Old N Approach No Longer Works

The typical recommendation used to be to apply 1.2 pounds of nitrogen per bushel of yield goal. Today, farmers are producing 200 bushels per acre of corn on less than those rates of N application. For 20 years, Iowa State University research has shown that yield-goal-based recommendations don’t work. For these reasons, ISU and other universities have thrown out the old way of making N recommendations.

“Our N recommendations are now based on using more precise methods, field by field,” says Tracy Blackmer, director of research for the Iowa Soybean Association’s On-Farm Network. “We’re using the fall cornstalk nitrate test, the early spring soil nitrate test and replicated strip trials in farmers’ fields.”

He says many farmers can cut N rates by 50 pounds compared to their normal rate by using the late-spring soil nitrate test and applying N side-dress.

The right rate frequently means a lower rate. With N at 30 cents a pound, cutting rates by 50 pounds can leave an extra $15 per acre in your pocket. Sometimes, however, the right rate is a higher rate. By sampling stalks in the fall, you can tell whether you applied enough N or whether you missed the target.

Consider side-dressing

Is 100 pounds of N really enough? The correct rate to apply depends on many things: the form applied, timing, application method, soil type, organic matter, growing season weather and management. Pinning down yield response is where replicated strip tests come into play.

In 2005, a number of farmers in the On-Farm Network trials compared yields from a side-dressed application of 125 pounds of N with 75 pounds (50 pounds less). Yield results show the optimal rate was usually somewhere between those two applications. “For corn after beans, applying 100 pounds of N is probably sufficient for most farmers if they are willing to side-dress,” says Blackmer.

More farmers are asking about side-dressing; they figure they can apply less N per acre by putting it on closer to the time when corn actually uses it. Side-dress application on a portion of your acres may offer some unique advantages this year. Often, prices drop slightly by side-dress time and application equipment may be more readily available.