It may be only a matter of time before counties in central and eastern Illinois suffer corn yield losses comparable to those experienced in 2005 in northeastern and western Illinois, according to a University of Illinois Extension study.

"Corn yields still have downside potential," said Gary Schnitkey, U of I Extension farm financial management specialist who co-authored the study with Department of Agricultural and Consumer Sciences colleague Bruce Sherrick.

"Many counties in northeast and western Illinois experienced yield deviations from trend yields of greater than 10 percent in 2005. This loss is the first time many of these counties had a large yield shortfall since 1995. Many counties in central and eastern Illinois have not had a large yield shortfall in recent years. This analysis suggests that it is only a matter of time before these counties have significant losses."

The entire study, "How Bad Were 2005 Corn Yields?" can be read online on farmdoc at:
http://www.farmdoc.uiuc.edu/manage/newsletters/fefo06_05/fefo06_05.html.

Schnitkey used as a starting point for the study the suggestion by some observers that genetic improvements have reduced downside yield potential in corn.

"Dry conditions prevailed over much of Illinois in 2005, thereby providing an opportunity to evaluate whether downside potential has been reduced," he explained.

The study uses yield deviations for county yields in 2005. These deviations are compared to deviations from 1955 through 2005. The figures were obtained for all Illinois counties from the National Agricultural Statistical Service (NASS) website.

"No county has a 2005 deviation greater than its 1988 deviation, a year that was the worst for many counties," said Schnitkey. "However, deviations for some counties in 2005 are large and in the 'second tier' of yield loses. While genetic improvements may have reduced yield losses, large negative yield deviations are still possible."

Percent yield deviations in 2005 vary across Illinois counties, he noted. "Nine percent of counties have yield deviations representing greater than 20 percent losses, 29 percent of the counties have yield deviations between minus-10 percent and minus-20 percent; 32 percent of the counties had yield deviations between minus-10 percent and zero percent; while 30 percent of the counties have positive percent yield deviations."

Geographical dispersion is the rule in percent yield deviations with the highest yield losses located around Chicago and a string of western Illinois counties south of the Quad Cities.

"Having a geographical dispersion to yield losses is fairly typical," said Schnitkey. "In 2002, for example, 42 counties located primarily in the southern part of Illinois had percent yield losses indicating greater than 20 percent losses. During that same year, 20 counties located primarily in the northern and western part of the state had positive yield deviations. The 2002 distribution is almost a mirror image of the 2005 distribution."

Schnitkey added that this study does not answer whether improved genetics aid in reducing yield shortfalls.

"It is clear that improved and genetically modified hybrids have increased trend yields over time," he said. "It is not clear that percent yield shortfalls are reduced by improved hybrids. Having said that, some of the yields in northern and western counties may have been worse had improved or genetically modified hybrids not been in use."