends, frost comes or something. Otherwise it just give it just gives it more days to grow and to gain weight. This can cause a considerable difference in the amount of yield we do receive compared to that that would appear if we had average growing condition."

Taylor says USDA does not use this calculation of stress on the crop to figure average yield, but only assumes average weather until harvest. According to Taylor, harvested acres could change in both models.

"When we get really harsh conditions and acres begin to drop out, since the yield is per acre sometimes the yield number will actually begin to increase," Taylor said. "But of course the total harvest is not increasing because the acreage is dropping."

USDA is forecasting over 84 million harvested acres this season, which Taylor doesn't expect to change much. Based on his yield of 149, that would produce a 12.6 billion bushel crop versus USDA's 12.9 billion bushel estimate, which USDA already pared by half-a-billion bushels from July's forecast.