*This is part 2 of 2 installments.
Farmers have to consider many factors when selecting nitrogen (N) rates and timing of application. The goal is to strive for optimum corn yields while trying to minimize nitrate-N loss and negative impacts to water quality.
Related: Why corn needs nitrogen
That was a key part of the message at the nitrogen management stop at the recent Iowa State University research farm field day at Kanawha in northern Iowa. With a wet June and drowned-out spots in many fields in Iowa this year, N application rates and timing were two of the topics generating questions. John Sawyer, ISU Extension soil fertility specialist, provided the answers.
What can you do to reduce nitrate-N loss from cornfields?
What can you do to reduce nitrate-N loss from cornfields and movement to surface waters? First, look at your management of N, Sawyer suggests. Are you applying the economic rate of fertilizer N or manure N per acre, and not over-applying? Are you applying in the springtime? Next, look at the options outlined in the Iowa Nutrient Reduction Strategy and decide what practice or practices would best fit your production system.
Do you want to use an in-the-field agronomic practice such as a cover crop or an edge-of-field practice such as a bioreactor or a wetland? Planting and managing a cover crop is an every year job, notes Sawyer. Would you rather take an engineering approach and install a wetland, a bioreactor, or a saturated buffer that lasts for multiple years? Maybe this is an opportunity for a change in crop rotation. Continuous corn acreage has increased in recent years, but it requires a higher N application rate than corn rotated after soybeans or alfalfa.
What's the best timing to apply nitrogen for corn?
At the June 25 field day, farmers asked questions about timing of spring-applied N. A number of ISU studies in the last 10 years haven't shown a big difference in optimal N rate or corn yield, whether N is applied preplant or sidedress. Keep in mind, says Sawyer, even sidedress N can be exposed to N losses and losses from soil derived nitrate-N occur also. If you applied N the first day of June this year, look at what happened in late June. Many areas of Iowa had above average rainfall. Some of the sidedress N was already in the nitrate form and exposed to potential losses.
Will corn benefit from a supplemental N application? The key factor is what the weather is doing that spring. Springtime precipitation data can be used to indicate the chance of excess wetness that would suggest the need for supplemental N.
Would a supplemental application of nitrogen be warranted?
Sawyer took precipitation data and compared it to corn yield response to N rate versus current suggested N rate (Maximum Return to Nitrogen, MRTN from the Corn Nitrogen Rate Calculator, CNRC) using long-term N rate research with continuous corn and corn rotated with soybean conducted at seven ISU research and demonstration farms in Iowa from 1999 to 2014. He used precipitation totals in either March through June (southeast Iowa) or April through June to see if it would give an indication corn might run short of N and that a supplemental N application would be warranted.
The precipitation totals (17 inches for southeast Iowa and 16 inches for the rest of Iowa) worked correctly about 75% of the time; that is, this approach determined if more N was needed or was not needed. "If you apply an optimal rate of N by ISU's recommendations, or a lower rate, and the weather gets excessively wet, this is a tool to help with decisions about additional N application," he explains. "It's another tool to provide some guidance."
Corn Nitrogen Rate Calculator and MRTN are on the website
The MRTN rate is the application rate that gives you the most profitable return, as the CNRC figures both corn prices and N prices into the equation. The CNRC and the MRTN approach are at the website extension.agron.iastate.edu/soilfertility/nrate.aspx and explained on the website agronext.iastate.edu/soilfertility. Also on that site, Sawyer suggests you read the article titled "Nitrogen Issues So Far This Spring" for observations and recommendations regarding how to use spring precipitation data to help make a supplemental N decision.
Summing up, Sawyer made these three key points at the field day:
• Due mainly to wet years recently, more supplemental N has been applied.
• Sidedressing N doesn't necessarily miss all of the loss conditions.
• There's no one best time to apply in the springtime; various factors come into play.