Corn field

SOIL SCIENTISTS with the University of Arkansas recommend corn growers vary their nitrogen rates depending on yield potential.

Six years of Arkansas research leads to updates for corn nitrogen recommendations

This past growing season, Arkansas had over 900,000 acres of corn. That spike in acreage meant the working group was tasked with updating corn nitrogen recommendations, which was recently completed. Among the factors considered: hybrids, soils where corn is grown, and weather.

Of all the crops grown in the Mid-South, corn uses the largest amount of nitrogen — and that means “nitrogen represents a very important portion of the production cost associated with growing corn in Arkansas,” says Leo Espinoza, University of Arkansas Extension soil scientist.

“Because our soils are fairly low in organic matter, we also know that most of that nitrogen needs to come from chemical fertilizer. Our soils don’t contribute very much N during the growing season, compared to the Midwest, where they can have soils with 5 percent to 6 percent organic matter.”

Also, there are environmental concerns associated with nitrogen, he says. “It’s critical for sustainable corn production in the Mid-South that our recommendations are based on research. Our farmers understand that, and they have supported our efforts.”

That’s why the university’s Fertility Working Group has such a strong focus on nitrogen, says Espinoza.

This past growing season, Arkansas had over 900,000 acres of corn. That spike in acreage meant the working group was tasked with updating corn nitrogen recommendations, which was recently completed. Among the factors considered: hybrids, soils where corn is grown, and weather.

Research has been funded by the Arkansas Corn and Grain Sorghum Promotion Board and the University of Arkansas Division of Agriculture.

“We’ve just finished a six-year study, looking at nitrogen recommendations for corn grown on clay soils,” Espinoza says. “We also have data from nitrogen rate studies in loamy soils.”

Previously, recommendations were based on the amount of N required to produce one bushel of corn on a particular soil texture. Recommendations also were based on yield goals.

“So, a farmer would choose a yield goal and we’d multiply that goal by a set number,” says Espinoza.

“Something we’ve since learned is that N is just one of many variables that affect yield production. So, rather than using the old, set number, the latest research results pointed us in another direction.”

While this is a bit of a change in conventional philosophy, “We’re now going with two sets of recommendations. First, is what we refer to as ‘relatively low’ yield goals, although they aren’t really all that low — perhaps up to 150 bushels per acre. The second set of recommendations is for higher yields — 150 bushels or more.”

So, the new recommendations are based on yield response to nitrogen rate equations the team developed. “We will continue using the same crop codes as before, as recommendations for P and K vary according to the expected yield,” Espinoza says.

“For nitrogen specifically, when the expected yield is up to 150 bushels, the recommendation is for 160 pounds and 230 pounds of N per acre for loamy and clayey soils, respectively.

“For yields of 150 bushels per acre or more, the recommended rates are 220 pounds and 290 pounds of N per acre for loamy and clayey soils, respectively.

“Nitrogen rates vary significantly from loamy to clayey soils. This may be related to higher microbial activity, and restricted movement of nitrate and ammonium in clayey soils.

“That approach fits the data more properly. Of course, we need to keep in mind that yields vary significantly across years and across locations. For instance, on the same field, a producer may see a yield variation of 20 percent to 30 percent.”

Much depends on weather, as producers are well aware.

“I know farmers who, in 2012, harvested up to 300 bushels per acre on fields where they typically harvest 180 to 200 bushels.

“Also important: We’ve found some variability among hybrids. Some hybrids yield more than others, and may have different nitrogen requirements. There are also differences between locations — say, between northeast and southeast Arkansas. There also appears to be rotational benefits.”

For those reasons, Espinoza reckons fertility recommendations are generic in nature. “They represent the average response to N rates in average growing conditions.

“Consider an established corn farmer, one who has been growing corn for years. By now, he probably knows what his crops need. These new recommendations are probably more useful for a new corn grower, so he can use them as a guide to gauge and modify his fertilizer program for his particular conditions.”

Another important change is testing for soil nitrates in corn. “We’ve stopped doing that because nitrates in soil can vary drastically, especially with changes in soil moisture. For example, soil nitrate information collected during the fall may not be relevant by the time a producer plants his crop in the spring.”

“That’s why we feel soil tests like N-Star — the nitrogen soil test being implemented on rice — may also have application for corn. University of Arkansas researcher Trent Roberts has initiated some work, looking at that soil test for corn.”

Does Espinoza expect Mid-South corn farmers outside Arkansas to consider this new information?

“Yes. While these recommendations were developed under Arkansas irrigated conditions, in reality, the same conditions may exist for other areas of the Mid-South. I’m sure Arkansas growers also look at recommendations from neighboring states.”

Yet another difference farmers outside Arkansas may notice is how nitrogen is split in the state. “Our recommendations call for a three-way split of N on corn,” he says.

“We apply one-quarter to one-third of the total N amount immediately before, or right after, planting. Then, we side-dress between the V-4 and V-6 stages. Last, many farmers would apply 45 pounds one or two weeks prior to tasseling. This last application is not an additional amount, but rather an additional split.

“That, we believe, is the most efficient way to apply nitrogen under our production conditions. Previously, we’d apply 100 percent of the N when the corn plant was only a foot tall. At that growth stage, corn only needs 25 percent to 30 percent of that amount.

“A good portion of the nitrogen is subject to leaching and volatilization losses. In some years, we’ve found that if we save that 45 pounds of N for application before tasseling it can really mean a big yield increase. It is difficult to predict when a yield increase is expected, because it all depends on the weather during vegetative stage — say 40-70 days after planting.

“I know there have been instances of a producer applying 130 pounds to 150 pounds of N and then getting three inches of rain a few days later. In that situaiton, he may lose a considerable amount of the nitrogen that’s applied. But if he has saved the extra amount for later, it can mean a yield bump, compared to a two-way split.”

Espinoza and colleagues are also looking at the relative performance of the different zinc sources available.

“Corn is being grown in areas with low zinc levels, and we know corn is a zinc-sensitive crop,” he says. “The list of zinc products varies according to formulation, solubility, concentration, presentation and price. The objective is to insure the corn crop receives the proper amount of zinc to maximize yield potential.”

Lately, there has been increased Mid-South interest in using in-furrow, liquid fertilizer — otherwise known as pop-up fertilizer. This is already a common practice in the Midwest.

“There are different products that we’re looking into,” Espinoza says.  “They’re good at increasing seedling vigor — we’ve found an obvious difference between corn plants that received fertilizer with the seed compared to those that didn’t. There is an obvious difference in height, and sometimes that can translate to a yield advantage.

“As a rule of thumb, when it comes to in-furrow fertilizers, we don’t want to apply more than 10 pounds of a specific nutrient per acre, and should avoid using fertilizer sources with a salt index higher than 20.

“Application rates should be lower for sandy soils, compared to clayey soils. Application equipment is critical — spraying liquid fertilizer on top of the bed is not as efficient as applying fertilizer on top of the seed. On my planter, I use a Keeton seed firmer, which has worked really well through the years.”

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