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.