Obtain Representative Sample when Testing for Nitrates

BROOKINGS, S.D. -- It's vitally important that producers obtain a representative plant tissue sample when testing for nitrate, a South Dakota State University specialist said.

SDSU Extension Crops Specialist Bob Hall said two options for using drought-stressed crops are to bale the forage or ensile it. But in either case producers should test the forage for nitrates that accumulate in drought-stressed plants and are potentially toxic to livestock.

Hall has written a publication for the SDSU Extension Service outlining some of the issues involved with sampling. SDSU Extension Extra 8145, "Obtain a Representative Plant Tissue Sample for Nitrate Testing," is available through county Extension offices. Or find it online at the SDSU Extension drought Web site, http://sdces.sdstate.edu/drought/.

"To get any representative field crop sample, you first need to consider the sample area, sample height, and the time of day you are taking the sample," Hall said.

A field that is relatively flat and uniform in soil type could be considered a single sample area. But a field that varies greatly in topography or soil fertility may have to be broken down into additional sample areas.

Hall added that since nitrate levels vary widely within a plant, it's important to cut your sample at the same height as you intend to cut the plants when harvesting for forage. The lower portion of the plant, especially the lower stalk or stem, is highest in nitrates.

"If you intend to harvest the plant 12 inches above the ground, you also need to obtain a test sample 12 inches above the ground," Hall said.

Hall added that the best time to obtain a sample is near sunrise when you can see which plants have recovered during the night and which plants are severely wilted. Those that are under the greatest stress are likely to accumulate more nitrates and a sample should include a representative number of those plants, which would give producers a worst-case look at nitrate levels.

The suggested method for obtaining a representative sample for nitrate testing includes these steps:

1. Determine the sample area and the sample stalk or stem cutting height.

2. Cut 20 representative whole plants for each sample area. Cut at the harvest cutting height and put them in a box or paper seed bag. Fold the stalks or stems if necessary. Samples sent Monday to Wednesday generally arrive in good shape. Samples sent late in the week often deteriorate after spending the weekend in the post office. If you are in a hurry to get your test results, mail by first class mail or UPS, not parcel post.

3. Complete if possible a SAMPLE INFORMATION SHEET FOR FEEDS, FORAGES AND FERTILIZERS form available at local Extension offices or on the web: www.abs.sdstate.edu/labs_services/anserv/fffinfo.htm.

4. Mail sample to: Analytical Services / Olson Labs, ASC 133 /SDSU Box 2170 / Brookings, SD 57007-1217.

Testing for nitrate can help producers decide what method to use in harvesting and preserving forage.

"Baling will not lower the nitrate levels in a stressed crop once it is cured," Hall said. "Ensiling, in contrast, generally lowers nitrate levels about 35 percent once fermentation is complete."

Hall added that in many cases, forage high in nitrates can be fed by diluting it with other feed in the ration. Ask your county Extension livestock educator for guidelines.

Hall said it's important to note that the more samples you obtain, the better your estimate will be for potential nitrate poisoning. Two or more samples for a large sampling area (a large field or a portion of a field) of many acres would result in more meaningful test results than a single sample from the same number of acres.

Nancy Thiex of Olson Biochemistry Laboratories at South Dakota State University added that there's always a risk with using crops as forage, and that any test can only show the nitrate levels in what was sampled. If producers aren't careful to take into account the factors that Hall mentioned -- such as topography, soil fertility, cutting height -- the sample might not reflect nitrate levels in the crop or forage that will be fed.

Contact: Bob Hall, (605) 688-4760
** For links to other sites showcasing SDSU's work in teaching, research, and Extension, visit http://sdces.sdstate.edu.

Lance Nixon, Editor
AgBio Communications Unit
South Dakota State University
ACC, Box 2231, Rm 200
Brookings, SD 57007
Telephone: (605) 688-4653
Lance_Nixon@sdstate.edu