Project News
Refined nitrogen rates put to test in high-yielding season
by Eric Palas, NPM coordinator, NEIDP

Since 1989, northeast Iowa water quality project staff have con ducted a series of demonstrations designed to help answer questions on refined management of one critical corn production input, nitrogen (N).

During the last 10 growing sea sons, continuous corn demonstra tions have been established at three separate locations representing Downs soil (Fostville and Garna villo) and Fayette soil (Gunder). The 1998 demonstration was conducted on land operated by Ron Mccartney near Gunder, and was in place since 1996. Similar demonstrations were established near Postville from 1992-1994, and near Garnavillo from 1989-1995.

Yields for 1998 were extremely high and were somewhat surpris ing given this year's weather. Heavy rains early in the season increased the potential of N losses due to leaching or denitrification. Treatments within the demonstra tion that received low rates of N began to exhibit deficiency symptoms during the third week of June. Corn plants in the low N treatments took on a pale green appearance and were significantly shorter prior to tasseling in mid July. However, abundant sunshine, above-normal temperatures, and low levels of stress during August and September provided for ideal conditions during grain fill.

To reduce the impact of field variation and environmental stresses such as insect, weed and disease pressure, the various N treatments were replicated four times within each demonstration.

The Gunder demonstration was hand-harvested in October. The corn was weighed from each plot and yields adjusted for grain moisture. The profit calculations compare the amount of nitrogen used and crop response to the zero nitrogen check (table 1). Long-term values of 20 cents a pound for the nitrogen fertilizer and $2.40 per bushel of corn were used.

Results include end-of-season cornstalk nitrate nitrogen data, which has been used in the demonstrations since 1994. The cornstalk nitrate test measures the amount of nitrate nitrogen (N0₃) in the corn plant after it matures. Low stalk nitrate levels (less than 250 parts per million, or ppm) indicate that additional N would have been likely to increase yields. The marginal range (250- 700 ppm) indicates a level very close to the minimal amount needed by the crop, but grain yield may not be reduced. The optimal range (700-2,000 ppm) indicates that N availability was close to the rate needed by the plant, while levels in excess of 2,000 ppm indicate a high probability that there was more N than needed.

Given this year's early season weather conditions, it was not expected that the 90 pounds per acre nitrogen rate would provide the highest yields in the demon stration. Stalk test results for the 90 pound rate also fell within the optimum range. Long term obser vations from the Postville and Garnavillo demonstrations had indicated optimum profit levels occurred with nitrogen rates for continuous corn at 120 pounds per acre, as shown in table 2.

The demonstration results provide a solid starting point for refine ment of nitrogen use and highlight tbe importance of conducting the trials over a number of growing seasons. "The results from the Gunder site in 1998 were a definite surprise," says John Rodecap, project coordinator. "The observed yields provide for added confidence in the results from the 120 pound per acre nitrogen treatment from previous demonstrations."

Producers who attempt to refine their nitrogen use should also make field observations over a number of growing seasons. The correct ainount of nitrogen to apply will vary with the production system specific to each farm in a given year, and a considerable amount of accuracy is lost with the typical equipment available for field applications of N. The end-of season cornstalk nitrate N test, the late spring test for soil nitrate and a careful assessment of corn yields can provide information specific to anv farm for refinement of N use.

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