You know the rule of thumb that it takes 1.2 pounds of nitrogen to produce a bushel of corn?
Well, it dates back to a misunderstanding over 50 years ago.
A USDA scientist published a research paper in 1973 that said N uptake required per bushel for maximum yield stayed relatively constant at 1.2 pounds of N per bushel.
There’s a catch, however, to this nitrogen use efficiency number. This was the total N uptake per bushel, not the actual N fertilizer requirement.
University of Nebraska researchers note this should never be the actual N fertilizer recommendation. The NUE often can be lower, since growing-season N can come from myriad sources that include:
- soil organic matter mineralization
- residual inorganic N
- legume N credits
- manure N credits
How low can you go?
This “free” N can help farmers reduce costs by lowering commercial N applications and help reduce the chances of N escaping into bodies of water.
In-season N applications also can help accomplish these goals. Keaton Krueger, who farms near Ogden, Iowa, aims for an NUE of 0.7 during his in-season N applications.
“Some years will be more, others will be less,” he said. “We’re not losing a lot of N in our system. If I applied some N in the fall, I would probably have a higher NUE because I could inherently lose some.”
Krueger added that his cover crops also enhance water quality by scavenging residual N.
“They eventually can release N during the growing season [for corn use] as they die into the season,” he said.
Doug Adams aims for an NUE of 0.5 pounds of N per bushel of corn via in-season applications. The Humboldt, Iowa, farmer initially sidedresses 75 pounds per acre of N in a window near emergence up to V6. He then sidedresses a 75-pound-per-acre application with y-drop machinery closer to tasseling.
He doesn’t always need the second N pass. A late-spring nitrate test this year indicated his corn was on track for N.
“Instead of applying 150 units [of N per acre], I only had to apply 75 units of N,” Adams said. “There was great weather for soil mineralization of N, and the corn was able to pull nitrogen out of the soil.”
Help is available
Switching to in-season N or reducing N rates can be nerve-wracking. Programs exist to ease the transition, such as:
Practical Farmers of Iowa. PFI offers an N Rate Risk Protection Program to help farmers find the optimal and economical N rate. Participants must farm in eligible counties in Iowa, Illinois, Minnesota, Missouri, Nebraska, South Dakota or Wisconsin. Farmers who meet these and other program parameters receive $5 per acre for every acre enrolled with reduced N. If corn yield drops below 95% of a 10-year average after lowering N rates, you’ll receive a payment of $30 per acre.
Nitrogen Soil Sampling Project. This initiative delivers free nitrogen soil testing for farmers in counties upstream from the Des Moines and Cedar Rapids metro areas. The Iowa Department of Agriculture and Land Stewardship and the Iowa Nutrient Research & Education Council project provide field-level data to help optimize fertilizer use and benefit water quality.
Nitrogen Fertilizer Application Consultation Tool. Users of N-FACT may enter factors such as location, previous crop and expected precipitation to help determine optimal N rates.