Fundamentally Speaking

Iowa Precip vs. Corn Planting Pace, Yields

Difficult to believe that a year after heat and dryness plagued the corn and soybean crops, these markets are now supported by persistent cold and wet weather that has delayed timely seedings of these crops.

The official NOAA May 2013 weather statistics have not yet been released but the Iowa state climatologist estimates that the spring (Mar-May) statewide rainfall was 17.30 inches this year, by far a record.

The accompanying graphic above tracks the Iowa spring rainfall from 1960-2012 as a percent of the 53-year average of 10.0 inches.

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The 2013 figure of 17.3 inches is 73% greater than average and that is well above any other figure on this chart.

This helps explain why the percent if the Iowa corn crop in the ground as of May 15 at 39% is the third lowest since 1979 trailing only the 35% in 1995 and 18% in 1993.

The graphic also points out the yield in 1993 at 40.5% below trend and the 11% deviation from trend in 1995 with Mar-May rainfall 20.7% and 35.1% greater than the 10-inch average.

Other wet years such as 1991 also had below trend yields but that was not the case for 2004, 2007, and 2008.

Though 1993 is remembered as being a huge flood year in much of the Corn Belt, the Iowa Mar-May precipitation that year was 12.05 inches, above the 10-inch average but not above 12.66 inches that is the 1960-2012 average plus one standard deviation.

A look at the data shows that the five wettest springs in Iowa since 1979 has average Mar-May rainfall 36.4% above average resulting in an average May 15 corn planting pace of 65% yet yields averaged 1.3% above the long-term trend.

The average for all years 1979-2012 showed average rainfall with a May 15 planting pace of 79% and yields at trend.

There is no doubt that this year’s late plantings may result in below trend yields if temperatures are particularly warmer in late as opposed to early July and a large part of the crop is not physically mature prior to the first fall freezes.

On the other hand, this heavy amount of precipitation has resulted in Iowa essentially now drought free, which is a very welcome change from the situation that prevailed just a few weeks ago and will obviously be of benefit to the corn crop as the growing season progresses.


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