High energy costs and low corn prices are making agronomists and farmers alike reconsider applying economics to the decision on nitrogen rates. New recommendations from the University of Illinois allow growers to apply their nitrogen on corn within a defined range of rates.
For corn following soybean, the range is 122 to 162 lbs. of nitrogen per acre when the corn price is $2.00 and nitrogen costs 30 cents per lb. For corn following corn, the recommended range using the same prices is 137 to 174 lbs. of nitrogen per acre.
"The proven-yield method used in recent decades to recommend nitrogen rates for corn in Illinois tends to set rates that are higher than the amounts needed to maximize the return to the investment in nitrogen," says Emerson Nafziger, crop scientist with U of I Extension. "This is especially noticeable when corn follows soybeans, yield levels are high, and the nitrogen price increases while corn prices stay low."
Nafziger explains that the recommended range changes when the price ratio between nitrogen and corn changes. "If the corn price stays the same, the nitrogen rates drop by about 1.5 lb for each one-cent increase in the cost of a pound of nitrogen. That's about one lb. of nitrogen per acre for each $10 increase in the cost of a ton of anhydrous ammonia," he says.
If the nitrogen price stays at 30 cents per lb., the corn price has to drop by about 33 cents per bushel to drop the recommended nitrogen rate by the same 10 lbs. per acre.
Because Illinois used data from different trials to define these ranges, there will be separate ranges for corn following soybean and corn following corn, Nafziger says. "The soybean nitrogen credit will no longer be subtracted. Because we use only data from trials where corn follows soybean in the calculations for this rotation, the soybean credit is already included."
He notes growers can still think of soybean as providing nitrogen to the next corn crop, since the recommended nitrogen rate for corn following soybean is lower than for corn following corn.
He suggests that growers increase the nitrogen rate by 0.4 lbs. of nitrogen for each bushel of yield expected above 150 bushels per acre. "This is a small adjustment, but it helps to assure high enough N rates in continuous corn fields with high yield potential," Nafziger says.
"Producers and advisers will need to decide where to set nitrogen rates within a recommended range," he says. "This decision can involve risk-management philosophy, previous experience with N rates, and possible considerations regarding nitrogen movement into the environment."