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.

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.

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.