Nitrate toxicity of cattle was noted as early as 1895 with corn-stalk poisoning. However, at that time nitrate was not recognized as the principle toxicant. In the late 1930s, after an outbreak of oat-hay poisoning in the high plains region, an indictment of nitrate was finally made. The term "Nitrate Toxicity" should actually be "Nitrite Toxicity". When nitrate is ingested by a ruminant animal, it undergoes a chemical reduction action to nitrite. This reduction is accomplished by rumen microorganisms. Nitrite is readily absorbed into the bloodstream where it oxidizes the ferrous iron of the red blood pigment hemoglobin to ferric iron producing a modified red blood pigment called methemoglobin. Methemoglobin is incapable of transporting oxygen to various body tissues so the animal exhibits a characteristic chocolate brown blood color prior to and during death which is caused by asphyxiation. Ruminant animals affected in this manner by high nitrate feeds are said to be suffering from methemoglobinemia. Simple stomached animals such as swine and poultry do not have the microorganisms which can make this rapid conversion and are not as susceptible to nitrate toxicity.
Plant Factors in Nitrate Accumulation
Practically all plants contain detectable amounts of nitrates. Excessive nitrate accumulation occurs when the uptake of nitrate exceeds its utilization in plants for protein synthesis. The following factors are related to nitrate accumulation in plants:
1. Plant species vary in their ability to accumulate nitrate. Even common barnyard weeds can cause problems.
2. Stalks are highest in nitrate content, followed in order by leaves and grain in decreasing amounts.
3. Immature or young plants have a greater potential for nitrate accumulation than older plants (such as those with seed in the hard dough stage). Management: Be cautious when turning livestock in on a field that is still immature in growth. Hungry livestock are not as likely to selectively graze leaves over stems so allow them to feed before releasing them on a field. Always test fields of concern prior to releasing livestock on them.
4. Any weather condition which reduces plant growth may increase nitrate accumulation. This includes drought and sometimes cool, cloudy weather.
5. Excessive use of nitrogen fertilizer may contribute to the problem.
Livestock Factors Related to Nitrate Poisoning
Despite a producer’s best efforts to avoid growing forages that contain dangerous concentrations of nitrate, occasionally, drought-stricken pastures of hay crops produce feeds that test high in nitrates. There may be methods of handling the high nitrate hays or pastures that reduce the risk of death or production losses. However, if the forage has extremely high concentrations of nitrate, such as 25,000 ppm, then the risk to livestock health is very great even when all known management techniques are employed. Burning, or burying that forage may be the only safe alternative. Knowledge of the following livestock factors will aid in a producers decision on how to either prevent or manage the effect of high nitrate feed sources.
1. Thin cattle in poor health or those suffering from respiratory disease are more susceptible to nitrate poisoning.
2. Allow hungry cattle to fill prior to release. If cattle are hungry, take time (1 to 3 days) to make sure they are consuming a significant quantity of a bulky forage such as good quality grass hay. Then, release the cattle in the afternoon when they are not as hungry.
3. Adapt cattle to nitrate. The objective is to give the ruminal microorganisms the opportunity to adapt to high nitrate intake. With high nitrate hay, this can be accomplished by blending with low nitrate feeds such as grass hay or concentrates. Grain feeding has the additional benefit of providing ruminal energy to stimulate the conversion of nitrate to nontoxic nitrogen compounds. With grazed high nitrate forages, palatable, low nitrate hay or concentrates can be used. Another alternative with grazed forage is to limit grazing for the first 6 to 8 days by increasing the grazing time each day. For example, cattle might be allowed to graze high nitrate forage for 2 hours on the first day and increase by 2 hours each day for 6 days after which cattle could be released full time onto the high nitrate forage. Another strategy with grazed forage would be to feed the animals several times a day (3 to 5 times) to disrupt grazing periods and provide ruminal fill to decrease the rate and extent of consumption of the high nitrate feed.
4. Dilute high nitrate feeds with now nitrate feeds. Dilution is one method that can be used to help ruminal microorganisms adapt to high nitrate feeds. But, it may also be the only practical method that can be used to feed high nitrate forage (less than 10,000 ppm). Dilution is most effective when the low nitrate feed can be blended or mixed directly with the high nitrate feed.
5. Release cattle in the afternoon when night-time nitrate accumulations have subsided. In addition, avoid grazing shortly after a drought-ending rain.
6. Stock lightly so animals can choose lower nitrate leaves over higher nitrate stems.
7. Provide large quantities of fresh drinking water. Water dilutes nitrate concentrations in the rumen and reduces the potential of toxicity.
8. Do not allow livestock to have direct access to fertilizer materials containing nitrates. Losses have occurred because cattle had access to storage areas, fertilizer spreaders, broken bags of fertilizer, or fertilizer spills in pastures.
9. Cattle in cold or inclement weather are more susceptible to nitrates and are more likely to consume stalks which contain more nitrate.
Treatment for Nitrate Poisoning
Due to the acute nature of this toxicity, for treatment to be successful it should be done immediately when symptoms are observed. Also, due to the fact that the oxygen transport system of blood is compromised, death by suffocation will occur, sometimes within minutes of observing symptoms especially if the animal becomes excited or exerted.
This toxicity may be distinguished by the chocolate color of the venous blood. The treatment recommended is methylene blue. This product is not readily available either commercially or over the counter for sale to producers. You should make plans with your veterinarian to have some material on hand in case of any emergency. The product is difficult to find and may have to be compounded by the practitioner. A well thought out and preplanned emergency plant with your veterinarian could save thousands of dollars in losses if an emergency should occur.
Summary
Careful use of nitrogen fertilizer, an awareness of plant factors, the effect of weather, and cattle management can help reduce losses from nitrate poisoning. Diagnostic techniques are available to determine nitrate levels in forages. Hay which has potentially toxic levels of nitrate should be fed only as a part of the total diet.