You plant corn this week. How fast will it grow? How long do you have to get anhydrous ammonia applied by sidedress application?

Corn responds to heat units, which are a measure of the amount of warmth and energy that accumulates during the season. Here are five steps to determine what stage of growth corn should be at during certain times. It depends upon heat units, which depend on the type of season Mother Nature dishes up.

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HOW LARGE IS SIDEDRESS WINDOW? This corn is at the four-leaf stage. At 82 GDDs that must accumulate per leaf, it needs 328 GDDs, or nearly 17 days at 20 GDDs per day, to reach the eight-leaf stage.

These steps are taken from the Purdue University Corn & Soybean Field Guide, 2016 edition. It is published annually by the Purdue University Diagnostic Training and Research Center. Corey Gerber is director for the center.

Step 1. Understand growing degree units

This is a way to quantity how corn plants should respond to the amount of energy that accumulates during the day. The formula is: growing degree days (GDDs) equals maximum daily temperature plus minimum daily temperature divided by two. After reaching that number, subtract 50. The only catch is that temperatures above 86 degrees F and below 50 degrees go in as 86 and 50, respectively.

Related: Have a smart phone? Download our Growing Degree Days app

Step 2. Work examples to understand how many GDDs can accumulate on a typical day.

Gerber includes this example to explain how GDDs work. If the daily high temperature was 80 degrees  and the nighttime low was 55 degrees, then the formula is: [(80 + 55) divided by 2] - 50 = 17.5 days.

So 17.5 GDDs, a measure of heat units, accumulated that day.

Step 3. Know how many GDDs corn needs to emerge after planting.

Purdue agronomists say there will be a range from 90 to 120 GDDs from planting to emergence. They recommend using 115 GDDs for emergence.

Step 4. Know how many GDDs it takes for the plant to produce one leaf.

From emergence to V10, the Purdue guide says it takes an average of 82 GDDs to produce each leaf. From V10 to tasseling, it only takes about 50 GDDs per leaf. These numbers are estimates .

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Step 5. Put the information to use to make estimates of when corn will reach certain stages.

Example: Assume you planted May 1 and want to know how long it will take corn to reach the eight-leaf stage. You want to make sure you have finished applying anhydrous ammonia by then. In other words, how much time do you have to get anhydrous applied?

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First, say it took 115 GDDs to get emergence. If you are picking up 11.5 GDDs per day, then that would take 10 days (115 ÷ 11.5 = 10 days). Each leaf requires about 82 GDDs. For eight leaves, you will need 8 × 82 = 656 GDDs. So if you planted May 10, it took 10 days to emerge to May 20, and you accumulated 20 GDDs per day, you would need about 33 days after emergence, or until about June 20, to reach the eight-leaf stage. That would help you plan application schedules so you can reach all fields before corn is too tall.