There has been a large change—downward—in corn prices this fall. How might that affect recommended nitrogen fertilizer or manure-N application rates? How might it affect your planning for the 2015 crop? "The answer depends on more than just the price of corn, but also the price of nitrogen," says John Sawyer, Iowa State University Extension soil fertility specialist. He provides the following explanation and recommendations.
It is the ratio of these prices (price ratio, where the $/lb. of actual N is divided by the $/bu. for corn. For example, $0.50/lb. N and $3.50/bu. corn is a 0.14 price ratio). Both prices are important and influence recommended N rates as the ratio reflects the last unit of N that can be paid for by the yield increase from that N application. In Iowa, recommended rates (Maximum Return to N, MRTN) come from the Corn Nitrogen Rate Calculator (CNRC), which is the online tool providing rate recommendations for corn following corn and corn following soybeans.
"The rates derived from the CNRC are adjusted for user specified N and grain prices," says Sawyer. "While a significant change in grain price may be troublesome, it may or may not affect recommended rates because of current N price."
Figure your nitrogen fertilization rates for corn production
Nitrogen rates determined from the online CNRC are directly the total fertilization amounts for each rotation, with no need to further adjust rate for previous crop. That is, for the soybean-corn rotation, there is no need to subtract a "soybean credit" as the rotation effect is already accounted for by the N rate trials that the database is derived from, says Sawyer. He provides the following example.
Tables 1 and 2 give example N rates for two different corn grain prices, along with four N prices. The N prices were kept constant between tables, but the corn price used in Table 1 was $5.00/bu. and was $3.50/bu. in Table 2. Therefore, the price ratios are not the same in both tables. For corn following soybeans, the reduction in the MRTN rate with the lower priced corn ranged from 10 to 17 lb. N/acre. For corn following corn, the reduction was from 12 to 23 lb. N/acre. These are not large N rate changes, but sizeable enough to be considered for determining N rates as corn prices change.
You input corn and nitrogen prices for your farming situation
Of course, potential N rate changes depend on specific N and corn prices, he notes. The CNRC allows users to input whatever corn and N prices are applicable to their situation. In addition, besides the MRTN rate, a range of rates is provided that gives flexibility in decision making about N applications and allows for rate adjustments based on factors such as uncertainty in corn selling price and issues such as water quality impacts from N application.
The MRTN rates provided by the CNRC are based on many sites and years of N rate trials conducted across Iowa, and provide a reasonable estimate of N rate required for optimal corn production. Actual need in any given year, of course, can be influenced by many factors, most uncontrollable. However, across time the MRTN rates provide adequate N in the majority of trials that have been conducted. When MRTN rates tend to be too low, that typically occurs with above normal springtime rainfall, which is not predictable either, says Sawyer.
Table 1. Nitrogen rates derived for different N prices with corn grain set at $5.00/bu.
Table 2. Nitrogen rates derived for different N prices with corn grain set at $3.50/bu.
Resources for making nitrogen rate decisions:
Click here to download Regional Nitrogen Rate Guidelines for Corn (PM 2015) or for printed copies, contact ISU Extension Online Store or (515) 294-5247. The ISU Agronomy Extension Soil Fertility website is located available here.