By Daniel Hudson, UVM Extension Agronomist
It is relatively easy to see the cost-benefit relationship associated with measuring milk production and/or quality in your dairy herd. When you learn that things are not going as well as they could be you identify the problem, fix the underlying issue, and get paid for your work with a larger milk check. In crop production, however, we have only a few chances to learn from soil or tissue measurements and make changes based on what we learn from testing. Properly practiced, these will also pay you back, most immediately in the form of reduced costs or improved crop yields.
- The first opportunities to identify agronomic problems come prior to planting the crop. If the fertility, drainage, and other soil quality parameters measure up, we are well on our way to a great crop. If not, we can take steps in the right direction. If we have good information about the quantity of nutrients that were applied in the manure, even better.
- Another opportunity comes when the corn is 6-12” tall and we can collect soil samples for pre-sidedress nitrate testing so we have a better idea if and/or how much additional nitrogen will be necessary to optimize our crop yield.
- In crisis situations, plant tissue testing can sometimes give information that can be used to make profitable mid-season nutrient applications.
- There is another very valuable opportunity that is overlooked by most farmers, and it comes just prior to silage harvest: the ‘late-season stalk nitrate test.’
What is the late-season stalk nitrate test and how might it enhance my bottom-line?
The late-season stalk nitrate test is an ‘almost-post-mortem’ look at the concentration of nitrate in the sections of corn stalks between 6” and 14” above ground level. This test gives farmers and agronomists insight into how soil/fertilizer/manure nitrogen was supplied to the corn plants under the conditions and management practices used in a particular year. The results of the test will indicate yield-limiting deficiency, an optimal concentration, or excessive nitrate levels. Stalk nitrate levels above the optimal range indicate that nitrogen was supplied at levels well above what the plants needed to give an optimal yield under those conditions. Because the conditions and practices used from one year to the next are often different, the optimal rate and ultimate fate of nitrogen often varies from year to year. Thus, the farmer’s task is to identify the lowest cost management practices that consistently deliver the results they need to have. Studying how stalk nitrate concentrations change under varying conditions and practices will help farmers refine their management and should increase profitability over time.
Why don’t more farmers use the late-season stalk nitrate test?
This test differs from other types of soil and plant testing that farmers are accustomed to. Unlike normal soil tests and pre-sidedress nitrate testing, this test yields an indication rather than a prescription. It is roughly analogous to testing milk for milk urea nitrogen (MUN) in the sense that it can help crop producers identify financial inefficiencies associated with supplying more or less nitrogen to the crop than is needed to consistently attain a ‘full’ yield. Using the results of these tests over several years, farmers can improve their understanding of how nitrogen behaves in their soils, which can help them make better decisions about fertilizer rates, timing, form, or incorporation practices and ultimately increase their net income via reduced expenses or higher crop yields.
As useful as this test is, it sometimes does not capture the farmer’s imagination as much as it could. The foremost reasons for this seem to be 1) ‘why start keeping score when the ball game is already over? i.e., ‘it is too late to do anything for this year’s crop anyway’; and 2) ‘I am getting ready for silage harvest and don’t have time to be walking around out in the corn(!).’ As true as these statements are, they overlook the fact that once an inefficiency is identified, it can later be avoided. If the ball game did not go well this year, it is better to know than not to know, because you are going to play the same game on the same soils next year and for years into the future.
What exactly does the stalk nitrate test report tell me? While several land-grant universities have slightly different interpretations of the test, the Penn State interpretation is the most conservative (from the perspective of the farmer) and is less likely to result in farmers concluding that they should reduce their nitrogen input rates when they really should not. Using the Penn State protocol, samples can be taken from 1/4 milk line through three weeks after black layer formation (physiological maturity). Penn State data suggests that optimal range for stalk nitrate concentration is between 700 and 2000 ppm. Nitrate concentrations below 700 ppm indicate that nitrogen deficiency probably limited economically optimum crop yield, while concentrations over 2000 ppm indicate that the crop almost certainly had more nitrogen available than was needed to produce a full crop. In contrast, Purdue and Cornell suggest that the ideal range is 450 to 700 ppm. Until we have more comprehensive data in the Northeast that suggests otherwise, I am more comfortable with the Penn State recommendations.
When it comes to using the late-season stalk nitrate test results, I often encourage farmers not to learn too much from a single year. Very rarely would it be appropriate to radically change a fertilizer program due to observations from a single year. Soil nitrogen is dynamic and a farmer’s goal should be to learn about its behavior on their farm over time so that they can be confident in and comfortable with future adjustments to their practices.
How to do the test: Do not think of doing this test on the comprehensive scale often used for routine soil testing of pH, P, K, Mg, etc. I recommend that farmers start by testing three fields or sections of fields. Test one field where you think you got the nitrogen rate was about right, another field where you think the plants might have been nitrogen deficient, and a third field where you think the nitrogen might have been applied at a higher rate than the crop needed. You might also consider testing sections of fields where you suspect leaching, denitrification, or think another important agronomic difference exists. The tests will give you an unbiased measurement of the nitrogen status of the corn plants in each of those fields.
The area tested should have had the same nitrogen fertilizer rates/timing, same (or very similar) hybrids, and be of a relatively uniform soil type and topography. Take stalk segments from 15 healthy plants in the area of interest, avoiding plants that are not normal (double-seed drop, skip, wide guess row, edge of field, unusual injury, etc). Segments from each plant should be collected by cutting the plant off 14 inches above the ground and again at 6 inches above the ground. Remove any portion of the leaves that remain attached, and place the resulting 8-inch segment in a paper bag with the other segments from the same field. Refrigerate the stalk segments if the sample cannot be sent or delivered to a test lab within 1 day. Do not use plastic bags as this will prevent drying and may cause spoilage. The testing lab will oven dry and grind the stalks prior to analysis. While several labs in the region offer this service at some limited level, the Cornell Nutrient Analysis Laboratory advertises this as a service they offer.
If you choose to send the samples to Cornell, send them to Cornell Nutrient Analysis Laboratory, G01 Bradfield Hall, Ithaca, NY 14853. The cost for one stalk nitrate test is $11.00. Make checks payable to Cornell University.
More detailed sampling and submission instructions and a submittal form can be found at: http://cnal.cals.cornell.edu/forms/pdfs/CNAL_Form_SN.pdf
For more information about the end of season stalk nitrate test visit:
End of Season Corn Stalk Nitrate Test (Purdue):
Questions? Contact me at firstname.lastname@example.org or 802-751-8307 ext