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12/3/2012 - 12/9/2012

Serious Drought Lingers

By Elwynn Taylor, Department of Agronomy

The drought of 2012 in the Midwest was a continuation of a weather anomaly that began in 2010. The historical indicator of El Nino and La Nina is the Southern Oscillation Index (SOI). The index is based on the 90-day standardized deviation of atmospheric pressure between Tahiti and Darwin, Australia. The standardized difference reached 0.8 on July 22, 2010, signifying the beginning of a La Nina event (Figure 1). By Oct. 23, 2010, the event was clearly the second strongest event in the 100+ years of record keeping. The young but potent La Nina resulted in an abrupt change in weather on a planetary basis that included record flooding in Montana, N.D. and adjacent Canada, and enormous amounts of water to drain into the Missouri river during 2011. The event ended a several year drought in the northwest United States and ended (with drought) a six year continuous string of above trend U.S. corn yields (figure 2).

Figure1.  The La Nina event that initiated July 22, 2010, reached peak strength on Oct. 23, 2010, second only to the mid-1950s event. The SOI reached El Nino threshold on Aug. 16, 2012, but did not establish. On Oct. 30, 2012, the SOI returned to the La Nina side of a neutral condition. Data from


Figure 2.  U.S. corn yield 1982-2012. The U.S. corn yield exceeded the trend for six consecutive years (2004-2009) and fell below trend in 2010. The “trend yield” for 2013 is near 160BPA (indicated by star).  USDA graphic:


Drought in 2011 was much like the drought in the mid-1950s: developing in the south central United States with rain becoming scant in the Corn Belt after early July.  Midwest crops depleted the subsoil moisture to the extent of rooting depth, and over winter precipitation did not bring full recharge to western Corn Belt soils.  Rooting conditions in 2012 were near ideal and observed corn and soybean roots to depths greater than 8 feet were reported in numerous locations. Deep rooting provided sufficient water to enable a greater than anticipated crop yield in numerous Corn Belt locations, but resulted in about 8 feet of moisture depleted soil and a resultant requirement of 16+ inches of moisture needed to replenish subsoil moisture. It is not likely that subsoil moisture will be fully replenished by the beginning of the 2013 planting season.

Moisture deficit in the subsoil increases the risk of crop yields being below trend and prevents the recovery of river, pond and well water to normal levels.

Measurements of soil moisture are made at 1-foot intervals to 5 feet at several locations. Figure 3 is a map of the “normal April 15” soil moisture. Observations of the moisture at selected locations were made in the fall of 2011 and 2012.  The fall observations are useful in that November water is retained throughout the winter and is indicative (to the extent they are short of normal) of the moisture deficit yet to be corrected.

Historically, severely deficit precipitation years of the magnitude of 2012 do not recover to normal annual precipitation in a single year (Figure 3).  Accordingly, an additional year of significant moisture stress is considered to be not unlikely and a fourth consecutive year of below trend U.S. corn yield a distinct possibility. The probabilities will become more definitive in the early weeks of 2013 as the likely phase of the El Nino/La Nina for the growing season becomes manifest.

Figure 3.  Normal subsoil moisture as of April 15 during the period 1961-1980. Soil moisture capacity is between 10 and 11 inches for the top 5 feet of soil for moist agricultural soils of Iowa. November moisture in 2012 exceeded observations for November 2011 in southeast Iowa and will likely result in normal values when augmented by spring rain. In northwest Iowa, the November 2012 observations were less than 2011 in locations sampled.


Figure 4. Precipitation received in central Iowa during the water year  (Oct. 1 through Sep. 30). The three historically driest years since 1950 are 1956, 1988 and 2012.  In both cases, the subsequent year also received below normal precipitation and experienced below trend yields in Iowa.


Elwynn Taylor is an extension climatologist and professor of agronomy. He can be reached at (515) 294-9985 or 3-mail Observations of subsoil moisture were provided by Joel DeJong, Paul Kassel, Jim Fawcett and Greg Brenneman, extension agricultural specialists.

New Options for Soybean Aphid Host Plant Resistance

By Michael McCarville, Erin Hodgson and Matt O’Neal, Department of Entomology

Host plant resistance for soybean aphid is the newest management tool for yield protection. In 2010, a single gene expression, called Rag1, was commercially released in the north central region. Aphids feeding on Rag1 plants do not live as long or produce as many offspring compared to when they feed on susceptible plants. In small plot evaluations of the Rag1 gene, there is a dramatic decrease in the seasonal accumulation of soybean aphid compared to aphids developing on susceptible varieties.

While soybean aphid populations were very low across much of Iowa in 2012, it is difficult to predict what we’ll see in 2013. In the past, low soybean aphid populations in even years (i.e., 2004 and 2006) were followed by high populations in odd years (i.e., 2005 and 2007). There is no guarantee that this trend will continue into next year, but farmers should still consider soybean aphids a potential pest for 2013.

The Department of Entomology at Iowa State University recently updated our Soybean aphid-resistant varieties for Iowa for the 2013 growing season. The publication lists currently available soybean seed with resistance to soybean aphid. The list is intended to assist farmers looking to adopt this new management tactic and possibly reduce their need for a foliar insecticide. The listing includes varieties in late maturity group 0 and maturity groups 1, 2 and 3. 

The list contains 13 varieties from four companies. It is organized by company, with varietal information provided on the relative maturity, herbicide resistance, source of aphid resistance and resistance to other pests. Three items of interest to farmers will be:

  1. One variety with resistance to both the soybean aphid and soybean cyst nematode (SCN): The SCN is a pervasive and serious pest of soybean in Iowa. Farmers with SCN-infested fields are encouraged to select an SCN-resistant variety.
  2. Three varieties carrying two different genes for soybean aphid resistance: Varieties containing two soybean aphid resistance genes provide significantly better aphid control than varieties containing a single resistance gene.
  3. Organic varieties with resistance to the soybean aphid: Organic farmers are limited in the insecticides they can use for effectively managing the soybean aphid. Organic soybean aphid-resistant varieties can provide effective control and yield protection.

The listing also contains Iowa State University’s recommendations for considering soybean aphid-resistant varieties. For more information on soybean aphid management, consult Soybean Aphid Management Field Guide 2nd edition. To see Iowa State University's annual evaluation of insecticide efficacy against soybean aphid, visit the soybean aphid website. This publication was funded in part by the Soybean Checkoff and the Iowa Soybean Association.


Michael McCarville is a Department of Entomology graduate student; he can be reached at 515-294-8663 or by e-mail at Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by e-mail at or phone 515-294-2847. Matt O'Neal is an associate professor in the Department of Entomology with teaching and research responsibilities. He can be reached at or at 515-294-8622.

This article was published originally on 12/10/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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