Forget the old rule of thumb: 1.2 lbs. of N/bu. of expected yield, minus credits for soybeans. That approach has been discarded.

The latest nitrogen (N) rate guidelines draw on a huge storehouse of N-response data to identify the most profitable application for any given ratio of N prices and corn values. “It's a new concept,” says University of Minnesota soil scientist George Rehm — one that is designed to produce the greatest return on fertilizer investment.

The profit-based approach to N management was developed by researchers at Iowa State University, and the Universities of Illinois, Minnesota and Wisconsin. It will be a significant change in thinking for many corn growers, Rehm says.

Yield-based N-rate guidelines have been used in most Corn Belt states since the early 1970s. Now, N management is getting renewed attention. Interest is being sparked by high fertilizer and natural gas prices, record corn yields — which have raised questions about appropriate rates — and inconsistent recommendations across states, says John Lamb, a University of Minnesota soil scientist. Concerns about the effects of N on water quality are also prompting scrutiny, Rehm adds.

The new recommendations evolved from two decades of N-response trials across a wide range of production environments in seven Corn Belt states. The field trials were conducted on non-irrigated sites using either spring preplant or sidedress N application. Data were gathered for both corn following corn (CC) and corn following soybeans (SC).

In a surprising finding, these experiments showed no relationship between optimum corn yields and the amount of N needed to produce those yields, Rehm says. Experiments in Minnesota, for example, found that in continuous corn, it took from 0 to 160 lbs. of N/acre to grow 150-bu. corn. Similar variation was seen across the Corn Belt. In other words, high corn yields don't indicate high N need.

How can that be? “That's the question everybody asks,” Rehm says.

The answer: The soil supplies unpredictable amounts of N. Plant-available N varies widely from year to year and field to field, depending on factors such as organic matter, crop rotations, drainage, tillage and weather. Productive soils can “supply a tremendous amount of N,” Rehm says. “The environmental conditions that contribute to high yields also help release N from soil organic matter.” Without adding any N, corn yields throughout the Corn Belt average about 55% of optimum in continuous corn and about 70% in a soybean-corn rotation, he says.

In addition, the new high-yielding corn hybrids are much better at using N, Lamb says. University of Minnesota trials from 2003 to 2005 showed that corn following corn used about 0.9 lb. of N fertilizer/bu., and corn following soybeans used 0.5-0.7 lb./bu.

“We've taken a lot of stress out of the corn plant's life,” Lamb says. New varieties have more extensive root systems, allowing increased nutrient uptake and better drought tolerance. Weed control has improved, cutting the competition for available N. And insect damage has been reduced, thanks to biotech varieties with built-in pest control.

Because it takes less N to grow corn today, the traditional yield-goal formula produces an N rate that's “too high, especially for corn following soybeans,” says John Sawyer, Iowa State University soil scientist. In Minnesota, for instance, “Surveys have shown that farmers have often been overapplying N at a range of 30-60 lbs./acre,” says Gyles Randall, University of Minnesota soil scientist.

As long as N fertilizer was relatively cheap compared to corn value, there wasn't a significant financial penalty for applying more N than the crop could use, Sawyer says. But when N becomes more expensive, economic losses can be significant. And there's more risk of nitrate leaching.

A five-year, continuous-corn study in southeastern Minnesota demonstrates the financial and environmental costs of overapplication, Randall says. “In this study, economic return to fertilizer N was greatest at the 150-lb. N rate.”

Average yields were 168 bu./acre. Increasing the N rate to 225 lbs./acre produced no yield increase. But profits were reduced by $27/acre. And nitrate-N concentration in the soil water below the root zone almost doubled.

The new N-rate recommendations are based on the optimum net return to N. That's the point where an application of N produces a corn yield increase large enough to pay for the fertilizer.

Net return to N — and optimum N rates — vary as N prices and corn values change. When the N price is high relative to corn value, it takes more yield to pay for the last pound of N, so the most profitable N rate drops. In contrast, when corn value is high compared to the N price, it takes less yield to pay for the last pound of N, so the most profitable N rate increases.

Researchers collected N-response data from over 700 experimental locations, then computed each site's net economic return to N using a range of N and corn prices. The resulting values were averaged to arrive at rates that provide the maximum return to N (MRTN). Suggested rates are given for CC and SC, at price ratios of 0.05 to 0.20 ($/lb. N:$/bu.).

Recommendations are now available for Illinois, Iowa, Minnesota and Wisconsin at Iowa State University's Web site, or from state Extension offices. Other Corn Belt states are now collecting N response data and will be developing similar guidelines in the future, Sawyer says.

The recommendations differ by crop rotation and region. For example, for SC, at a 0.10 price ratio (22¢/lb. N:$2.20/bu. corn), the MRTN rate is 163 lbs. N/acre for Illinois, 123 for Iowa, 101 for Minnesota and 107 for Wisconsin. For CC, at the same price ratio, the MRTN rate is 176 lbs./acre for Illinois, 174 for Iowa, 136 for Minnesota and 139 for Wisconsin.

The new guidelines also offer a range of N rates that provide similar profitability. This gives growers flexibility to adjust the rates for local conditions, price fluctuations, and their own risk tolerance. Net returns to N in the range fall within $1/acre of the optimum. For instance, the profitable N-rate range for Illinois CC at a 0.10 price ratio is 156-199 lbs./acre.

But will there be enough yield?

Farmers worry about getting a good crop if they fertilize on the basis of economic goals rather than yield goals, Sawyer acknowledges. “Farmers like to look at what they produce, so it's a bit of a struggle to get away from that,” he says.

A Minnesota crop consultant attending an Extension meeting last fall commented: “I know lots of fertilizer dealers who would be afraid to recommend these guidelines. They look 20% low to me.”

Yet, Sawyer says the best way to reduce the risk of over- or under-application is to choose an N rate within the profitable range. “While it may seem logical or desirable to have N sufficiency near 100%,” there is little chance of N deficiency with the new guidelines, he says. That's because near the MRTN, “it takes relatively large increases in N application to move yield higher,” he says. “The N response curve really flattens out near the optimum. So when you back off from the maximum N rate to the economic rate, there's a big N-rate savings, but little yield loss.”

And when corn values are high relative to N prices, as they are expected to be in 2007, the most profitable N rate approaches the rate for maximum yield, Sawyer adds. In northern Illinois, for example, a price ratio of 0.08 (25¢/lb. N: $3/bu. corn) produces a suggested N rate of 212 lbs./acre for CC, enough to realize 99% of maximum yield potential.

Glenn Arfstrom is sold on the new N guidelines. The central Minnesota farmer tried them in 2006, the first season they were available. “It was recommended that we could reduce our N application and our yields would be just as good,” he says. “Also, there would be less chance of N getting into the water table.”

Arfstrom, a long-time seed dealer and retired college plant science instructor, grows food-grade soybeans and corn on 460 acres of highly productive land in Kandiyohi County. His farm is hemmed in by housing developments, which prevent him from expanding, so “it's very important to get as much return off each acre as possible.”

He first heard about fertilizing for maximum economic return in late 2005, “and I thought it made a lot of sense.”

Previously, he had applied N fertilizer based on a 175-bu./acre yield goal. In 2006, using the new guidelines, he cut his N use by about one-third.

In his rate calculations, Arfstrom valued N at 25¢/lb. and corn at $2.40/bu. (including LDP), for a price ratio of 0.104. At that ratio, the MRTN for productive soil in Minnesota is 140 lbs./acre for CC, with a profitable range of 120-165 lbs. Arfstrom chose the lower end of the range, applying 130 lbs./acre in the spring.

For corn following soybeans, Arfstrom decided to go with the upper end of the recommended range. At a price ratio of 0.104, the MRTN is 110 lbs./acre, with a range of 90-125 lbs. “Being a little skeptical about what would actually happen in the field, I applied 120 lbs./acre,” he says.

Severe drought pushed Arfstrom's corn yields down about 15% in '06, to 145 bu./acre. “But if rainfall had been normal, I don't think yields would have suffered at all,” he says. “I've come to the conclusion that we definitely do not need to apply 1.2 lbs./bu. of yield.”

For more on the new nitrogen rate guidelines, see “Concepts and Rationale for Regional Rate Guidelines for Corn,” at www.extension.iastate.edu/Publications/PM2015.pdf.

New Guidelines Help Adjust N Management

How do N prices and corn values affect nitrogen management under the new N-rate guidelines?

“Both corn price and N cost affect the return to N,” says John Sawyer of Iowa State University, “and it's their ratio that directly influences the net return and point of maximum return (MRTN).” When N prices are high, relative to corn values, he says, expect:

  • Reduced net return
  • Lower MRTN rate
  • Narrower range of profitable N rates
  • Lower percent of potential maximum yield
  • Greater economic penalty with N rates above MRTN.

When corn values are high, relative to N prices, expect:

  • Increased net return
  • Higher MRTN rate
  • Wider range of profitable N rates
  • Greater percent of potential maximum yield
  • Smaller economic penalty with N rates above MRTN.
Web-Based Tool Calculates Return To N

A new Web-based tool lets corn growers in Illinois, Iowa, Minnesota and Wisconsin find the most profitable N fertilizer rates.

The calculator is at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

Computations can be done for corn following corn, or corn following soybeans. Just enter your region and your estimated N prices and corn values. The calculator gives you a handy table of recommendations, and you can even graph the results.