Does your corn N strategy deserve an A or a D?

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Taking time this fall to conduct a stalk nitrate test can help producers "grade" their current nitrogen application rate and provides key information on which to base any changes for next spring.

Taking corn stalk samples now can help determine if the corn was under, adequately, or over fertilized with nitrogen. If the nitrogen applied this year was greater than that recommended by the University of Nebraska–Lincoln and this fall’s stalk nitrate samples indicated excess nitrogen, consider reducing nitrogen rates for next season.

Use the corn stalk nitrate test in irrigated fields where moisture was not limiting. Fields that tend to have high stalk nitrate tests are those where manure or excess nitrogen was applied and fields following alfalfa. Iowa State University developed the corn stalk nitrate test, and its usefulness has been verified in other states.

click image to zoom What Does the Test Show?

The results of the corn stalk nitrate test indicate whether the corn was over fertilized during the season. The test shows low, optimal and excess stalk nitrate values (Table 1). Low values indicate nitrogen may have been deficient. Excess values indicate that there was more nitrogen than the plant needed to produce grain. The scientific basis for this test is the fact that corn will continue to accumulate nitrogen past the level at which grain yield is increased. Since corn does not show visible symptoms of excess nitrogen, analysis of the stalk tissue can determine when this occurs. This test is probably best used for finding excess nitrogen since deficiencies can be spotted visually by leaf yellowing.

This season, if the test comes back in the “excess” range, that indicates that reductions in nitrogen may be possible next season. (For more information on recommended rates, see the NU Extension NebGuide, Fertilizer Suggestions for Corn (EC117) or visit Soil Management to Optimize Crop Production in CropWatch, where there are worksheets and an on-line calculator to help determine recommended nitrogen rates.) 

How to Take the Test

Take corn stalk samples up to three weeks after black layer formation in 80% of the kernels. Newly published information indicates that the stalk test can be taken as early as when the milk line is one-fourth of the way down the kernel. To take the test, remove an 8-inch segment from 6 inches to 14 inches above the ground.

Remove the sheaths. Don’t take diseased stalks or stalks damaged by hail or insects. Take 15 stalks per sample, keep them cool and send to the laboratory immediately. Samples should be sent in paper wrapping and not plastic since plastic wrapped samples may mold. Have the samples analyzed for nitrates.

Research Supporting Use of The Stalk Nitrate Test

An article (Fox et al., July 2001) in the Agronomy Journal compared the stalk test, late season chlorophyll meter, and green leaf count techniques.

click image to zoom Based on this article, I have summarized their analysis of the results in Table 2. The authors used experimental data to determine the error rate of using different critical levels to interpret the test results. Because the tests were conducted on corn grown in replicated experiments, they could determine if the diagnostic test level accurately matched the plant response. Their criteria for whether the test was valid was whether the yield was at 93% of maximum yield. For example, with the chlorophyll readings taken at one-fourth milk line they used a critical value meter reading of 52. They derived the 52 reading from their previous research.

Once the criteria was set, they determined if the treatment correctly predicted sufficient nitrogen. They also divided the errors into two groups: one predicted the plant was nitrogen deficient when it wasn’t while the other predicted the plant had adequate nitrogen when it was deficient. Using the chlorophyll meter reading to determine if the plant had adequate nitrogen wrongly predicted the crop was deficient 13.4% of the time. The plant actually had adequate nitrogen even though the meter suggested it was low. Using the same meter reading criteria, 1.7% of the time it falsely suggested the plant had adequate nitrogen when it was low.

When the authors lowered the criteria from 52 to 48, the total error rate actually decreased from 15.1% to 7.3% because the percent the meter falsely predicted deficiency decreased from 13.4% to 2.7%. There was not a corresponding increase in the false prediction of adequate nitrogen.

The data on the stalk nitrates also shows the change of error rates when the criteria for predicting deficiency changes. The Fox et al. data indicates that using 250 ppm would keep prediction errors to 7.2%. Using the 700 ppm critical value used by Iowa had a 0% error rate for falsely predicting nitrogen sufficiency and a 12% overall error rate.

The Fox et al. data provide more evidence that corn stalk nitrate tests are a useful tool in nitrogen management. They are best used to determine if adequate nitrogen was available. They would be especially useful in fields with manure history where the producer needs reassurance that reducing fertilizer nitrogen will not affect yields. This year they may also help producers determine if reducing nitrogen rates decreased yields.

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