Growing Degree Days are commonly used to track crop development. For "105 day" corn, about 2600 GDD units are necessary to reach maturity. The cool and wet spring combined with a early May snowfall made it difficult for corn to get into the ground on time in Iowa. Planting corn deep into May and June puts the crop at risk of not accumulating enough GDDs prior to the fall freeze. The featured chart attempts to provide probabilities of accumulating GDDs based on the combination of growing season begin (planting / crop emergence) and end (first freeze) date. The left hand plot shows the overall frequencies based on yearly data for Ames since 1893. The right hand plot combines what has already happened this year with scenarios based on all previous years for 7 August and on. For example, for a corn crop emerging on 15 May, about 20% of the years got to 2600 GDDs by 15 September prior to the first fall freeze. Given this year's condition, for a corn crop emerging on 15 May, there is near no chance that 2600 GDDs can be reached by 15 September and only about 50% of all years will get us to 2600 GDDs at all. One mitigation is that as planting dates get later, farmers will plant hybrids with shorter maturities (less GDD requirements). Although, this seed change is not always logistically possible. The chart would indicate that we need to make it through much of September without a freeze to give the longer maturities a chance.

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