Drought Damage Believed Severe in Illinois Soybeans

September 12, 2011

Drought is the biggest worry for soybeans from very low August rainfall.   Agronomists claim that August dryness is very damaging for the yield.  "Adequate moisture during the seed-filling period may completely overcome the effects of moisture stress in flowering.   In Urbana, Illinois, one inch of rain in excess of the normal for an 8-day period during the seed filling stage increased yields by at least 1.5 bushels per acre.  The seed filling period is the most critical time in the life of the soybean plant " (Modern Soybean Production, Walter Scott and Samuel Aldrich).

By contrast, very low August rainfall produces tiny seeds in the pod that shrink the yield.  

The USDA Illinois soybean yield from USDA was held steady at 48 bushels per acre on the September report, and only 3% below trend.   A near average yield is not likely with severe dryness in August.   Statewide, Illinois rainfall was only 1.85 inches in August and 52% of normal.

Drought Rare

The National Climatic Data Center has a different way to evaluate rainfall, comparing the current precipitation in the state to the other years in the 116 year history. Illinois August rainfall was found to be the 8th lowest on record. That would be the 7th percentile. Iowa and Indiana, other key soybean states, were very dry also though not as extreme as Illinois, ranking among the driest 25% to 30% of years in the record.

On temperatures, Illinois was not extremely hot, though still "above normal".  Temperatures in the Upper Midwest were warmer, relative to the 116 year record, making the top quartile in  Minnesota, South Dakota and Wisconsin. Southern soybeans  were "much above normal"  in Arkansas and Louisiana.

The United States soybean yield in September was raised .4 bushel from August to 41.8 bushel per acre.    That seems rather  optimistic given low August rainfall in major Midwest soybean states Illinois, Iowa, southern Minnesota and Indiana.