Chad Lee, Ray Smith, Michelle Arnold and Cynthia Gaskill
University of Kentucky
June 29, 2012

Dry conditions often cause nitrates to accumulate at high levels in forages. Nitrate levels need to be checked in drought damaged forages before feeding to livestock or horses.

Sample Technique

A proper sample must be taken for accurate determination of nitrate levels. The results from the test are only as good as the sample and handling of that sample. The following are some guidelines to follow.

Harvest at least one (1) pound of fresh weight for each sample. The plants should be cut at the intended harvest height. (Note: We suggest harvesting forages at three (3) inches or more above the soil surface. Nitrates tend to accumulate in the lower stems. By keeping the cutting height above three inches when harvesting forages, nitrate levels in the harvested plant should be reduced).

Collect a representative sample from each field. Usually five or more locations across a field will serve as a representative sample. Plants from the five or more locations in the field should be combined into one sample for nitrate testing.

If multiple fields are in question, or crops at different growth stages are to be tested, submit samples from the different fields or crops as separate samples. Growth stage, the date when fertilizer nitrogen was applied, and the extent of freeze damage all could affect nitrate levels in the plants.

Relatively dry plants (eg, corn stalks, hay) should be stored in paper bags and mailed in cardboard boxes overnight to the testing laboratory. Moist plants (eg, silage, fresh forage) should be placed in plastic bags and immediately put in a cooler with ice packs. Either deliver the samples directly to the laboratories the same day or ship overnight on ice packs. If moist plant samples will be stored overnight before shipping, then they should be stored in a freezer in plastic bags.

Note: Storage of moist plant samples in plastic bags at room temperature will result in bacterial growth and reduction of nitrate to nitrite, resulting in inaccurate nitrate results.

During the handling process, nitrate levels could decrease in the plant sample, especially if they are stored overnight at room temperature. If a period of time has occurred between harvesting and testing the samples, then you could expect that nitrate levels reported would be less than nitrate levels in the field.

Testing Laboratories in Kentucky

The two veterinary diagnostic laboratories in Kentucky that perform nitrate testing on forages are the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) and the Murray State University Breathitt Veterinary Laboratory. Both are accredited by American Association of Veterinary Laboratory Diagnosticians. However, the two laboratories use different methods for their analyses. The UKVDL methods and techniques are consistent with most testing laboratories across the country and the feeding thresholds are consistent as well (Table 1). The UKVDL recently passed a national nitrate/nitrite proficiency testing program, which confirms the accuracy and repeatability of their testing methods. Click here for more information on that proficiency test. Breathitt Veterinary Laboratory uses a different method and has different feeding thresholds and guidelines (Table 2).

Note: All sources of dietary nitrate, including feeds, forages, supplements, and water should be taken into consideration when determining total dietary nitrate concentration. Representative sampling is crucial for proper interpretation of results.

Also: Nitrite, a breakdown product of nitrate that can be found in forages, is much more toxic than nitrate, and much lower levels of nitrite can cause poisoning and death.

Note: The guidelines from Breathitt are specific to samples tested at Breathitt and cannot be applied to samples tested at any other laboratory.

Costs for Testing

The UKVDL in Lexington, KY performs a forage nitrate panel that includes both nitrate and nitrite analyses for $15 per sample. Starting July 1, 2012, there will not be an accession fee, so the total cost will be only $15 per sample for the panel. Also, as of July 1, there will be no out-of-state fees for nitrate tests from farms outside of Kentucky. Turn-around-time for test results is generally 1 to 3 business days after receipt of samples. The UKVDL accepts samples directly from veterinarians, farmers, and extension agents.

The Breathitt Veterinary Laboratories in Hopkinsville, KY provides nitrate and nitrite testing on forages for a fee of $12 per sample for nitrate, $12 per sample for nitrite, and a $10 accession fee for each accession or submission. The turn-around-time for test results from the Breathitt lab is usually 1 to 2 business days. Contact the Breathitt Veterinary Laboratory for more information about their sample submission process, specific guidelines testing guidelines and interpretation of results.

Other Laboratories

Several commercial laboratories, such as Dairy One Forage Laboratory, conduct the nitrate testing as well. However, be aware that nitrate levels can be reported a variety of ways and the method of expression can differ between laboratories. Nitrate can be reported as nitrate (NO3), nitrate-nitrogen (NO3-N), or potassium nitrate (KNO3). These numbers are NOT equivalent, as they represent different chemical structures. Make sure the feeding guidelines used for a particular result match the type of analysis performed. To convert between the different methods of reporting, use the conversions in Table 3.

Forage nitrate results can also be reported using a variety of units. The most common units of measurement are parts per million (ppm) or percentage (%). Results are usually reported on a dry matter basis. To convert from ppm to %, move the decimal point four places to the left (eg, 5,000 ppm = 0.50%)

Points

1. Keep moist samples frozen or on ice until shipped, and ship samples as soon as possible. This avoids the possibility of nitrate reduction during storage and transportation.
2. Be sure to know the specific guidelines of the testing laboratory you are using.
3. Note: Horses are much less sensitive to nitrate than are cattle or other ruminants, and can tolerate much higher concentrations of nitrate, but exact threshold values have not been established. Horses are extremely sensitive to nitrite, so any preformed nitrite in forages can pose a significant risk. Please consult with a veterinary clinical toxicologist for interpretation of nitrate/nitrite concentrations in horse feeds.

Laboratory Contact Information

University of Kentucky Veterinary Diagnostic Laboratory
1490 Bull Lea Road
Lexington, KY 40511
Phone 859-257-8283
Fax 859-255-1624
Contact for nitrate questions: Dr. Cynthia Gaskill, cynthia.gaskill@uky.edu, 859-257-7912

Murray State University Breathitt Veterinary Center
PO Box 2000, 715 North Drive
Hopkinsville, KY 42241-2000
Phone 270-886-3959
Fax 270-886-4295
Contact for nitrate questions: Dr. Ramesh Gupta, 270-886-3959