The ratio between corn price and nitrogen price has a lotto do with your most economical nitrogen (N) rate on corn.

With the volatility in those prices, that ratio can change enough in a year's time to justify an adjustment of 30 lbs. of N/acre or more.

University agronomists have developed corn N rate calculators based on that ratio to give you the recommended N rate for maximum return to nitrogen (MRTN).

One calculator is available for Illinois, Iowa, Minnesota and Wisconsin growers. Another is available for Nebraska. The calculators suggest N rates for corn in a corn-soybean rotation and for continuous corn. The multi-state calculator can be used online at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

The Nebraska calculator can be used online or downloaded from http://soilfertility.unl.edu/.

Historically, high N prices and up-and-down corn and N prices in recent years have heightened the focus on the N:corn price ratio. The same is true of Nebraska's N rate calculator.

The calculator for the multi-state region of Iowa, Illinois, Minnesota and Wisconsin enables users to use up to four different N:corn price ratios at one time to determine and compare suggested N rates that provide maximum economic return to nitrogen. The calculator draws on corn yield response trials — most conducted since 2000 — from more than 700 sites over a number of years in specific regions in Iowa,Illinois, Minnesota and Wisconsin.

University agronomists in seven Corn Belt states developed the calculator, according to John Sawyer, Extension agronomist at Iowa State University. Inclusion of recommendations in the calculator for areas in Indiana, Ohio and Michigan awaits more N response trial data.

In the multi-state calculator, the lower the ratio, the higher the suggested MRTN rate. For example,at an N:corn ratio of 0.20 (let's say $0.40/lb. nitrogen and $2/bu. corn), the suggested N rate for corn in a corn-soybean rotation in Iowa is about 100 lbs. of N/acre. But at a ratio of 0.05 (let's say $0.20 nitrogen and $4 corn), the calculator shows a suggested N rate of about 145 lbs./acre — a 45-lb. difference.

The Nebraska calculator reflects a more arid climate than other Corn Belt areas. For example, Nebraska's calculator uses a producer's yield goal based on average yield for the last five years (excluding years with unusual stress) plus 5%. Among other things, the calculator takes into account soil nitrate levels in the root zone and N mineralization rate — factors that address wide variation in soil organic matter content and rainfall.

While the Nebraska calculator and the one for the multi-state region don't follow exactly the same approach, both incorporate the corn-nitrogen price relationship as a critical part of determining the most economical N rates for a variety of soils and climatic conditions. The two calculators tend to arrive at similar recommendations under generally equal growing conditions, according to Sawyer and Richard Ferguson, Extension agronomist at the University of Nebraska-Lincoln (UNL).

Nebraska's calculator recommendations reflect price ratio stated as corn price/bu. to N price/lb., the reverse order of the N price:corn price ratio used in the multi-state calculator. Stated either way, the impact of the ratio on economic N rates is the same.

Higher corn prices and some decline in N prices last spring pushed the ratio to as high as 14:1. But that ratio would drive N rates artificially high and lead to environmental risks such as nitrate losses from the root zone, Ferguson says. The Nebraska calculator now limits the price ratio to a range from 5:1 to 10:1.

Both calculators take yield drag into account for continuous corn. For corn following corn — even second-year corn after soybeans — the calculators suggest higher N rates than for corn after soybeans.

Although maximum corn yields have climbed dramatically over the years, the N rates that produce maximum corn yields have remained relatively unchanged. “We can go back to the late 1970s and find a very similar range of suggested N rates,” Sawyer says. “Yield levels have gone up, but required N fertilization rates haven't really changed.”

It's the changes in the ratio of corn and N prices that affect the recommended N rate for MRTN, say Sawyer and Ferguson.