1,000 Growing Degree Days: Protecting Nitrogen for When Corn Needs It Most
For many operations, nitrogen application occurs before the timer even starts on the growing season, with a significant dose of nitrogen fertility going down in the fall or very early spring – before the seed is even planted.
“Across much of the country, our application window is well ahead of when the crop is actually going to be rapidly pulling in water and nutrients and experiencing that rapid growth,” said Evan King, technical agronomist for Koch Agronomic Services (KAS). “When you think about nitrogen applications, even UAN or urea applications intended for crop uptake will be heavily loaded to our plan's pre-plant side. This includes fall ammonia applications, which happen months before we even start putting seed in the ground.”
When Does a Crop Truly Need Nitrogen
Understanding the nitrogen uptake phases in a corn crop is crucial for informed decision-making. A corn crop requires nitrogen from the beginning, but the uptake is not uniform throughout the plant’s growth cycles. It occurs in three major phases, each with its unique characteristics and requirements:
Early Growth: Less than 20% of nitrogen uptake occurs between germination and V6. This is when corn is pushing its growing point above the ground and working to elongate the nodes.
Rapid Growth: About 60% of total nitrogen uptake occurs between V6 and VT. This is the phase where major vegetative and early reproductive growth occurs, including ear establishment and ear size determination. Nitrogen availability is critical during this time.
Reproductive Growth: The crop’s last 20% of nitrogen use occurs here, and what it can’t pull from the soil, the plant will now repartition from stalks to grain. Corn continues to use additional nitrogen through R6 maturity.
“What can happen from the time that we put on our nitrogen to the time we reach the finish line is that we could start running out of nitrogen via a loss pathway,” said King. “If that happens, we can start to see plants cannibalize nitrogen from the stalks and blades, leading to weak stalks and corn that can experience lodging. It can also just run out of gas and not reach its full yield potential.”
When these high-nitrogen-use phases occur is based on Growing Degree Days (GDDs), or heat units. GDDs measure heat accumulation and can estimate the timing of crop development. In simpler terms, GDDs measure the amount of heat a plant has been exposed to, which helps predict when certain growth stages will occur.
GDDs are calculated by subtracting a base temperature from the average daily temperature (GDD = [ (T MAX + T MIN )/2 ]−T BASE). Very little growth occurs below the base temperature. While plants have different temperature requirements, corn has a base temperature of 50 degrees Fahrenheit. The rapid growth phase in corn occurs just before 1,000 GDDs.
Farmers in the Corn Belt can use this tool from the Midwestern Regional Climate Center to see the average days to 1,000 GDD for their county.
The “Nitrogen Uptake and Partitioning in Corn” graph visually represents how corn plants absorb and distribute nitrogen at different growth stages. This can help farmers understand when their crops most need nitrogen.
*Reprinted by permission. Modern Corn Hybrids’ Nutrient Uptake Patterns (2013. Bender, Haegele, Ruffo and Below.)
How Applied Nitrogen is Lost
Nitrogen can be lost from the soil through three main mechanisms: volatilization, denitrification and leaching. Volatilization occurs when urea is hydrolyzed to ammonia and escapes to the atmosphere. UAN is 50 percent urea, making it susceptible to volatilization losses as well. Denitrification occurs when anaerobic bacteria in saturated soils reduce nitrate to nitrous oxide or nitrogen gas. Leaching occurs when nitrate is dissolved in water and moves below the root zone.
The rate and extent of nitrogen loss can vary significantly depending on the combination and intensity of these factors. Estimates from Iowa State University and the University of Illinois suggest that volatilization can range from 1 to 75 pounds nitrogen/ac/season, denitrification can range from 2% to 5% per day, and leaching can range from 10% to 30% of the applied nitrogen. This variability underscores the need for caution and attention when managing nitrogen loss.
Stopping Loss Before It Starts
Nitrogen stabilizers can help reduce nitrogen loss by inhibiting the enzymes or bacteria involved in converting urea or nitrate to gaseous forms. In simpler terms, they help keep the nitrogen in a form the plant can use rather than letting it escape into the air or leach through the soil profile.
However, not all nitrogen stabilizers are equally effective. Some low-rate stabilizers may not provide enough nutrient protection to get the crop to 1,000 GDDs. Therefore, it is important to choose a strategy that provides the longest protection for nitrogen applications—in fall or spring.
Some factors to consider when choosing a nitrogen stabilizer are the active ingredient, rate, timing and placement of the nitrogen fertilizer, as well as the soil and weather conditions. A nitrogen stabilizer should match the nitrogen source and the loss mechanism that is most likely to occur. For example, a urease inhibitor such as ANVOLTM nitrogen stabilizer or AGROTAIN TM nitrogen stabilizer can help prevent volatilization from urea, while a nitrification inhibitor like CENTUROTM nitrogen stabilizer can help prevent denitrification and leaching.SUPERUTM premium fertilizer is another option, protecting nitrogen through both urease and nitrification inhibitors.
“We have two patented active ingredients in Duromide and Pronitridine,” said King. “They are the next generation stabilizers on the market, proven to bring added benefits beyond what the competitors can, with a more stable active ingredient and a longer application window across soil environments.”
Nitrogen stabilizers are crucial in protecting the nitrogen investment and ensuring its availability for crop consumption. It is essential to choose a strategy that gives the most extensive protection to get your crop through its busiest stages.
The decision of when to apply nitrogen is often one of operational efficiency, striking a balance between having the time, equipment and manpower to make an application and when the most value will likely come from doing so.
