Is Managing Nitrogen Fertilizer for Corn Getting More Difficult?

By Jeff Vetsch

If farmers don’t have enough challenges lately with trade issues and low commodity prices, recent wet growing seasons have made nitrogen management of corn more difficult.

While annual precipitation has been trending upward across Minnesota, excessive rainfall during the growing season has been especially pronounced in south-central and southeast Minnesota the last few years.

In 2016, when Minnesota set the state’s annual precipitation record, Waseca recorded over 56 inches of annual precipitation. In 2018, Caledonia and Harmony both recorded over 56 inches. Last year, several weather stations in the region reported 50+ inches. Harmony has recorded 50+ inches in three of the last four years.

What does all of this rainfall mean for agriculture?

For corn growers, greater precipitation means fewer days available for field work, delayed field operations, compacted soils, nitrogen loss and often yellow corn. With all of this rain, farmers and their advisors may be wondering: should I change my nitrogen management practices?

While I still recommend, first and foremost, following the University of Minnesota’s nitrogen rate guidelines for corn and using the Nitrogen Rate Calculator, recent research in southeast Minnesota suggests that split application of nitrogen may be worth trying.

The University of Minnesota has partnered with the Minnesota Department of Agriculture, Minnesota farmers and their agricultural advisors to conduct research and education on how to optimize nitrogen rate and timing for corn. Since 2015, 16 on-farm research trials have been conducted in southeast Minnesota. The data are shared with participants and entered into the Nitrogen Rate Calculator, the U of M’s database and are a source for nitrogen rate guidelines for corn grown in Minnesota.

The optimum nitrogen rate for corn varied considerably among trial locations. For corn following soybean sites, the optimum nitrogen rate ranged from 103 to 203 lbs. per acre and averaged 154 lbs. Corn after corn sites ranged from 100 to 270 lbs. per acre and averaged 186 lbs. For comparison, 71% of the 98 sites in the Nitrogen Rate Calculator database were optimized between 76 to 150 lbs. per acre for corn following soybean.

What is causing the wide range and uncertainty in optimum nitrogen rates?

Seventy-five percent of the sites studied had at least one month with excessive rainfall (greater than 150% of normal) and 56% had three or more months of excessive rainfall during the growing season. Excessive precipitation during the growing season is resulting in nitrogen loss and inefficient use by corn plants.

What can farmers do to minimize these losses and maximize economic return on nitrogen fertilizer?

Farmers certainly can’t stop the rain. However, they can use Best Management Practices (BMPs) adapted to their region and soils to minimize nitrogen loss. Split application of nitrogen fertilizer was one BMP tested in this study. A portion of the nitrogen fertilizer was applied in the spring prior to planting and the remainder was applied when the corn was about 10 inches tall, usually in early June. In 25% of research sites, a split application of N increased corn yield 11 bushels per acre compared with applying all nitrogen prior to planting. Although a 25% success rate is not huge, split application never reduced corn yield, and it occasionally reduced the total amount of fertilizer required when compared to applying all fertilizer prior to planting. Furthermore, split application generally worked best at the wettest sites.

When split-applying nitrogen, the amount of nitrogen applied prior to planting can be as little as 20% of the total with corn following soybean. However, corn following corn and small grains research suggests that applying at least 50% of the total nitrogen rate prior to planting is optimal.