Know Your Numbers & Test Forage

Reports of ranchers who have lost cattle from nitrate poisoning have surged recently across the American Midwest.  Although environmental conditions in Wyoming can be contributed to increased levels of nitrates in certain forages, nitrate’s effects on livestock can be minimized if the conditions for nitrate accumulation are known and understood, and appropriate management steps are taken.

Nitrate poisoning is a potentially life-threatening disease caused by the presence of nitrite in the blood at a level capable of causing anoxia, or internal suffocation.  It commonly occurs when ruminant animals, or animals that obtain their nutrients from plant consumption, ingest forage or feed with abnormally high nitrate content.

Under normal conditions, the nitrate ingested by ruminant livestock like cattle and sheep is converted to ammonia by bacteria in the animal’s first stomach, or the rumen (which receives food from the esophagus and partly digests it with help from bacteria before passing it along to reticulum).  But, when ruminants consume nitrates at an abnormally high rate, nitrate can convert to nitrite faster than nitrite converts to ammonia.

Nitrate poisoning is a potentially life-threatening disease caused by the presence of nitrite in the blood at a level capable of causing anoxia, or internal suffocation.

Nitrate poisoning occurs when the nitrite level in the rumen exceeds the capacity of the microbes to convert it to ammonia.  When this happens, nitrate and nitrite absorb through the wall of the rumen and into the bloodstream.  It is the nitrite that causes the toxicity.  However, it’s the process of the nitrite combining with the hemoglobin to form methemoglobin, and methemoglobin’s inability to carry vital oxygen to body tissues, that’s ultimately deadly to livestock, hence the disease’s namesake.

Trate vs. trite; which is right?  Technically, nitrate (NO3) alone is non-toxic to animals.  It’s true that almost all forages do contain some nitrates.  But, at regular levels, these aren’t necessarily toxic and are generally acceptable for animal (and even human) consumption.  Conversely, nitrites (NO¯2), the intermediate byproduct produced in cattle and other ruminants when nitrates are broken down into ammonia after consumption, can be extremely toxic.

The amount of nitrate being recycled back into the rumen, along with the rate of nitrite breakdown, influences nitrate toxicity levels for different animals.  Individual animals have different levels of tolerance to nitrites because of the breakdown and recycling rates.  This is reflected in the variability between animals, specifically in the amount of methemoglobin that can form before production or reproduction is affected, or death occurs.

Sheep and cattle are the most susceptible to nitrate poisoning because of the microbes found in their digestive systems, which lend to the conversion of nitrate to nitrite.  In contrast, monogastric animals convert nitrate to nitrite in the intestine, nearer to the end of the digestive tract, where there is less opportunity for the nitrites to be absorbed into the blood.  It is this difference—in the site of conversion—that makes nitrate poisoning far more concerning for ruminants than for monogastric animals, such as horses and pigs.  For this reason, ruminants are especially vulnerable to nitrate poisoning.  And, while many species are susceptible, cattle and sheep are affected most frequently.

Cause

When absorbed into the bloodstream of a bovine, nitrites can change hemoglobin into methemoglobin.  Unlike hemoglobin, which carries oxygen from the lungs to other tissues of the body, methemoglobin is incapable of carrying oxygen.  When enough methemoglobin is produced that the blood cannot carry the oxygen necessary for a body to survive (approximately 80 percent of total hemoglobin content), the animal dies from a lack of oxygen.

When enough hemoglobin is converted to methemoglobin the animal begins to suffer from oxygen starvation.  This change in the hemoglobin, or red blood cells, is influenced by several factors, which can generally be predetermined and/or monitored.

Hemoglobin conversion factors:

  • Rate of nitrate intake (amount of feed and how quickly it is consumed)
  • Rate of conversion of nitrite to ammonia in the rumen
  • Rate of digestion of feeds and the subsequent release of nitrates
  • Movement of nitrite out of the rumen

Most commonly, nitrate poisoning occurs when producers are feeding hay – specifically, hay containing a high concentration of nitrates.  Eventually, most cattle producers who feed hay will encounter hay with high nitrates, especially when using millet, sorghum, oat, or other cereal-based hays.  But, that needn’t necessarily equate to livestock or feed losses.

Symptoms

Knowing what physical ailments to look for – those that are commonly associated with nitrate toxicity and which can help to uncover toxic nitrate levels — will help in early detection and extended treatment opportunities and options.

  • Brownish, chocolate-colored discoloration of the blood caused by the presence of methemoglobin (and lack of oxygen)
  • Difficult and rapid breathing
  • Muscle tremors
  • Low tolerance to exercise
  • Incoordination
  • Diarrhea and/or frequent urination
  • Collapse and sudden death

Resulting from tissue hypoxia, signs of nitrate poisoning become apparent at a rapid rate.  The animal affected by nitrate poisoning may experience a rapid, but weak heartbeat with subnormal body temperatures, muscle tremors, weakness, and fatigue.  Animal may also die suddenly without warning, and without appearing ill.

Effects of nitrate poisoning include abortions and stillbirth may also be seen in some cattle 5 – 14 days after excessive nitrate exposure, but likely only in cows that have survived a methemoglobin count of over 50 percent for 6 – 12 hours, or more.  Other effects commonly associated with nitrate toxicity, include: retarded growth, lowered milk production, vitamin A deficiency, and increased susceptibility to infection.

Nitrate reduction—and nitrite production—is present in other ruminant mammals like horses and other equids, including asses and zebras, but not to the same extent.  Young pigs also have a gastrointestinal system capable of reducing nitrate to nitrite, but mature monogastric organisms (animals with simple, single-chambered stomachs) are considerably more resistant to nitrate toxicoses.

Because the digestive tracts of non-ruminants lack a mechanism for converting nitrates to nitrites, they’re not at risk of nitrate poisoning from nitrate intake.  That said, it is possible for non-ruminants to get nitrite poisoning from ingesting increased levels of nitrites, because they’re incapable of converting nitrites to ammonia.  For this reason, non-ruminants should not consume feed high in nitrites, for example moldy hay.

Differential Diagnosis

Differential diagnoses include poisonings by cyanide, pesticide, and toxic gases (like carbon monoxide and hydrogen sulfate), drugs like acetaminophen, as well as disease (grain overload, hypocalcemia, emphysema), and other ailments which lend to sudden, unexpected death in livestock, which can include: anaplamosis, blue green algae toxicity, perilla mint, and prussic acid.  Prolonged exposure to excess nitrate, when combined with cold stress and inadequate nutrition, may also lend to Alert Down Cow Syndrome, or the Downer Cow.

Management

To manage and minimize nitrate’s potentially fatal effects on livestock, the Cooperative Extension Office of the University of Wyoming College of Agriculture recommends that you first familiarize yourself with the commonly-known conditions for increased nitrate accumulation.

Nearly all plants contain some nitrate, but some are more prone to excess nitrate accumulation than others.  Such “high-risk” plants are listed below, all of which are commonly grazed and prone to nitrate buildup. 

Weeds:

  • Blindweed
  • Pigweed
  • Kochia
  • Lambsquarter
  • Sunflower

Crops:

  • Corn
  • Small grains
  • Sudangrass
  • Sorghum

Vegetables:

  • Sugar beets
  • Lettuce
  • Cabbage
  • Potatoes
  • Carrots

If you have an abundance of the abovementioned where your cattle pasture or graze, be cognizant of their presence and adapt livestock grazing habits and additional and/or supplemental food sources accordingly. 

Plants stress, too.  Excess nitrates can also accumulate in plants (including, but not limited to the abovementioned) when they are stressed.  We recommend singing and yoga for stressed plants.   We’re kidding.  But, seriously… Drought or hot, dry winds put forage under water stress, and can result in nitrate accumulation.  Similarly, damage caused by hail or frost impairs photosynthesis, and can also result in excess nitrates.  Cool, cloudy weather is yet another contributing factor to elevated nitrate levels in plants.  When any of these conditions occur within a few days of harvest or grazing, the potential for nitrate poisoning exists.  It’s worthy of note, though, that if the stress is removed and the plants allowed to recover, nitrate levels should return to normal within days. 

If there’s concern regarding increased toxicity in your plants—whether it stems from plant type, environmental or circumstantial stress—a test of the feed should be performed to reduce the risk of disease and the potential for livestock losses.

The stage of plant growth can also be a factor in increased nitrate levels.  During a plant’s initial growth period, much of the nitrate taken up by the plant is used for root and shoot development.  At this early developmental stage, the roots take up more nitrate than the plant requires, lending to excess nitrate accumulation in the stems and leaves.  As the plant continues to grow, its leaves are able to convert more nitrate into plant protein, and less surplus nitrate is needed.  Hence, less nitrate is found in a plant as it matures.

Contributing factors to boosted nitrate in plants

  • Stress
  • Extreme temperatures/weather
  • Growth stage
  • Use of nitrate-containing fertilizers
  • Use of heavy manure applications
  • Areas where stock have congregated & urinated/defecated
  • Wilting
  • Herbicide application
  • Presence of sulfur-deficient soils

Treatment

Animals with nitrate poisoning can be treated by slow, intravenous injections of one percent methylene blue (the blue dye commonly used by hospitals in cancer detection), which is not approved by the FDA for use in food-producing animals.  Injections should be given in distilled water or isotonic saline should at 4 – 22 mg/kg or more, depending on exposure.  Lower doses may then be repeated, as needed, in 20 – 30 minutes, and re-treatment considered again at six to eight hour intervals should exposure/absorption continue.  Lavage rumen with cold water and medicated, antibiotic solution to prevent ongoing nitrite production.

Prevention

Nitrate poisoning prevention is best achieved by controlling the type and quality of forage offered to livestock.  Avoid using forages with toxic level of nitrates, or dilute them with low-nitrate feeds, as per below.  Again, forages with sub-lethal nitrate levels can be used to feed livestock with the appropriate precautions.  And, no single level of nitrate is toxic under all conditions.  When in doubt, always test your feed’s nitrate levels.

Field Tests

Field tests for nitrate are presumptive, and should be should be confirmed by standard analytic methods at a qualified lab.  However, the following field tests can be used to demonstrate a need for further testing.

  • Diphenylamine test (one percent in concentrated sulfuric acid); is best-suited to in determining the presence or absence of nitrate in suspected forages.
  • Dipstick test; or nitrate strips, are effective in determining nitrate values in water supplies and can be used to evaluate nitrate/nitrite content in serum, plasma, ocular fluid, and urine.

Hay with a low level of nitrates can be safely fed to non-pregnant animals, such as bulls or yearlings, or mixed with other hay and fed to pregnant cows.  For detailed nitrate numbers, reference the table (left): Nitrate (NO3) Safety Levels.

Lab Tests

If you suspect hay or feed to be high in nitrate, testing and analytical services are available through the Forage Analysis program of the Testing Division at the Wyoming Department of Agriculture.  The program is designed to assist Wyoming ranchers, feeders, and hay producers in determining the nutrient value of various hays, silages, and forage, including: fiber, calcium, phosphorous, trace metal nutrients, and toxic constituents such as cyanide and potassium nitrate.

For Feed, Forage, and Grain Analyses (including nitrate analysis), contact the lab at (307) 742-2984.  They will be able to generate a price quote and sampling instructions for you prior to your submitting samples.  More information on testing is available online at wyagric.state.wy.us. 

There are also private agriculture testing labs with national capabilities, such as the Alliance Analytical Labs of Coopersville, Michigan, that can be contacted for nitrate testing services at (616) 837-7670.  When in doubt, always check the nitrate levels in forage with questionable levels to avoid loss of livestock due to nitrate poisoning.

Labs can test for nitrate exposure in pre- and post-mortem specimens.  Plasma is the preferred pre-mortem specimen, not serum.  The preferred post-mortem specimens are fetal fluids, ocular fluids.  These fluids should be frozen in clean, clear, glass or plastic container.  Blood alone is not a good indicator of nitrite toxicity because 50 percent of methemoglobin converts back to hemoglobin in under two hours.

Conclusion

While nitrate poisoning is not necessarily toxic to ruminant animals, it’s common.   And, learning the symptoms, causes, preventions, and treatment of nitrate poisoning in your livestock, including how to detect high levels of nitrate toxicity, can prevent potentially devastating losses.  Questionable feed, forage, or hay?  Know before you feed.  Test is best.  How better can we say this?  The best way to prevent nitrate poisoning is to simply know your numbers and test.  Period.

Leave a comment