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# How long before this corn tassels?

Crop Watch: Growing degree formula can help predict development schedule.

The corn in the photo was at the V3, or three-leaf, stage when the photo was snapped on May 27. The short bottom leaf counts as the first leaf. There are two more leaves with exposed collars. The fourth leaf coming out doesn’t count. It counts if using the “droopy leaf” method, but most agronomists and pesticide labels are based upon the leaf collar method. Only leaves with collars exposed are counted.

This information is laid out clearly in the Purdue University Corn & Soybean Field Guide, published by the Purdue Crop Diagnostic Training and Research Center. The center is directed by Corey Gerber.

The guide also explains how to predict when corn will reach a certain stage of development. Here are the steps.

Step 1: Understand the growing degree days concept.

This method bases expected growth on the number of heat units accumulated each day. The formula used for growing degree days most often is: GDD equals daily maximum temperature in degrees F plus daily minimum temperature. Divide the result by two; then subtract 50. Temperatures above 86 degrees F are entered as 86, and those below 50 degrees F are entered as 50.

Step 2: Try an example of GDD formula.

Suppose the high is 90 degrees F and the low is 55 degrees F on June 20. The formula calculates out to 17.5 GDDs for the day.

Step 3: Calculate growth through the V10 stage first.

Previous Purdue research says it takes 82 GDDs on average from emergence through V10 to add one leaf collar. This field was at V3 on May 27. Suppose an average of 17.5 GDDs will occur from then through the 10-leaf stage. Then it will take 4.7 days to add each leaf, or about 33 days to go from V3 to V10. That would mean the plants would reach V10 about June 29, assuming normal weather.

Step 4: Calculate growth stage from V10 to tasseling.

From leaf stage V10 to final leaf, it only takes about 50 GDDs. Suppose the plant tassels at leaf 18. Assume the average GDD increases to 22. Then the plant could reach tasseling in about 18 days. That would mean this corn would tassel around July 16.

Some corn will likely tassel earlier this year, and some might tassel later. Predicting growth stage is helpful for planning potential pesticide applications, including fungicide applications.