Applying fungicide to corn mid-way through the growing season has become quite common over the past several years. Gray leaf spot disease, which overwinters in Kentucky and diagnosed in corn every year is generally the disease targeted for control.

Southern corn leaf rust, which does not overwinter in Kentucky, is the disease of greatest concern for yield loss. SCLR arrived in Daviess County this year. Early-mid April corn was in the milk-dough stage and made it through with minimum yield loss whether or not fungicide was applied. May planted corn required fungicide to avoid losses. There were reports of a second fungicide application benefiting an additional 30 bushels in mid-late May planted corn.

One of the associations with fungicide use is higher harvest moisture. This is not an annual phenomenon but September of this year held excessive moisture in April planted corn through the end of the month. Dr. Kiersten Wise, Extension Plant Pathologist at the Grain and Forage Center for Excellence, reported a 2-3 point difference in moisture between fungicide treated and non-treated plots in her trials. The fungicide treated plots decreased very little in moisture over three weeks, while non-sprayed treatments decreased at a much steadier pace. The same scenario held true for our region.

The expense of drying corn led to several discussions this fall regarding the cost of fungicide application, the cost of drying corn, and the return on fungicide investment. I used a local field case study and the UK budgets found online at to present two scenarios. The field used in this example was 40 acres harvested on Sept. 23 that produced an average dry yield of 240 bu/acre. The average harvest moisture was 20% and the average market price was $3.50. Elevator drying charges averaged $41.11 per acre. The fungicide cost $17.00, helicopter $15.26, and $10 for a gallon of foliar fertilizer used to increase spray carrier volume equaled a total fungicide cost of $42.26.

Based on observation and research it can be assumed that a fungicide application in optimum growing conditions could protect as much as 10% of the final yield but increase harvest moisture 2.5%. Using these assumptions, the final dry yield without fungicide for this field would have been 219 bu/acre and 17.5% moisture. Drier corn and fewer bushels would cost an average of $19.59/acre to dry.

The purpose of this illustration is to determine the return on investment of fungicide for this particular field. Land is the one variable cost different from the others. I always suggest using a third of gross revenue as the opportunity cost of farming the land, whether owned, rented or mortgaged. Land cost at 240 bu/acre was $266/acre. Land cost in the $219 bu/acre scenario was $255/acre.

For this field, the return above all variable costs per acre including the fungicide application expense, additional drying, and land cost was $123/acre. It was $114/acre in the comparison with no fungicide expense and a lower land cost and drying charge. Overall, the fungicide return on investment was $9/acre.

The takeaway is that despite the much higher yield obtained with fungicide, the additional expenses prevented a runaway margin gain. This corn was early allowing it to be far into kernel development before the onset of SCLR. I am not suggesting that the narrow margin advantage is reason to forgo fungicide, nor am I suggesting it will always be a positive return on investment. The extended cost of grain drying this fall is unusual but I think it is a function of high-quality test weight corn that was achieved with fungicide use protecting leaves from SCLR during grain fill.

The field in the study had an exceptional yield. I have always said that unlike most production inputs, the decision to purchase fungicide can be made on a field-by-field basis mid-way through the growing season based on growing condition observation. If a field is suffering from flood or drought, forgoing the $42.26/acre cost is probably wise. If tasseling initiates in a solid, consistent growth field enjoying optimum growing conditions, the 10% or greater yield benefit associated with fungicide is likely obtainable. In a year when SCLR occurs any time before the dough stage, a fungicide will easily protect 10% or more of field yield.

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