How to Manage Nitrogen

February 1, 2017 02:14 PM
 
Corn - Nitrogen Management

A 200-bu.-per-acre corn crop consumes 330 lb. to 350 lb. of nitrogen per acre. Know your soil’s nitrogen-supplying power to fine-tune application rates.

The need to protect water supplies while feeding a burgeoning world population will spur farmers to become the best nitrogen stewards ever. It won’t be easy, says Farm Journal Field Agronomist Ken Ferrie, and it will take time. But he predicts farmers will meet this challenge, just as they have responded to every other call to feed more people while optimizing nutrient use.

“Farmers will have to become more efficient than ever to meet goals like the 45% reduction in nitrogen loss called for in Illinois’ nitrogen reduction strategy,” Ferrie adds. The purpose of the strategy is to reduce nitrate pollution in the Gulf of Mexico, Chesapeake Bay and other water bodies. States are adopting their own strategies and targets, as required by the Environmental Protection Agency.

The tools, knowledge and information for razor-sharp nitrogen planning are already available. In the January issue of Farm Journal, we told you how to lay out nitrogen management zones based on each soil’s nitrogen-supplying power and risk of loss. The next step is to understand when your corn plants need nitrogen because not all hybrids have the same requirements. 

Ferrie calls the concept “just-in-time nitrogen management.” It means having nitrogen available when needed, without maintaining a large inventory in the soil where it can be leached away into water supplies. It’s similar to the way auto manufacturers operate, with parts arriving as needed rather than sitting for long periods in a warehouse. Of course, the inputs, whether nitrogen or auto parts, must be on hand when needed or the factory will shut down, Ferrie cautions. With plants, if the factory shuts down, yield potential is lost and it can’t be regained.

Another way to look at it is you must have enough nitrogen available so corn plants never have a bad day but limit the excess supply to protect the environment. “It’s a tightrope we’ll have to learn to walk,” Ferrie summarizes. “It is based on the 4Rs—right product, right rate, right time, right place, which growers already are implementing.”

The right-time aspect requires understanding how much nitrogen (per day, week or month)corn plants take up at each stage of growth. “Fortunately, there’s a wealth of published information farmers can use for guidance,” Ferrie says. “Using computer models, we can input our planting date and weather data and the model can predict when plants will reach various growth stages and how much nitrogen must be taken aboard each day.”   

Here’s what a nitrogen manager needs to know about season-long uptake. “A 200-bu. per acre corn crop consumes 330 lb. to 350 lb. of nitrogen per acre,” Ferrie says. “On many soils, we only need to apply about
200 lb. of nitrogen, or less, because the soil provides the rest of the nitrogen. Our challenge is to make sure sufficient nitrogen is present when the plant needs it throughout the growing season. That requirement is small at the beginning and becomes very large later in the season. And it varies
by hybrid.”  

As shown in the graph on page 14, right after emergence, corn takes up only a very small amount of nitrogen. By the time a plant reaches the V5 growth stage (five leaf collars showing), it might contain only 8 to 10 grams of dry matter in its leaves, stalks and roots, and that dry matter is only 1.5% to 2% nitrogen. So at 36,000 plants per acre, one acre of corn takes up only about 1.2 lb. of nitrogen through the V5 stage. 

When corn needs nitrogen

Although uptake is low from emergence to V5, it can’t be neglected. “Poor placement, one of the 4Rs, can restrict plants from finding even
1 lb. of nitrogen,” Ferrie says. “If you applied anhydrous ammonia 7" to 8" deep the previous fall, the nitrogen might still be there, but it will be out of reach for the plant.”

Another factor is the carbon penalty, in which a large volume of old
crop residue stimulates microorganism populations and causes soil nitrogen to become tied up and unavailable until later in the season. 

Ferrie’s studies show the carbon penalty can tie up 100 lb. per acre of applied nitrogen. “So you must manage for the soil environment,” he says. “It will be different for continuous corn than for a corn/soybean rotation, and it will be affected by various cover crops and tillage systems.”

At the V8 stage, corn plants shift into rapid uptake. From then through the rest of the season, plants take up
5 lb. to 10 lb. of nitrogen per day.

During the V5 through V8 growth stages, sufficient nitrogen is critical because that’s when many hybrids begin adjusting their potential ear size. “If a plant suffers serious nitrogen deficiency between the V5 and V8 growth stages, it might cut back from 18 rows of kernels to 14 or 16,” Ferrie says. “Once a plant scales back its ear girth, we can’t get it back.”

From V12 to R3, plants store nitrogen in their stalks. If at any time a plant can’t meet its nitrogen needs, it translocates nitrogen from its stalk to the grain.

At about R3, the plant begins heavy translocation of nitrogen from the stalk to the grain, as plants work on filling kernel depth. Through R4 and R5, entering the dent stage, the plant continues to translocate nitrogen from the stalk into the grain. “If the stalk is empty of nitrogen at this time, it will affect grain fill,” Ferrie says.

At V12, growth becomes so rapid that, as farmers often say, you can hear the corn grow. “At this stage, the nitrogen uptake rate is steep, and the supply is critical,” Ferrie says. “This is the crucial period in which maximum ear length still is being negotiated inside the plant. It continues all the way to grain fill. After V12, if we stress the plant very long, without enough nitrogen, it might start to abort kernels. 

“Kernel abortion can continue into the dough stage, and, once it happens, you can’t get those kernels back. Our studies have shown, by the time we see lighter green color in nitrogen-
deficient strips, we usually have given up some yield. We can turn those plants green again by applying nitrogen—and we have to, to avoid losing much more yield—but we can’t make up the lost yield potential. 

“If corn plants change color, you were not just-in-time with your nitrogen application; the corn plant has already slowed down,” he adds.

The final factor in your nitrogen is your hybrids’ response pattern. “Companies are starting to provide information about whether their hybrids prefer nitrogen up front, at the back end of the season or broken up with split applications,” Ferrie says. “If this information is not available for your hybrids, you can incorporate nitrogen timing into a hybrid test plot and observe the response.” 

To analyze a hybrid’s response to nitrogen timing, consider how it flexes its ear. “Our studies convince me all hybrids flex,” Ferrie says. “They flex only one direction—downward. In other words, if you plant a hybrid at a very low population, it will maximize ear size to its genetic potential; with enough sun and nutrients you may get multiple ears. As we crowd plants in the row, ears flex downward in size.” 

Based on three years of hand-harvested hybrid studies, in which Ferrie and his crew counted and weighed every kernel, they found the hybrids in their plots flexed three ways: number of rows of kernels, length and depth of kernel. “Some hybrids flex all three—girth, length and depth; I call them full-flex hybrids,” Ferrie says. “Others, which we call determinate, flex only in depth of kernel. In between are some semi-flex and semi-determinate hybrids.

 

Accuracy of Nitrogen Models

Computer models that help farmers keep track of corn’s nitrogen needs will also help farmers master just-in-time nitrogen management. Farm Journal Field Agronomist Ken Ferrie tested these models in his own research studies.

Corn College

The studies involved analyzing thousands of ears from multiple plots to learn how plant density and nitrogen programs affected ear flex.

Ferrie’s staff at Crop-Tech Consulting and client-volunteers hand-harvested corn from test plots, weighed ears, counted kernel rows and row length and shelled the grain. Ferrie had the grain analyzed by a laboratory to determine nutrient content.

“That told us how many pounds of nitrogen were contained in the grain,” Ferrie says. “But we still didn’t know when nutrients were picked up by the plants or how many nutrients were contained in the roots, leaves and stalks at various growth stages.”

To learn more, Ferrie’s staff grew corn hydroponically, using only water and nutrients. They harvested the plants at the V5, V8 and V12 growth stages. Laboratory analysis of root masses and above-ground tissue revealed the amount and location of nutrients in various parts of the plant at each growth stage.

“Our study confirmed most of the computer models used to predict nitrogen uptake by corn plants are pretty accurate,” Ferrie concludes.

Corn College

As a sidelight, the tests showed, as one would expect, that more mobile nutrients, including nitrogen, were higher in the above-ground plant tissue than in the roots. With less-mobile nutrients, there was much less difference between the amount in the roots and the amount in leaf/stalk tissue.

“I think the fact there is more nitrogen in the above-ground tissue might explain some of the challenges we’ve encountered in managing the carbon penalty after some cover crops,” Ferrie says. “The root systems contain a higher carbon/nitrogen ratio than the above-ground residue. If we look only at the carbon/nitrogen ratio of the above-ground tissue, we might not realize the extent of the carbon penalty created by the cover crop.”

 

“Planting what I call full-flex hybrids is somewhat higher risk,” he continues. “While they can produce high yield at low populations, they don’t handle stress as well as determinate hybrids. The determinate hybrids maintain their kernel count—the size of the ear gets smaller, but the number of kernels on the ear stays the same. These hybrids handle stress better, especially at the front of the growing season.”

From Ferrie’s research, here’s what ear type means for nitrogen management: 

  • A full-flex hybrid, if stressed at the V5 growth stage, will flex back from 18 rows of kernels to 14 or 16 rows. “Every two rows of kernels is 20 bu. per acre you won’t get back,” Ferrie says.
  • If you have sufficient nitrogen and no early season stress, a full-flex hybrid will maintain its row number; but if you stress it from the V12 to R2 stage, it will abort kernels. A determinate hybrid is less likely to abort tip kernels. “Every kernel aborted at the tip of the ear is worth 5 bu. per acre of yield,” Ferrie says. “If you fall from 45 kernels to 35 kernels per row, that’s a loss of 50 bu. per acre.”
  • Determinate-ear hybrids must finish strong—they can’t run short of nitrogen at the end of the growing season. “From R2 to R6, plants gain depth of kernel,” Ferrie says. “That is the only area in which determinate plants can flex. If a determinate hybrid runs out of nitrogen, its ears might be the same kernel length as those of a full-flex variety, and have the same number of rows, but its kernels will be shallower. So, it can cost you a lot of bushels. You need to have a backup plan for applying nitrogen, such as aerial application.”
  • Knowing how a hybrid flexes or handles stress helps you plan how to keep nitrogen available. “Management is more critical with a full-flex hybrid; but late-season nitrogen is more important with a determinate hybrid,” Ferrie says.
  • The issue of nitrogen timing is why a hybrid sometimes yields for one farmer but not for another; they follow different nitrogen programs. “When you study hybrid plots, look for those where the nitrogen program matches yours,” Ferrie says.

Timing is one of the keys to avoiding yield loss as you work on just-in-time nitrogen management. “As you scale back nitrogen rates, you must understand how hybrids respond and what running short of nitrogen will cost you in yield,” he adds. “Fortunately, you have many tools in your toolbox, including computer modeling, soil nitrate testing and tissue testing, which can help keep you from falling behind schedule. High-clearance applicators, aerial application and irrigation allow you to apply nitrogen late in the season, if the crop needs it.”

Complicating the challenge, as we explained in our January article, is the fact much of the nitrogen used by corn comes from the soil itself. How much nitrogen becomes available is influenced by soil type and weather.   

Moving to just-in-time nitrogen won’t be easy. “But we may have to do it in order to meet nitrogen loss reduction goals,” Ferrie says. “And I’m confident we will.”

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