Elmore: USDA crop estimates for Iowa too optimistic
By Jean Caspers-Simmet
Date Modified: 09/09/2010 9:25 AM
NASHUA, Iowa —Roger Elmore, Iowa State University Extension corn agronomist, thinks USDA's Aug. 13 yield estimate of 179 bushels per acre for Iowa is too high.
"It's too optimistic," Elmore said last week at Iowa State University's Northeast Research Farm field day.
USDA's August numbers are actually based on ear counts taken in 400 fields around Iowa in two random spots per field.
"USDA should say it's an estimate of ear counts, not yield," Elmore said. "The September estimate will be more accurate."
In September, estimates will be based on ear girth and ear length. When the crop is mature, reporters will harvest it, scale it out and the results will go in the final report.
An important component that is impacted by the weather is kernel weight, Elmore said.
"There is no provision for kernel weight in the current estimate other than the ear count is taken the end of July and multiplied by a kernel average for the last five years," Elmore said. "You get a 179-bushel statewide average. There are a lot of holes in that. There's a lot that can happen between the end of July and maturity —drought, flood, hail, disease, frost. All those things impact kernel weight. If it frosted tonight, we'd see dramatic reduction in kernel weight. These numbers are very optimistic, and you could say that for the federal numbers, too."
Elmore said that south central and southeast Iowa have been hammered by wet weather all summer.
"Northeast Iowa looks better than the rest of the state," Elmore said. "It was so wet in southern Iowa. Some guys lost their crop two to three times. There was nitrogen leaching. All those rain systems just stayed over southern Iowa."
Elmore said that the hot weather in early August will impact yields.
"Corn likes 50 to 86 degrees," Elmore said. "You get above 86 degrees and it speeds up development, especially when night temperatures are high. It speeds up maturity, and that's a drain on seed weight. Early maturation is not positive."
Night temperatures were in the 70s and 80s in early August with daytime temperatures in the 90s. Due to the heat, corn matured early and missed out on the tail end of the season.
Last year's 182-bushel yield came about because as soon as the corn silked, the weather brought cool nights and moderately warm days, which stretched out the seed fill period.
"Having that time to increase kernel weights is important in maximizing yield," Elmore said.
A crop analyst from Chicago calls Elmore every couple of weeks to talk about crops, and the analyst told him he's hearing that yields aren't as good as farmers would like.
"He said yield reports he's hearing are up to 20 percent lower than expected in southern Illinois, Missouri and Kansas," Elmore said.
Elmore had farmers participate in an exercise where they estimated corn yield. They determined the average number of ears in 1/1000 of an acre. They then determined the average number of rows of kernels per ear and the number of kernels per row. They multiplied the ears per 1/1000 of an acre times the rows per ear times the kernels per row and then divided by 90 to get bushels per acre.
"The estimate could be off by 30 bushels plus or minus if ear samples are not representative or if kernel weights are not normal," Elmore said.