(DTN) USDA ignored growing degree days in its calculations, leading to the drastic decrease in yield expectations between the August and October crop production reports, said Iowa State Climatologist Elwynn Taylor at a meeting of agriculture lenders here Tuesday.

USDA doesn't take growing degree days into consideration, which may explain their large corn yield estimate before estimators actually went out into the fields. (DTN file photo by Katie Micik)USDA takes growing degree days into consideration in its weekly crop progress reports, but doesn't consider GDDs in formulas that estimate yield prior to actual sampling from the fields.

Growing degree days are heat units available to the crop that help growth progress. Different hybrids require different GDD totals to reach maturity.

During his speech, Taylor mocked a conversation between himself and USDA:

ET: "The USDA came out with some estimates, or forecasts, of crop yield. They started in August and then in September they gave estimates that were high, like they were a year ago. And, then out came the estimate a month ago that was substantially lower. So why did they make this big change?"

USDA: "We got out there and sampled the fields and the corn just wasn't there. Beans were fine but the corn wasn't there so we had to tell them what we found."

ET: "Why were you so wrong before?"

USDA: "We did the best we could."

ET: "That's not quite true. They did the best they were willing to do. They weren't willing to use the growing degree days."

Taylor said Tuesday's crop report makes him smile because he predicted 154 bpa yield back in August. USDA's August estimate was 165 bpa, its October estimate was 155.8 bpa and its November estimate, released Tuesday, was 154.3 bpa.

Taylor pulled up a map of Minnesota's crop conditions and corn was rated 90 percent good or excellent, the best in Minnesota's history. Then, Taylor pulled up a chart that showed the USDA's percent error and highlighted years where USDA was off. "And I looked at what these big mistakes were and they all were for the same reason; they didn't consider the growing degree days. Even though they've got the data on the bottom half of the page."

Then he looked at the dates when corn began silking and reached dent stage this growing season compared to a normal year. "Silking to dent took less time than normal. Less time from silking to dent, less time for the corn to gain weight. How much less time? 15 percent less. 15 percent tapped from the yields and that's from the best state of all."

In addition to pointing out an unconsidered factor in USDA's estimates this year, Taylor turned to La Nina and droughts. He has been calling for drought for several seasons now, and said this year's La Nina development parallels La Nina developments in 1954 and 1974. He said that 1974 "was a disastrous crop after 20 years without a trace of drought."

"We shall see. We don't know exactly what will happen but there are some cautions there."

Katie Micik can be reached at katie.micik@telventdtn.com