By Elwynn Taylor, Department of Agronomy
Corn yield has been less variable during the past 12 years than the previous 20. However, the yield of the past 5 years in Iowa exceeded trend. The national yield has a run of 5 above-trend years; this is an historical record. A run of 4 above trend years occurred in 1984-87 (with a major drought at each end).
On average, a new record corn yield is achieved every 4 or 5 years. Improved genetics and management get the credit for the consistent improvement, and weather is the reason that yields are somewhat erratic over a period of years. Weather is the major uncontrollable risk factor in agriculture. The weather is not controllable, but the risk is manageable.
Widespread drought in the Corn Belt is occasional, the most recent being in 1988. Even the 1988 drought did not decimate the crops of the western Corn Belt, but it reduced yields by 30 percent over much of the central and eastern portion. Serious drought tends to follow a 19-year cycle and the next 4 years fall into the higher risk portion of that cycle. The majority of seasonal weather indicators imply increased crop production risk for the 2008 crop season. Attention to soil condition has benefited crops the past few years and will remain important.
Indicators for 2008:
Southeast Drought: Residual moisture deficiency persists in the southeastern U.S. Some level of drought in South Carolina precedes initiation of widespread Corn Belt drought (greater than 90 percent of all major Corn Belt droughts are preceded by drought in South Carolina). The risk of serious drought in Ohio is greater than 60 percent if June drought occurs in South Carolina, and there is a 36 percent chance of Corn Belt-wide drought within 15 months.
60-Year Cycle: The “dry” conditions of the desert Southwest may, with some justification, cause concern for Corn Belt weather. Drought originating in the Southwest is not known to sweep the Midwest; however, a series of dry years in the Southwest may precede below normal moisture years developing in the eastern half of the U.S. Tree ring studies from Arizona to Virginia show that wet and dry intervals of approximately 30 years are not uncommon and do not appear to be random.
During the past 100 years the wet/dry relationship is discernable in weather records. The similar pattern seems to lag somewhat, moving from west to east. The “dry” trend in Arizona may be an indication of a 30-year trend toward more arid conditions there. The trend appears to have initiated about 1980. It is possible that much of the Midwest reached the “peak” of moisture in the mid-90s and may experience diminishing annual moisture over the next two decades.
ENSO: There are weather factors that influence Midwest weather more than does the so called ENSO (El Nino/Southern Oscillation). However, the SOI (Southern Oscillation Index) is one of the “best” indicators in that it provides some “lead time.”
A negative SOI (El Nino) tends to persist 6 to 14 months and the Midwest has, historically, not experienced widespread drought during these conditions. The SOI does have an influence on crop production risks in the Corn Belt. The movement from “El Nino” to neutral did influence the 2007 growing season. The 90-day SOI average reached La Nina on Dec. 25, 2007. Some researchers state that the event may begin to dissipate before June 2008. This indicates above average risk for 2008 crops, but is a lesser risk than a strong La Nina that persists into mid-summer.
Appendix I: 19-Year Drought Cycle
By Louis M. Thompson 1989 (revised 9/24/2006)
Adapted by E. Taylor (9/27/2006)
*Lunar declination reaches the minimum (18.5 degrees) during the wet phase and the maximum (28.5 degrees) in the dry phase. Some evidence relates weather trends to lunar gravitational influence.
Very High Yield 2004++
High Yield 1937¤ +
Low Yield 1995¤-
Very Low Yield (drought) 1988)
Very Low Yield (combined) 1974!) Very wet spring, summer drought, early freeze
Very Low Yield (flood) 1993))-
Low yield years in “wet” phase: 10
Low yield years in “Dry” phase: 20
Good yield years in “wet” phase: 16
Good yield years in “dry” phase: 14
The chance of a “good” year is about the same in either phase.
The chance of a “poor” crop doubles during the “dry” phase.
The average is 4 good and 4 bad crop years in each 18-19 year cycle.
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Elwynn Taylor is a professor with responsibilities for developing and implementing of extension education and information programs in agricultural climatology.