Feeding Nitrate-Containing Forages

Any time there is a summer drought that limits production of corn silage/forage and other forages, plant tissue accumulation of nitrates is a risk. Corn silage produced from forage grown under drought conditions is often poorly eared and low in energy. Adequate forages are essential for feeding ruminants and other herbivores. Forages provide an excellent source of nutrients, fiber for proper rumen functioning and milk fat test, and make use of renewable forage resources. Feeding drought-stressed forage that is high in nitrates is challenging, but with good management and some proper precautions, can be successful.

Nitrate in the Plant

During a drought, nitrates will accumulate in a plant. This is most often seen in corn silage and other corn forages but can occur in other plants too.

Nitrates are most likely to accumulate when plants are stressed. This may coincide with excessive amounts of nitrate in the soil. This could come from the release of nitrates in the soil from organic matter or manure, or it could be a result of heavy applications of fertilizer. Some research has shown that heavy fertilization can result in increased accumulation of nitrates in plants that do not normally accumulate (tall fescue, orchard grass). During periods of drought, the plant will continue to take up nitrate, but moisture stress will reduce the conversion of nitrate into protein in the plant. Anything that reduces the rate of plant photosynthesis or protein biosynthesis will also result in greater nitrate accumulation; frost, low temperatures, cloudy weather, and herbicide applications can all result in greater plant accumulation of nitrates.

Since moisture is required for nitrate uptake, the greatest nitrate accumulation during a drought will occur immediately following rainfall. Whenever this happens, forage should not be harvested, nor should animals be allowed to graze. Concentrations of nitrates are generally highest in stalk tissues and lower in leaves and vegetative tissues. One strategy for management is to set forage harvesters to cut higher on the stalk so that higher nitrate material is not harvested.

Nitrate in the Animal

The ruminant animal can convert nitrate to nitrite and ammonia in the rumen and detoxify the nitrate. Problems occur when the level of nitrates builds up in the rumen due to higher levels in the diet and rumen microbes cannot convert all the nitrate present to ammonia. Conversion of nitrate to nitrite occurs more quickly than the conversion of nitrite to ammonia. If levels of nitrate are great enough, nitrite will accumulate in the rumen and will be absorbed through the wall of the rumen into the blood supply. When this happens, nitrite combines with hemoglobin in the blood and converts it to methemoglobin, which will carry very little oxygen to the tissues. When methemoglobin reaches toxic levels, death can occur due to suffocation (oxygen deprivation). The presence of methemoglobin in the blood is easy to recognize because of the chocolate brown color of the blood. Blindness and abortion can also occur.

Acute poisoning can result in increased respiration and pulse rate, muscular incoordination and staggering, diarrhea, frothing at the mouth, and cyanosis (a darkening of mucous membranes resulting from oxygen deprivation). Sublethal poisoning may result in a loss of appetite, lowered milk production, slow growth, abortions, and poor fertility.

Contact Your Veterinarian for Treatment

The most common treatment is the administration of 1–2% methylene blue in an IV injection to provide 4–15 mg of methylene blue/kg. of body weight. According to the Merk Veterinary Manual, ruminants can tolerate higher levels of methylene blue than other species. Injection of cold water and antibiotics into the rumen may halt the microbial production of nitrite.

Ruminants can be adapted to higher levels of nitrate. This can be done by slowly increasing the level of nitrate-containing forage in the diet. Due to the large population of microbes in the rumen, cows, sheep, and other ruminants can convert nitrate to ammonia. However, there is a threshold of how much they can convert. They can be fed forage containing <1.5% nitrate provided they are slowly adapted and provided the forage is only a portion of the diet. (See guidelines in Table 1 below).

Testing Forage for Nitrate

The only reliable method to determine if nitrates may be a problem is through testing. The diphenylamine test can be used as a field screen to determine if testing is necessary. The test solution consists of .5 g of diphenylamine dissolved in 20 ml of water. Add sulfuric acid to a total volume of 100 ml. Cool the solution, store in a brown bottle, and keep refrigerated. Split the stem or stalk to expose the inside and add 1–2 drops of the test solution to the cut surface of the plant. An immediate color change to intense blue or black is a positive reaction indicating more than 2% nitrate. Samples that react in this manner should be submitted for quantitative analysis at a laboratory. Proper sampling to ensure that a representative sample is tested is very important. Contact your local County Extension Agent or the State Division of Animal Health for more information.

Pointers for Forages and Feeding Management

1. Applying high amounts of nitrogen fertilizer or manure late in the season can result in increased nitrate uptake and accumulation by plants. Split fertilizer applications in the spring. Consider using the pre-sidedress soil nitrate test (PSNT) to better manage soil nitrate levels.
2. Harvest when plants have outgrown nitrate accumulation. As plants mature, nitrate accumulation will decrease.
3. When harvesting, raise the cutter bar to avoid the high nitrate levels in the lower stalk (about 8–10 inches). The highest levels of nitrate accumulation are in the stalk. Raising the cutter bar will prevent including the lower stalk with corn to be ensiled.
4. Do not allow animals to graze and do not harvest forages following heavy rains. The period following heavy rain is often the period of greatest nitrate uptake by the plant.
5. As always, pay attention to proper silage production, moisture content, packing, and length. Corn silage should be 60–70% moisture at harvest. Consider adding water to the silo if less than 60%. Drought- damaged corn can be chopped at 1/4 to 3/8 inch in length. This will help in packing to exclude as much oxygen as possible. Make sure all equipment, including sharp knives, is in good condition prior to starting the harvest season. The silo should be filled quickly and packed as tightly as possible to exclude oxygen.
6. Order of feeding priority: Silage > Hay > Grazing > Greenchop. Ensiling will destroy 40–60% of nitrates. Therefore, silage crops will have the lowest levels of nitrate due to bacterial destruction. Producing forage for dry hay does not destroy nitrates. Greenchop will be the riskiest to feed. If nitrate levels are high enough, ensiling may be the only way to salvage the forage.
7. Never feed a forage containing greater than 1.5% nitrate. See the table below with guidelines for including nitrate-containing forages.
8. Mix with other feeds when feeding. Managing the total nitrate intake is the goal when formulating diets with nitrate-containing forages. Diluting with concentrates and nitrate-free forages can help to accomplish this. Whenever grazing nitrate-containing forage, feed (especially grain) the animals prior to turning out to graze to ensure the dilution of nitrate.
9. Supplement with Vitamin A. Previous research has shown that forage containing high levels of nitrate can also be low in Vitamin A or with reduced Vitamin A availability. Vitamin A is inexpensive and should be added to all ruminant diets.
10. Test water supplies for nitrates.
11. Test forages whenever nitrate accumulation is likely due to drought.

Nitrate and Lethal Silo Gases

Whenever silage is made from high nitrate forage, toxic gases (oxides of nitrogen) are produced in the first few days following fermentation. These gases are pungent, may smell like bleach, are yellowish or reddish-brown in color, and may leave a yellow stain on wood or other surfaces. These gases can be lethal at levels too low for you to see them. Forages containing high levels of nitrates are especially prone to this problem.

Gases may occur at any time during silo filling. Although gases may occur up to 10 days after the last silage is put in the silo, the danger is greatest between 12–72 hours after first filling. The gases are heavier than air and will accumulate above the silage in the silo, in the chute, and in the silo room, and will flow out the silo juice drain. They may also pose a risk in adjacent feed rooms and could disperse through the barn, loading areas or milk houses. Throat irritation may be the first indication you have been exposed to low levels.

The risk is greatest for someone entering the silo alone. The silo blower should be run for 15–20 minutes before entering a partly filled silo and the blower should continue to run when anyone is inside. Never enter the silo unless there is at least one person available to help remove you in case of a problem. Stay out of the silo for at least a week, or preferably two, after the silo is filled. Keep the silo room and adjacent feed and barn rooms well-ventilated by keeping doors and windows open for at least two weeks after filling. Remember that the presence of high nitrate forage will result in a greater risk of silo gas formation.

Nitrates in Water

Consumption of well water is unlikely to cause nitrate toxicity, which is more likely when livestock have access to contaminated water sources such as ponds, ditches, or other surface contamination. Although nitrates in feed and water can have additive effects toxicity is unlikely when water containing less than 100 ppm NO₃-N is consumed.

Please see the accompanying worksheet below for determining nitrate levels in the diet.

References

1. Amoral, D., B. Crest, G. Herschel, J. Johns, K. Olson, M. Biter, F. Benson, L. Meyer, and D. Shurley. 1988. Using Drought-Stressed Corn. Cooperative Extension Service. University of Kentucky, Lexington, KY.
2. Barnes, R. F., D. A. Miller, and C. J. Nelson. 1995. Forages. Volume I and II. The Science of Grassland Agriculture; An Introduction to Grassland Agriculture. Fifth Ed. Iowa State University, Ames, IA.
3. Drewnoski M.E., Anderson, B.E., Kononoff, P.J. (University of Nebraska), and M.B. Reynolds (Iowa State University). 2019. Nitrates in Livestock Feeding (PDF). University of Nebraska. NebGuide. G-1779.
4. Fjell, D., D. Blasi, and G. Towne. 1991. Nitrate and Prussic Acid Toxicity in Forage: Causes, Prevention, and Feeding Management. Cooperative Extension Service. Kansas State University, Manhattan, KS.
5. Snifffen, C. J. and L. E. Chase. 1981. Nitrate in Dairy Rations. Cooperative Extension Service. Cornell University, Ithaca, NY.
6. Thompson, L.J. 2021. Nitrate and Nitrite Poisoning in Animals. Merck Veterinary Manual. Merck and Co., Inc. Whitehouse, NJ.
7. Townsend, J. Animal Considerations for Nitrate Toxicity When Using Whole Plant Corn as a Feed (PDF). Purdue University School of Veterinary Medicine.

Nitrate Worksheet

Example Farm

Your Farm

July 2024