Weather Watchers Say Odds Favor Dry Summer

La Nina as well as the 19-year drought cycle both indicate increased chance of drought this summer.

Tile lines are running with plenty of water in Iowa this spring which means we will start out with a full tank of subsoil moisture as crops are planted this spring. But we’re overdue for a drought this summer. That’s how a couple of well-known Iowa climatologists size up the weather outlook for the 2008 crop season.

La Nina, caused by a cooling of the surface water in the Pacific Ocean off the coast of Ecuador, is the biggest factor that will likely determine if a drought develops during the upcoming growing season. Elwynn Taylor, Iowa State University Extension climatologist, says the newest computer models show the La Nina event lasting into June. If the La Nina does indeed continue into June, conditions could turn quite dry during the summer.

Harry Hillaker, climatologist with the Iowa Department of Agriculture, agrees. He says 17 times out of the past 23, La Nina years have been drier than the long-term average for April to October. Taylor notes that there are other factors besides La Nina that could increase the drought risk—such as the 19-year drought cycle. Typically there is a major drought in the Corn Belt about every 19 years. That last major one that occurred was in 1988. We are overdue to have a major drought in the U.S. Corn Belt, he says.

Downtrend in yield predicted for 2008

Taylor says a strengthening of LaNina increases the changes for drier-than- normal conditions during the 2008 growing season and a 70% or greater chance of yields below the trend line. The national corn yield trend line is 151 bushels per acre. Taylor is currently predicting a national average corn yield of 142 bu. per acre, which is 9 bushels below the trend line. He says December futures at harvest this coming fall may climb above $7 per bushel.

During the last major drought in 1988, market volatility was due to reduced supply. Market volatility in recent months is due to increased demand. In 2008, Taylor suggests we could have volatility compounded by supply/demand imbalances.

One positive this spring is the current soil moisture levels, he notes. Drainage tile are running indicating that soil moisture levels exceed field capacity. When the attraction of the soil particle to water is stronger than the pull of gravity (field capacity), most Iowa soils contain 2 to 2.2 inches of plant available water per foot of depth. A 5-foot soil profile would contain 10 to 11 inches of available water or almost half of the water needs of a corn crop. The available water in sandier or shallower soils is less and the risk of drought stress is greater.

Can you start planting corn by April 20?

Many Iowa farmers like to start planting corn by April 20. "If the cool, wet soil conditions continue, it will be difficult to be patient and wait for optimum planting conditions," says George Cummins, an ISU Extension field agronomist. "Mudding the crop in will have negative effect if surface or sidewall compaction develops and the weather turns hot and dry."

A good root system is needed in a dry year. "It is imperative that corn plants have a good root system to pull them through a dry summer," says Cummins. He shows a table of data from ISU’s statewide summary of 12 site years of corn rootworm management strategies. The data was sorted by dry and wet (normal) years. In the dry years on the left side of the table, even with only modest rootworm injury to the corn roots, the result was significant yield loss.

In the normal years on the right side of the table, all of the rootworm treatments yielded the same statistically even though the root injury varied. Root injury is generally measured in July after the corn rootworm larvae stop feeding and pupate. In a normal year root regeneration may occur and yield-limiting moisture stress will not occur, notes Cummins.

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