The corn plant knows best when it needs more nitrogen and other nutrients, but too many times growers or consultants aren’t listening to it at the right time.

“We don’t do enough of trying to analyze the plant when it’s taking in the majority of its nitrogen, and that is around the silking period,” says Tony Vyn, University of Purdue agronomy professor. “At that stage, it could be taking up to 5 pounds of nitrogen per day. During that time, the plant can tell you if it has sufficient nitrogen along with other nutrients.”

(In this second of a two-part series, Tony Vyn, University of Purdue agronomy professor, analyzes the nutrient requirements of modern corn hybrids and how they have changed in recent years.)

Vyn advises a late-split nitrogen application, and he encourages crop consultants and producers to spend more time in the fields at flowering.

“It’s the worst time to have to walk through a corn field but the best time to get ear-leaf samples to indicate if you’re in the right concentration range with nutrients. Phosphorus also is determined in this ear-leaf test,” he says.

Researchers also need to focus more on achieving nutrient balance in corn, he adds. “It’s time for us at the land grant institutions to spend more time determining the sufficiency level of ear-leaf nutrient concentrations in modern hybrids at substantially higher yield levels.”

Vyn participated in a worldwide study looking at ratios of nitrogen to phosphorus and nitrogen to potassium.

“This is a question of balance. In your personal life, your balance tells a lot about who you are and how you react to stress. The same goes for a corn plant. If it’s better balanced in terms of its nutrient composition for the critical growth stages, it’ll be better able to tolerate temperature or moisture stress.”

The ratio that is ideal in a corn plant for nitrogen to phosphorus is 5:1 for higher yield levels, says Vyn. For nitrogen to potassium, it’s essentially 1:1.

“So if you hear presentations that say we should be putting on 250 pounds of nitrogen, you also should have about 250 pounds of potassium at maturity in the corn plant.”

Most of the focus on late applications on corn is with nitrogen, and this can be done in a number of ways, says Vyn.

Options for late nitrogen applications

“It can be done with a bulk spread of urea, but in our program we’re focusing more on using injected UAN using coulter systems between the rows. The hypothesis here is that modern hybrids will be more responsive to that late split of nitrogen.

“We also need to be thinking about a late split as a pre-planned program. Whenever we have excessive rain, we have losses due to de-nitrification as well as due to leaching. A sidedress program already is a big step forward compared to a pre-plant nitrogen program, which is what the majority of our growers do. But even then, we should move towards the intentional late application of 30 to 50 units of nitrogen.”

If the recommendation for nitrogen for a particular yield level is 200 pounds, Vyn says he would apply it later, after the V10 or after the V12 stage, because of the higher amount of nitrogen that goes into the plant in later stages.

“We’re looking at how much yield benefit there will be with newer hybrids by intentionally going in late with the nitrogen application. In comparing the older with the newer hybrids, we got the highest yield with 180 total pounds of nitrogen: 140 pounds as a side-dress application at V5 followed by another 40 pounds at V12. This is the first year, and we’ll continue looking at this.”

There are quite a few hybrids today that are responsive to a late split of nitrogen, he says, and from a stewardship point of view, it makes sense to plan on a late application rather than to apply late nitrogen only in a rescue situation, where you lose nitrogen due to excessive rain.

“We have to move away from the rescue situations because we’re stewards of the land, air and water, and we have a responsibility from an air quality and water quality point of view.”

Modern hybrids are entirely different, he says, and as a group take up more of their nitrogen post-silking. They probably take up at least 40 pounds more in 200-bushel-plus corn, or sometimes as much as 8 pounds per acre or more. Post-silking nitrogen accounts for 56 percent of final nitrogen in grain, says Vyn.

More research needed

More rate/timing research is needed on intentional late-season nitrogen applications and the associated yield and nitrogen loss risks, says Vyn.

“We know higher plant densities can be hurt by nitrogen deficiencies. Also, no-till production can help limit nitrogen losses, and strip-till provides new nitrogen timing and placement opportunities.”

Side-dressing some or all nitrogen is certainly superior to preplant only, says Vyn.

“Consider late-season applications, especially when high rainfall has resulted in deficient soil mineral nitrogen relative to corn’s requirement. Consider a strategy of never putting on more than 50 percent of your nitrogen application at any one time, and make that application when the modern hybrids take up their nutrients.”

The goal, says Vyn, is to do the best job possible on the production side without forgetting the environmental part of the equation.

“Regardless of whether we’re looking at nutrients or hybrids, it’s important to be part of a sustainable system. We want to ultimately enhance the soil’s health.”

Trials conducted since 1975 show there’s nothing better than growing corn in a rotation, says Vyn. Continuous corn yields continue suffer at a rate of at least 10 to 20 bushels or more.

“Any system on the farm affects soil properties, not just structure and the relative amount of wind and water erosion, but also nutrient stratification. So as we think about trying to push for higher corn yields, we need to think about nutrient stratification and whether or not the new roots for any particular stage of corn growth are actually in the zone where nutrient supply is adequate.”

No-till approach

In a continuous no-till program for corn and soybeans, he says, there’s about a 3:1 ratio for soil phosphorus in the top 4 inches compared to the lower 4 inches.

“Anything other than a mold-board plow led to stratification. In sandier soils, you wouldn’t have this extent of stratification, and it also depends on how much rainfall you receive or irrigation water you apply. The influence on soil-test potassium is almost the same.”

That’s something to think about, he says, when you’re considering nutrient application programs. “If your only program is one of broadcast applications, then for a given soil and given rainfall and a given program, the amount of stratification is not just dependent on yield levels but very much dependent on where you’re placing the nutrients themselves.

“For several years, we’ve been looking at nutrient placement options other than broadcast, and sometimes that’s part of a strip tillage program where we’re essentially trying to do some fall strip. In a fall program in Indiana, we do not recommend a fall anhydrous application. Sometimes, it’s a response to the price of nitrogen in the fall.”