Weatherproofing for Critical Stages

If corn color and growth show a visual response to sidedressing, it would have grown faster with more nitrogen applied preplant or with the planter. Strip-tests can help you determine the ideal rate to apply. ( Darrell Smith )

Yield is what results when corn plants negotiate with their environment, and the process becomes especially intense from emergence to tasseling. Just as when humans negotiate with each other, stress during the bargaining process can have lingering aftereffects.

“During this period, plants can’t afford to have a bad day,” says Farm Journal Field Agronomist Ken Ferrie. “If yield potential is lost early, it is very hard to recover.”

Weatherproofing at this critical stage emphasizes two of the 4Rs of fertility management—right time and right place. Plants must never go hungry, even if weather is uncooperative.

From emergence to tasseling, a corn grower is managing a factory that produces starch. Ferrie explains how the process, which is fueled by photosynthesis, impacts number and size of kernels:

  • Every acre must manufacture enough kernels to contain the amount of starch produced based on those growing conditions.
  • Yield consists of the number of kernels per acre and the amount of starch per kernel.
  • Genetically a corn plant is set to produce so many kernels per ear.
  • Kernel size flexes more for some hybrids than others, but maximum size is limited.
  • The number of kernels is first set by population or ear count. If the ear count is too low to hold an acre of starch output, the kernels will expand to the maximum possible size. So when you have big ears filled to the tip, it might mean you didn’t produce enough kernels to hold an acre’s worth of starch.
  • An ear of corn typically produces 600 to 650 kernels.
  • Shortly after emergence, the ear starts to adjust its kernel set to the environment. If a plant only produces enough starch for 450 kernels, it’s better to abort 250 kernels than to have 650 that don’t mature.
  • So early-season stress causes plants to adjust kernel set per ear. Once an ear adjusts from 20 rows to 16 rows, or from 45 kernels long to 35, it will not increase kernel count later, even if the stress passes (although some loss can be made up in kernel size).

“The bottom line,” Ferrie concludes, “if a corn plant is stressed hard enough for even one day, it might shed kernels it can’t get back.”

Sufficient nutrients must be available every day to keep plants happy. “The amount of nutrients consumed early is small, but its importance is high,” Ferrie says.

Most nutrients taken up by plants come from the soil, released by soil microbes. These microorganism populations are sensitive to soil temperature, moisture and other factors.

“As we study soil biology in our test plots, we see microbial activity really crank up above 65°F soil temperature,” Ferrie explains. “Then microbial activity doubles with every 10°F increase up to 90°F. The level of fertility on your soil test actually  predicts what will become available when the soil temperature rises above 65°F and biological activity is running wide open—and not necessarily what is available early in the season.

“We can get a lot of corn growth between 50°F and 65°F soil temperature if plants have adequate nutrients. But if nutrients are not available because microbial activity has not yet taken off, plants will grow slowly and may adjust their number of kernels.”

Crucial early nutrients, such as sulfur, nitrogen, phosphorus and zinc, might not be available in cold soils. “At this stage of growth, fertilizer timing and placement are important,” Ferrie says. “Applying phosphorus with the planter places phosphorus, which drives cell elongation and division, or growth, close to small plant roots. That phosphorus isn’t sensitive to soil temperature.

“This is why the visual response to starter fertilizer is more impressive in cold springs,” Ferrie adds. “Farmers often report height and color differences after a starter fertilizer tank runs dry or before the pump kicks in on the ends. That visual response illustrates weatherproofing—keeping nutrients available for plants might keep the ear kernel number closer to the plant’s genetic potential.

“If you save 50 to 75 kernels per ear, and have the photosynthetic capacity to fill them, you will have a higher yield. That’s where weather plays a role—if you increase the kernel count but don’t produce enough starch to take advantage of it, there will be no yield increase. That’s why in years with early soil warmup and no weather stress, we see less response to weatherproofing. But it’s like crop insurance—you buy it for protection when things go against you.”

Planters offer weatherproofing options. “With in-furrow application, be sure to consider fertilizer rate and salt content to avoid seedling damage,” Ferrie advises. “Placing fertilizer beside and below the seed lets you increase the rate without risking seed damage, but it also slows the response rate. A light in-furrow application for pop-up, in addition to fertilizer beside and below the seed couples a rapid response with a longer lasting response.”  

Don’t let plants suffer from the carbon penalty.

“When soil warms above 65°F, the microbial population explodes,” Ferrie explains. “Depending on the amount of carbon (such as old crop residue) and the quality (corn residue versus soybean residue, for example), the microbes consume many soil nutrients as they break down the carbon. This makes the nutrients temporarily unavailable to plants. The process is called immobilization, and it happens especially with nitrogen, phosphorus and sulfur.

“The tie-up of soil nutrients might be slow if the soil warms slowly, or it might be faster and more severe if the soil warms quickly. In that situation, corn plants might look green on Monday but take on a pale yellow color by Friday. Nutrients immobilized by microbes will be released later in the growing season, but before that happens, young corn plants might be starved for nutrients and may abort kernels. I call that the carbon penalty.”  

There are two ways to keep corn plants happy and well-fed during the carbon penalty. “One option is to broadcast enough nitrogen on the surface prior to planting to feed the microbes and the crop,” Ferrie says. “Our studies indicate a spring preplant application requires 60 lb. of nitrogen per acre or more in a corn/soybean rotation and at least 100 lb. per acre in continuous corn. Incorporated crop residue requires more nitrogen than residue left on top of the soil. While this is the easiest way to apply nitrogen, if you get a lot of rain it might move the nitrogen deeper into the soil where the young roots can’t reach it.

A second option is to focus mainly on the plants by banding nitrogen close to their roots. “Banding nitrogen is at least twice as efficient at paying the carbon penalty because the fertilizer is close to the roots when plants need it,” Ferrie says. “You can band fertilizer as you freshen rows in the spring or as you plant. Fall-applied anhydrous ammonia isn’t much help with the carbon penalty. The nitrogen is too far away from the small roots.”

Although sidedressing is a form of banding fertilizer, sidedressed nitrogen will be applied too late, and too far away from the plants, to manage the carbon penalty.

Confirm your nitrogen planning with strip tests. “To make sure your corn never has a bad day, put out some strips with rates higher and lower than your present rate,” Ferrie says. “Watch the crop from emergence to sidedressing. If a higher rate looks better, you are applying too little nitrogen. If the lower rates look just as good as your present rate, you are applying too much. Testing soil for nitrate can help analyze the response.”

If corn color and growth shows a response to sidedressing, it would have grown faster with more front-end nitrogen.

Nitrogen uptake increases during the rapid-growth stage. For most hybrids, that period is from V8 or V10 through tasseling. “During this period, plants can take up as much as 10 lb. of nitrogen per acre per day,” Ferrie says. “Fortunately, this is when most soils are at net mineralization, making nutrients available to plants.” (Note: Mineralization and immobilization actually occur at the same time, alternating between net mineralization and net immobilization, which creates the carbon penalty.)  

“The question for a crop manager is whether the soil can supply enough nitrogen fast enough,” Ferrie says. “Many soils can supply more nitrogen than a crop requires, but not in the time frame when it’s needed. Weather plays a big role in the rate of mineralization, as well as the amount of mineralizable nitrogen available.

“Lab tests can reveal a soil’s potential mineralizable nitrogen rate,” he adds. “This information can help you weatherproof your fertilizer plan. Soils with high potential mineralization rates give you more flexibility. On those soils, watch the weather. Mineralization will be slow if it’s hot and dry. If it’s warm and wet, you might lose nitrogen. Use nitrate testing and weather modelling to tweak your plan.”
Soils with low nitrogen mineralization capacity are tougher to weatherproof. “With soils low in mineralizable nitrogen, computer modelling is not very helpful,” Ferrie says. “Programs that calculate maximum economic return to nitrogen are based on average results from many soil types.

“While a bushel of corn requires the same amount of nitrogen on any soil, the amount applied must be higher on soils that can’t mineralize much nitrogen of their own. You may need to make multiple applications.

“Wet or dry soil can cause problems; more nitrogen needs to be present when rapid growth starts because the soil can’t keep up with the crop’s daily uptake,” Ferrie continues. “Often, you will need to apply some nitrogen later to finish grain fill.

“These soils respond more strongly to placement of nitrogen because meeting the plants’ daily need is a challenge. Tools such as Y-Drop nozzles, which place nitrogen in the stemwater area, are one way to weatherproof these soils. Even with Y-Drop nozzles, applications must be timely, or you will sacrifice yield.”

Irrigated soils are easier to manage because you can spoon-feed them through the season.

Many tools can help you with weatherproofing. They include high-resolution aerial images using NDVI (normalized difference vegetation index) or thermal imagery, tissue testing, nitrate testing, the Illinois Soil Nitrogen Test (ISNT) and field-by-field weather tracking. “But the most important one is boots-on-the-ground scouting,” Ferrie says.

Don’t feel guilty about neglecting soybeans to concentrate on corn nutrition during this critical part of the growing season. “Unlike corn, soybeans can stand a few bad days early in the season,” Ferrie says. “Their yield potential is determined during the flowering period.” 

Keep Your Pest Boss Busy

Although weatherproofing involves managing nutrients, don’t let that distract you from other threats during the critical period between emergence and tasseling. “Watch for insects and disease,” advises Farm Journal Field Agronomist Ken Ferrie. “Cutworms or corn borers can wreak havoc on the best nutrient plan.

“Once corn is into rapid growth, take care with pesticide applications. If you apply pesticides with drop nozzles, be sure to follow label restrictions and warnings. Goosenecking during the rapid growth stage can reduce ear size.”

Too dry, too wet and everything in between. This story is the sixth in an eight-part series on weatherproofing your crops. Follow along at