Nitrate and Nitrite Poisoning

By David L. Morris, DVM, PhD.

From Fence Post 9-23-96, “Vet Column”

            Many species are susceptible to nitrate/nitrite poisoning, but cattle are affected most frequently.  Ruminants are especially vulnerable because the organisms in the rumen (or first stomach) convert nitrate to ammonia.  To accomplish this biochemical conversion, nitrite, an intermediate product, is produced.  The problem is that nitrite is approximately 10 times more toxic than nitrate.  Nitrate reduction and the associated nitrite production also occurs in the caecum of the horse, but not to the same extent as is ruminants.

            Acute intoxication results in the inability of the animal to transport oxygen in the blood.  The nitrite reacts with the red blood cell and prevents the cell from adequately carrying the needed oxygen to the body tissues.  The nitrite also affects the smooth muscle of blood vessels by resulting in a secondary lowering of the blood pressure.

            Signs of nitrite poisoning usually appear suddenly due to the oxygen deficit and lowered blood pressure.  Rapid, weak heart beat, subnormal body temperature, muscular tremors, weakness and in coordination are early signs of toxicity.  Rapid breathing, anxiety and frequent urination are common.  The clinical signs can vary from acute death to signs exhibited over days and weeks.  Abortion and stillbirths may occur in some cattle.  Prolonged excess nitrate exposure coupled with cold stress and inadequate nutrition may lead to the alert downer cow syndrome in pregnant beef cattle; sudden collapse and death can result.

            Toxicity occurs most commonly in those animals unaccustomed to eating plants that contain excess nitrate.  Crops that readily concentrate nitrate include cereal grasses (especially oats, millet and rye), corn, sunflower and sorghums.  Weeds that commonly have high nitrate concentrations are pigweed, lamb’s quarter, thistle, Jimson weed, fireweed (Kochia), smartweed, dock and Johnson grass.  Anhydrous ammonia and nitrate fertilizers and soils naturally high in nitrogen tend to increase nitrate content in forage.

Nitrate levels may be hazardous in ponds that receive extensive feedlot or fertilizer runoff; these types of waters may also contaminate shallow, poorly cased wells.  Although nitrate concentrations are increasing in groundwater in the U.S., well water is rarely the sole cause of excess nitrate exposure.  Water with high nitrate levels and significant coliform contamination  has great potential to affect health adversely and lower productivity than do either nitrate of bacteria alone.  Livestock losses have occurred during cold weather due to the concentrating effect of freezing which increases nitrate content of remaining water in stock tanks.

            Excess nitrate in plants is generally associated with damp weather conditions and cool temperature (55 degrees F.), although high concentrations are also likely to develop when growth is rapid during hot, humid weather.  Drought conditions, however, particularly if occurring when plants are immature, may leave the vegetation with high nitrate content.  Decreased light, cloudy weather and shading associated with crowding conditions can also cause increased concentration of nitrates within plants.

            Nitrate, which does not selectively accumulate in fruits or grain, is found chiefly in the lower stalk.  Anything that stunts growth increases nitrate accumulation in the lower part of the plant.  Nitrate in plants can be converted to nitrites under the proper conditions of moisture, heat and microbial activity after harvesting.  Nitrate levels of covered hay will not change once the hay is cut, but grazed forages still change concentration on a daily basis.  Large, uncovered bales with excess nitrate are potentially dangerous if stored outside; rain or snow can leach and subsequently concentrate most of the total nitrate present into the lower third of these bales.

            The following table is a guideline for interpreting nitrate analysis of feed samples:

                        Ppm Nitrate                         Effects on Cattle

                               0-3,000                                    Considered safe.

                        3,000-6,000                           Moderately safe in most situations; limit to                                                                                     50% of the total ration for stressed animals.

                        6,000-9,000                           Potentially toxic depending on situation;                                                                                  should not be the only source of feed.

                        9,000 and above                    Dangerous and will often cause death.

            Guidelines to follow to reduce the risk of nitrate toxicity include:

1. Avoid excessive application of manure or nitrogen fertilizer.
2. Do not graze potentially dangerous forage on cool, cloudy days.
3. When harvesting forage sorghum and Sudan grass, raise the cutter bar 6-12 inches to exclude the base of the stalk.  This will also minimize harvesting many weed species that accumulate nitrate when shaded.
4. Delay harvesting any stressed forages.  A week of favorable weather is required for plants to reduce accumulated nitrate.

RMLA Webmaster copied 7-27-05

Return to Health                    Return to RMLA Home