Frequently Asked Questions About Nitrates in Grazed and Harvested Forages

Stressful growing conditions may result in high concentrations of nitrates in forage and plant species commonly found in pastures and hayfields. Animals consuming high-nitrate material will experience symptoms associated with nitrate toxicity or poisoning. This resource answers the most-asked questions about nitrates in forage systems.

What Are Nitrates?

Nitrates (NO₃) are compounds composed of nitrogen and oxygen. These compounds can be found naturally in soil and water. They are also found in many commercial fertilizer products, since nitrates are a highly soluble form of nitrogen. Nitrogen is an essential nutrient for plant growth, and nitrates are the primary form that plants absorb nitrogen from.

How Do Nitrates Accumulate in Plants?

Nitrates are highly soluble—so their uptake is relatively high—and nitrates may become concentrated in a plant. When plant growth is slow, the plant will continue to take up nitrates, but does not convert the nitrates into amino acids and proteins. This will cause a buildup of nitrates in the plant.

Factors Driving High Nitrates

Nitrate accumulation will occur following excessive nitrogen fertilization or during plant-stress events when plant growth is slow. Here are scenarios in which plants will accumulate nitrates.

Excessive Fertilization

Each plant species has an optimum amount of nitrogen required for plant growth. The plant will continue to take in nitrates beyond what is required but will not convert them into proteins for new growth. Therefore, the concentration of nitrates in the plant will increase.

Drought, Cold/Frost, or Prolonged Cloudy Weather

Periods of drought, cold, or prolonged cloudy weather will slow the rate of photosynthesis and result in little to no plant growth. This may cause an increased concentration of nitrates in the stressed plants.

Rain Following Drought

Rapid uptake of nitrates during the first few hours after a drought-ending rain may also lead to high nitrate levels. The conversion of nitrates to amino acids within the plant may take several days. Thus, expect nitrates to remain very concentrated for at least 7 days after the crop has received enough rain to sustain growth. As drought severity and duration increase, so does the rain and time needed to initiate a plant growth response.  

Herbicide Application

Nitrates may accumulate in plants following herbicide applications that slow or stop plant growth. The plant cannot distinguish between the stress caused by the herbicide activity from an environmental stressor, and the response will be the same as when there is drought, cold, or prolonged cloudy weather.    

Can All Plants Accumulate Nitrates?

Technically, all plant species can accumulate high levels of nitrates. This includes grasses, legumes, and forbs. Some species are more likely to contain high nitrate concentrations, especially in forage crops that require high levels of nitrogen fertilization. Thus, it is common for summer annuals, perennial grasses, and small grains to have high nitrates (see callout box below). In addition, many common weed species naturally accumulate high levels of nitrates.

What Level of Nitrates is Considered Toxic?

Laboratories may report nitrates in different units, including forage nitrates (ppm NO₃), nitrate nitrogen (ppm NO₃-N), and nitrate ions (% NO₃). All reports should be read on the dry-matter basis regardless of the unit. Once you have a forage test report, look up the feeding guidance in Table 1 associated with the level of nitrates in the matching unit measured on your report.

Table 1. Feeding Guidance for High-Nitrate Hay and Baleage Based on Lab or In-Field Testing.

Feeding guidance	Test result from laboratory report			Test result from in-field nitrate test strip (mg/L NO3–)
	Forage nitrate (ppm NO3)a	Nitrate nitrogen (ppm NO3-N)b	Nitrate ion (% NO3)c
Safe to feed with adequate feed, mineral, and water	Under 4,500	Under 1,000	Under 0.45	Under 100
Limit to 1/2 of the ration (50%) for pregnant animals	4,500 to 6,500	1,000 to 1,500	0.45 to 0.65	100 to 150
Limit to 1/2 of the ration (50%) for all animals	6,500 to 9,000	1,500 to 2,000	0.65 to 0.90	150 to 200
Limit to 1/3 of the ration (33%) for all animals	9,000 to 15,000	2,000 to 3,500	0.90 to 1.50	200 to 350
Limit to 1/4 of the ration (25%) for all animals	15,000 to 18,000	3,500 to 4,000	1.50 to 1.80	350 to 400
Potentially lethal; very risky to feed; dilute carefully in total mixed ration	Over 18,000	Over 4,000	Over 1.80	Over 400
Note. Laboratories may report nitrates in different forms. All reports should be read on the dry-matter basis regardless of the unit. Below is a list of commonly used state and commercial laboratories and the units they report in: a The University of Georgia Feed and Environmental Water Lab and Tennessee Forage Lab report in forage nitrates in parts per million. b Clemson University’s Agricultural Service Lab, Dairyland Laboratory, and Cumberland Valley Analytic Services report in nitrate nitrogen in parts per million. Multiply the test result by 4.426 to get forage nitrates parts per million. c Waters Agricultural Laboratories report in nitrate ions as a percentage. Multiply the test result by 10,000 to get forage nitrates in parts per million.

How Do I Test for Nitrates in Standing Forage or Pasture?

It is difficult to test for nitrates in standing forages or pastures because of variation across the field and selective grazing by livestock. Quantitative laboratory analysis is more accurate, but the report represents the forage at the time of testing, not the current field conditions. In-field testing is available but should only be used as a diagnostic or screening tool. Any positive result should be quantified with laboratory testing.

Laboratory Testing Method

Check with the specific laboratory you plan to use before submitting your sample. They may require a certain sample size, delivery day, etc. Below is general guidance for submitting fresh forage samples for laboratory testing.

1. Cut at least 15 to 20 samples from random locations in the field. Make the cut at the desired harvest or grazing height.
   • One sample = one plant for large, stemmed grasses such as sorghum, millet, or johnsongrass.One sample = a handful or bunch of plants for most other species.
2. Use scissors to cut the plants into 1- to 2-in. segments.
3. Put material into a clean plastic bucket and mix well.
4. Put approximately 1/2 lb in a gallon plastic bag for submission.
5. Deliver samples directly to the laboratory. If samples cannot be hand-delivered, place them in a freezer for 24 hr, then overnight-ship them in an insulated container. 

In-Field Testing Method

In-field testing should be used only as a diagnostic or screening tool, as it provides only a visual or qualitative measure. Nitrate test strips are available from many different retail stores and online distributors. These are commonly sold for pool or water testing.

1. Cut at least five to 10 samples from random locations in the field. Make the cut at the desired harvest or grazing height.
   • One sample = one plant for large-stemmed grasses such as sorghum, millet, or johnsongrass.
   • One sample = a handful or bunch of plants for most other species.
2. Press the sample onto the designated square on the test strip. Many strips will have two squares, one for nitrites and one for nitrates. Refer to the product’s packaging for more information.
   • Large-stemmed species can be easily pressed onto the test strip to release plant fluids from the stem. Press the cut end of the stem directly onto the test strip.
   • Smaller-stemmed species may need to be crushed to release the plant fluids. Tap the cut ends of the stems with a small hammer or mallet on a clean and stable surface. Then press the crushed part of the stem directly onto the test strip.
3. Wait a few minutes to allow the test square to change colors. Most toxic materials will show a response in under a minute.
4. Compare the color of the test square to the scale on the product’s packaging (see Figure 1).
5. Note that overmature or excessively drought-stressed forages may be too dry for in-field tests to work!

How Do I Test for Nitrates in Hay and Baleage?

Follow the same procedure as you would for collecting routine samples for forage quality of hay or baleage. For more information, please review C 1287, Unless You Test, It’s Just a Guess: How to Take, Interpret, and Utilize a Forage Sample.

Can Nitrates Decrease in Harvested Forage?

Nitrate concentrations DO NOT change substantially when stored as hay. Ensiling as baleage or silage can reduce the nitrate content but there is no guarantee that it will be reduced to a safe feeding level for your class of livestock.

Please note that forage that is too dry, poorly packed, or fed too early may have not fermented or reduced nitrate levels as expected. The riskiest harvest method is green chop (material cut and fed immediately), as the harvested forage will often undergo a heat cycle that increases the conversion of nitrates to nitrites.

How Do I Prevent Nitrate Toxicity or Nitrate Poisoning?

It is difficult to predict when high nitrate levels will occur, because of the various rates of fertilizer application, variable weather conditions, and the variety of crops grown for pasture, hay, and silage. Therefore, when weather or management are suspected factors in the potential for toxicity, assume the material is toxic and evaluate nitrate levels in the forage.

Split Fertilizer Applications

Nitrogen fertilization is critical to good forage yields. However, splitting the nitrogen applications throughout the growing season—rather than applying the full rate at the beginning of the season—reduces the risk of high nitrate concentrations. Furthermore, plants may also have higher nitrate concentrations when soils are deficient in other major nutrients such as phosphorus, potassium, etc.

Delay Harvesting and Test Stored Forages

Consider delaying harvesting for hay, baleage, or silage until the nitrates have declined. Generally, nitrates will be more common in vegetative growth that is not a suitable height for harvesting. If so, wait until the plant begins growing and reaches the target harvest height. Regardless of harvest timing or method, request a nitrate test when you submit the forage sample for quality analysis.

Adjust Grazing Management

The best way to avoid nitrate toxicity or poisoning is prevention. If possible, avoid grazing high-nitrate material.

If animals must graze these areas, use the following strategies to help mitigate the risk of nitrate toxicity or poisoning. Implementing these strategies does NOT remove all risk associated with high-nitrate material. Monitor livestock carefully to avoid acute toxicity.

• Provide plenty of clean water and supplement with sufficient vitamins and minerals.
• For lactating, pregnant, or sick animals, avoid or minimize feeding forages that are high in nitrates.
• Avoid turning out hungry animals. Feed a low-nitrate hay and supplemental feedstuffs that are high in energy (e.g., corn gluten) prior to turning animals into pasture that is high in nitrates.
• Limit animals to 3 or 4 hr in high-nitrate material per day to keep intake approximately 50% of the normal amount.
• Limit animal grazing heights, as nitrates are more concentrated in the bottom of the plant stem. Remove animals before the canopy reaches 4 to 6 in.

How Does Nitrate Toxicity or Poisoning Happen?

Nitrate is reduced to nitrite by microbes in the digestive tract. In most cases, the microbes further convert the nitrite to ammonia and ultimately to amino acids and proteins.

However, ingestion of high concentrations of nitrates will build up nitrite levels, and nitrite is absorbed into the animal’s bloodstream. Once in the bloodstream, nitrites bind with hemoglobin (creating methemoglobin) and prevent the normal transfer of oxygen.

What Are the Signs of Nitrate Toxicity or Poisoning?

All ruminant livestock can be impacted by nitrate toxicity or poisoning. It is rare to see symptoms in horses since they digest forage differently than ruminant livestock.

The severity of symptoms and the time required for them to appear will depend on the concentration of nitrates and the animal’s health.

Low nitrate intake levels may cause lower milk production, abortion, breeding problems, or symptoms that mimic a nutritional deficiency (vitamins A and E, rickets, phosphorus, or calcium imbalance).

Signs of acute nitrate toxicity result from severe inhibition of oxygen transfer and are very obvious. Symptoms include the mucous membranes taking on a bluish color, rapid and difficult breathing, a rapid pulse above 150 beats per minute, tremors, staggering, collapse, and ultimately death.

A key diagnostic feature of acute nitrate toxicity is dark brown or “chocolate-colored” blood. Unfortunately, acute toxicity may appear within 1 to 2 hr of ingesting toxic levels of nitrates, and animals are often found dead.

Can Nitrate Poisoning Be Treated?

In most cases, treatment is impractical because of the rapid onset of symptoms. Treatment must be given under the guidance of a veterinarian. Veterinarians can counteract acute nitrate poisoning by slowly administering an intravenous solution of methylene blue in isotonic saline (1% w/v). Methylene blue reduces methemoglobin to oxyhemoglobin, restoring the blood’s ability to exchange oxygen. The treatment may need to be repeated.

Acknowledgments

The author would like to acknowledge Dr. Dennis Hancock for his work on a previous version of this publication.