Rethink Nitrogen Needs

Is it Time to Adjust Your Fertilizer Program?

Purdue agronomist Tony Vyn says modern hybrids capture more applied nitrogen, requiring lower rates for top yields. (Progressive Farmer photo by Tom Campbell)

How late can corn utilize a nitrogen application? New university information finds you can delay the final 20% to 25% of your nitrogen as late as the V10 to V12 growth stage. "That delay is possible because modern corn hybrids have the ability to take up more of their nitrogen in the postflowering period than older hybrids," explains Tony Vyn, a Purdue University agronomy professor.

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That new information comes from a research review by Purdue scientists released last spring. They analyzed 86 field experiments in the U.S. and China, comparing corn hybrids from before 1990 to those released after 1990. Scientists found that hybrids after 1990, which they refer to as modern hybrids, take up about 36% of their nitrogen after silking compared with about 30% in pre-1990 hybrids.

One surprise in the review was how much better modern corn hybrids can recover from mid-season nitrogen shortages. "A somewhat pale-green corn leaf color due to nitrogen shortage occurring mid-season is no longer a death sentence for yields," Vyn explains. "Nitrogen-stressed corn can recover very nicely when more nitrogen fertilizer becomes available as the soil dries or as late-season nitrogen is applied."

How much recovery? Vyn says trials show that temporary nitrogen-stressed corn can sometimes recover to produce yields equal to corn where adequate nitrogen kept leaves dark green every day of the growing period. "You don't want to intentionally cause nitrogen deficiency by underapplying nitrogen in the preplant and/or sidedress applications," he says. "However, with modern hybrids, the yield risk from underapplying in early applications is lower than with older hybrids."

CAPTURE N

The review also found modern hybrids capture more applied nitrogen fertilizer. "What this means," Vyn says, "is that higher yields in today's hybrids don't necessarily require more nitrogen than was applied 10 or 20 years ago."

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That hybrids today are more effective in utilizing nitrogen is one reason why land-grant universities have urged farmers to move away from the old rule of thumb that says it takes 1.1 to 1.2 pounds of nitrogen to produce 1 bushel of corn.

A free online program can help calculate nitrogen needs. It's called the MRTN (Maximum Return to N) corn nitrogen rate calculator (see cnrc.agron.iastate.edu) supported by Midwest land-grant universities. Recommendations are based on ongoing experiments with modern hybrids that take their improved efficiency into account.

ENVIRONMENTAL BENEFIT

It's good for your bottom line -- and the environment -- when hybrids capture a higher percentage of applied nitrogen.

How much applied nitrogen fertilizer actually makes it into corn plants? Vyn's guess is around 50%, or, in some cases, even less. He thinks 70% recovery is a good goal.

There are many factors why nitrogen recovery is so low. Some, such as weather, can't be controlled. But other factors can. For example, nitrogen recovery percentage is less when N rates are high (especially when above the agronomic optimum) and when all nitrogen fertilizer is applied the fall before planting," Vyn says. And, it's higher when nitrogen is split-applied at different stages of corn plant growth.

He cautions not to assume if plant nitrogen recovery is 50% that the other 50% is lost. Some nitrogen remains in the field. For example, some is retained in roots, some becomes long-term organic matter, some is used to feed microbes and some is lost to the atmosphere."

HYBRIDS MORE RESILIENT TO HIGH POPULATIONS

The research review by Purdue University scientists also found modern hybrids are more resilient in high-plant populations than older hybrids. "That's one of the reasons modern hybrids are achieving higher yields," says research agronomist Tony Vyn, noting that farmers have already figured this out. "Plant populations have been increasing, on average, about 300 seeds per acre every year," Vyn says, "with many Indiana farmers now planting around 32,000 seeds per acre."

He cautions, however, "It's not true with all corn hybrids that you get a yield boost with higher plant populations. Different hybrids vary in their responsiveness to high plant populations."

Hybrids that don't yield more in high plant populations aren't necessarily poorer yielders. "Hybrids utilize different strategies for higher yield," Vyn explains. "Some hybrids have increased kernel number per unit area, and others get a boost in yield from having more kernel weight with not very much change in kernel number."

PLANT DENSITY DOWNSIDE

He adds there's a downside to relying too much on increasing seeding rate to boost yields, even if you're using a hybrid that responds well in higher plant-density situations. The most obvious downside, Vyn says, is the increased cost of planting more seed per acre.

"The other downside is that corn planted at higher plant density tends to have higher earlier nitrogen uptake where more energy is put into leaves and less into the stem," Vyn says. By the time the corn reaches silking, it has more leaves and less stem even though there are more plants. "Corn in these higher plant-density situations can be more sensitive to nitrogen deficiencies than corn in moderate plant densities," he cautions. "You need to be careful about pushing plant populations too high in situations where there are substantial risks of nitrogen loss, because higher plant densities will then yield less."

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