Did Applying Supplemental Nitrogen Fertilizer to Corn in South-Central Minnesota Pay?

By Jeff Vetsch and Kyle Holling

After a very rainy spring, many farmers and ag advisors in southern Minnesota had the same question in June 2024: Does my corn need supplemental nitrogen (N) fertilizer? At the U of M’s Southern Research and Outreach Center (SROC) in Waseca, we decided to put supplemental N to the test. Here’s what we found.

Rain, rain, and more rain

Much of southern Minnesota received excessive spring rainfall which likely led to N loss due to leaching and denitrification. At the SROC in Waseca, we recorded 7.48 inches of rainfall in May and 12.45 inches in June. These values are 3.01 and 7.07 inches greater than normal for those months. Tile drains were flowing rapidly. Fields, drainage ditches and rivers were inundated with water. Nitrate concentrations in tile drains at the SROC drainage research facility were about 80% greater in May and June than in April. Observations like ponded water in fields, yellow corn, heavy tile flow and increasing nitrate concentrations in tile drainage supported the theory that N loss was significant in many fields.

Impromptu research project

On July 12, SROC researchers applied supplemental N treatments to two corn fields. Nitrogen treatments included three supplemental N rates (0, 40 and 80 pounds per acre) and two N sources/methods of application (urea, 46-0-0, with NBPT applied as a surface broadcast, and UAN, 32-0-0, applied at the base of the corn row). The corn was in the V13 growth stage at the time of application. The small plots were randomized and replicated three times. The corn in these two fields (A and B) had received 140 and 160 pounds N per acre, respectively, as anhydrous ammonia applied in late April. Both fields were soybean the previous year.

Our results

Not surprisingly, the two fields had different yields. Field A averaged 226 bushels per acre while field B averaged 210 bushels per acre. These yield levels were surprising considering how poor the corn in these fields looked in late June and July (short, uneven growth and pale color). Supplemental N application increased yield when averaged across the two fields and N sources. Corn yield averaged 210, 220 and 224 bushels per acre for the 0, 40 and 80 pounds N per acre supplemental treatments, respectively. The 80-pound treatment was NOT statistically greater than the 40-pound treatment but both were greater than the control (0 pounds of supplemental N). The N source/method of application did not affect corn yields.

Economic return

Supplemental N application in these two SROC fields would have given a relatively small return on investment. At $0.50 per pound of N, the fertilizer cost for the 40-pound N treatment would be $20 per acre, plus a urease inhibitor and an application cost that could range from $15 to $25 per acre depending on product, method of application, equipment ownership or custom hire. At this late growth stage (V13), high clearance equipment would have been required. In these two fields, the 40-pound treatment would have been more profitable than the 80-pound treatment, assuming $4.50 corn.