Nitrate Toxicity

Toxic levels of nitrates in forages are always a possibility whenever the normal growth of plants is disrupted by events such as hail, drought, spray drift or frost. The number of animals affected by acute nitrate poisoning in Saskatchewan is usually low; however, when deaths do occur, they occur suddenly and can be devastating. The impact of these deaths on the industry as a whole is not great, but the impact on the individual producer can be disastrous. High levels of nitrate need not be a problem as long as the feeding program is managed correctly.

What is Nitrate Toxicity

When growing conditions are favourable, plants take up nitrogen largely in the form of nitrate. The nitrate is rapidly converted to ammonia which is incorporated into plant protein. Unfavourable growing conditions can interfere with nitrate conversion and cause it to accumulate in the plant. Under normal conditions, cattle convert the nitrate in forage to nitrite. This is then converted to ammonia and used by microbes in the rumen to make protein.

The problem arises when nitrate converts to nitrite faster than nitrite converts to ammonia in the rumen. When this occurs, nitrite accumulates and is absorbed into the bloodstream where it binds to hemoglobin, thus reducing the oxygen-carrying capacity of the blood. The animals die by asphyxiation.

Symptoms

Symptoms of lethal nitrate poisoning include laboured breathing, frothing at the mouth, rapid pulse, weakness, diarrhea, frequent urination, muscle tremors, incoordination and convulsions, collapse and death. Death may occur in three to four hours. Post-mortem examination reveals dark, chocolate-coloured blood. Sub-lethal doses may result in loss of appetite, reduced milk production, slow growth and abortion.

Lack of oxygen to the fetus due to nitrate poisoning can cause abortions. Abortion due to nitrate poisoning is accompanied or preceded by some other evidence of nitrate problems in the adult animal, such as chocolate-coloured blood and bluish discolouration of unpigmented areas of the skin or mucous membranes.

Treatment

Nitrate poisoning can quickly become fatal. When high nitrate is suspected, remove the contaminated feed and provide a high-energy feed such as barley. A veterinarian should be called immediately to confirm the tentative diagnosis. Because death is a result of oxygen shortage, handle cattle as little and as quietly as possible to minimize their oxygen needs. A common veterinary recommended treatment is a methyl blue solution delivered intravenously. If nitrate poisoning is suspected, consult your veterinarian for dosage rates and the optimum course of treatment. Treatment may have to be repeated every six to eight hours because the rumen may be full of forage or feed that contains nitrate, and nitrites will continue to move from the rumen into the blood stream. Mineral oil may be given orally to protect the irritated mucous membranes.

Understanding Safe Nitrate Levels

Methods of reporting nitrate values in feed are shown in Table 1. The amount of nitrate in water usually is expressed as parts per million (ppm) of nitrate ion (NO₃) or nitrate nitrogen (NO₃N). In Western Canada, the nitrate ion (NO₃) is the most commonly used value for the measurement of nitrate.

Table 1. Methods of nitrate analysis and reporting (Dry Basis).

Deaths have been reported when the nitrate content in the overall diet was as little as 0.93 per cent nitrate NO₃; however, rations containing substantially more than 0.93 per cent nitrate NO₃ have been fed without harm. Researchers in Missouri fed steers rations containing 1.24 to 1.46 per cent nitrate NO₃ from sudangrass hay. The steers slowly adapted to the high nitrate feed and performed satisfactorily. It is not uncommon that cattle and other ruminants have been fed levels higher than 0.5 per cent, however feeding higher levels should only be permitted after a period of adaptation to the high nitrate feed.

The toxic level depends both on the amount of nitrate in the feed and how fast that feed is consumed. For example, it takes about twice as much nitrate to kill a ruminant when the nitrate comes from forages that are eaten over a long period and have been gradually introduced as compared to a feed that is consumed quickly and to which the animals have not been previously exposed.

Managing High Nitrate Feed

Stored Feeds

Test the stored feed for the presence and amount of nitrate. Frequent intake of small amounts of a high nitrate feed increases the total amount of nitrate that can be consumed daily by livestock without adverse effects, and helps livestock adjust to high nitrate feeds. For example, feeding limited amounts of long-stemmed, high nitrate feed several times a day, rather than feeding large amounts once or twice daily, is a good strategy. Animals should not be allowed to consume feeds containing more than 0.5 per cent nitrate if they have not been exposed to such feeds prior to this.

Animal Safety

To help animals safely make the transition to high-nitrate forages, these feeds should be physically mixed with low-nitrate forages so that the overall nitrate level is less than 0.5 per cent. This can be accomplished easily in feedlot rations where grain is fed and forages are chopped and mixed as a complete ration. Putting out one bale of high-nitrate feed along with one or more bales of low-nitrate feed is not acceptable as it would be possible for some animals to eat only from the bale containing high levels of nitrate. Therefore, the feed must be physically mixed together to blend the nitrates down to a safe level. Be sure to make the transition to the questionable feed over a period of one to two weeks and to avoid moving cattle on and off high-nitrate feeds.

If mixing is not possible, put out low-nitrate feed at the beginning of each feeding and follow up with the higher-nitrate feed to ensure each animal has consumed the low-nitrate feed first.

A balanced ration tends to reduce problems from nitrates in the ration. Feeding adequate levels of energy, vitamins (A and E) and trace minerals reduces the risk of toxicity. Feeding grain in combination with high-nitrate feeds helps reduce the effect of the nitrate content. Energy from the grain apparently helps complete the conversion of nitrate to bacterial protein in the rumen. This can be another tool in managing high-nitrate forage which cannot be physically mixed with low-nitrate forage.

If the animals are in good body condition, are receiving a balanced ration and have been introduced to high-nitrate feed over a period of time, it is possible for them to maintain normal levels of growth while consuming feeds with nitrate levels of one per cent or higher. Livestock should have access to clean water at all times.

Do not feed damp hay, straw or fodder suspected of being high in nitrate. Damp feed seems to be more toxic because some of the nitrate has already been converted to the more toxic nitrite.

Don't panic when you get a high nitrate level in your forages. Proper sampling and a nitrate test will allow you to make use of the forage through proper feeding management.

Pasture

Allowing livestock to graze pastures suspected of having high nitrate levels is not without risk. Implementing one or more of the following management practices will reduce the risk of livestock losses to nitrate toxicity.

• Don't overstock suspect pastures to limit consumption of lower stem material which is highest in nitrate content.
• Don't strip-graze suspect pastures to limit consumption of lower stem material which is highest in nitrate content.
• Provide other feeds that contain little or no nitrate during grazing.
• Graze suspect pasture during the day and remove the animals at night during the first week to reduce the amount of pasture consumed and to acclimate cattle.

Observe livestock frequently when they begin grazing a new pasture that is suspected of having high nitrate levels to detect any signs of nitrate toxicity. Recent research indicates that beef heifers can safely graze sudangrass that contains high levels of nitrate (0.93 per cent nitrate NO₃- ), but cattle grazing these pastures tend to selectively graze the leaf portion of the plant that is not high in nitrate, and slowly adapt themselves to these feeds. If stocking rates are heavy for a pasture suspected of high nitrate concentrations, it may force the cattle to consume high-nitrate plant parts, increasing the possibility of nitrate poisoning.

Formulas for Converting Methods of Reporting.

• Potassium Nitrate = Nitrate Nitrogen X 7.22
• Nitrate = Potassium Nitrate X 0.613
• Potassium Nitrate = Nitrate X 1.63
• Nitrate Nitrogen = Potassium Nitrate X 0.139
• Nitrate = Nitrate Nitrogen X 4.43
• Nitrate Nitrogen = Nitrate X 0.226

(The nitrate ion (NO₃) will be used to evaluate and discuss nitrate concentrations in feed and water in this article.)

Causes of High Nitrate Levels in Forage

Nitrate taken up from the soil through plant roots is normally incorporated into plant tissue as amino acids and proteins. The primary site for this conversion is the actively growing green leaves. Nitrate accumulates in the stalk or stem of plants when factors interfere with normal plant processes.

All plants contain some nitrate, but excessively high amounts are likely to occur in forages that have been grown under stress conditions such as shade or low light intensity, detrimental weather (including drought, frost, hail, low temperatures), herbicide applications, insect predation and diseases. The amount of nitrate in plant tissues also will depend on the species, stage of maturity, part of the plant and nitrogen fertilization.

Shade - Conversion of nitrates into amino acids and proteins is linked closely with photosynthesis. Light is the energy source for these activities, so shaded plants or lower leaves may be higher in nitrates than plants grown in full light.

Weather - Not all drought conditions cause high nitrate concentrations in plants. Some moisture must be present in the soil along with the nitrate to permit absorption and accumulation. If the major supply of nitrate for the plant is in the dry surface soil, very little nitrate will be absorbed by plant roots. In plants that survive through drought, nitrate levels are often high for several days following the first rain.

The nitrate level in plants can rise and fall. If the source of stress is drought, and timely rain allows the plant to recover, the nitrate level falls as the plant uses the nitrate in the production of new plant tissue. If drought persists, so does the nitrate.

Frost, hail and low temperatures all interfere with normal plant growth and can cause nitrate to accumulate in the plant. Frost and hail may damage or completely destroy the leaf area of the plant. A decrease of leaf area limits photosynthetic activity, so nitrate absorbed by the roots accumulates in the stem or stalk.

Most plants require temperatures above 13°C for active growth and photosynthesis. Nitrate can be absorbed quickly by plants when temperatures are low, but conversion to amino acids and protein occurs very slowly during times of low temperature.

Herbicides - Herbicides, such as 2, 4-D tend to disrupt normal plant processes and can result in temporary high nitrate content in plants. However, spraying pastures and silage crops to control weeds may actually reduce the nitrate hazard of these feeds, especially when weeds high in nitrates are killed.

Disease - Plant diseases interfere with normal growth and development. This can cause nitrate to accumulate by interfering with nitrate reduction, protein synthesis or manufacture and translocation of carbohydrates.

Insect Predation - Leaf damage caused by insect predation can result in reduced photosynthesis and may result in accumulation of nitrate.

Plant Species. Plants vary in the amount of nitrate that accumulates in various tissues. Certain weeds, such as pigweed, kochia and lambsquarter, often are high in nitrate. Oats and millet cut for hay at an immature stage can also have high nitrate concentrations. Sorghum and sudangrasses often store high amounts of nitrates. Brome and orchard grass store very little nitrate under normal growing conditions. Legumes generally do not contain high nitrate concentrations. Alfalfas, vetches, trefoils, peas and clovers generally do not accumulate nitrates. A list of common crop and weed species that can accumulate nitrates is shown in Table 2.

Stage of Growth - Stage of growth markedly changes the nitrate content of forages. Nitrate concentrations usually are higher in young plants and decrease as the plant matures. However, plants grown in soils with excessive nitrates or grown under stress may be high in nitrate content at maturity.

Plant Parts - Plant parts closest to the ground contain the most nitrate. Leaves contain less nitrate than stalks, and the seed (grain) and flower usually contain little or no nitrate. Most of the plant nitrate is in the bottom third of the stalk.

Nitrogen Fertilization - Nitrates in the soil are the source of nitrate in plants. While a positive relationship exists between soil nitrate and nitrate in the plant, the effect of nitrogen fertilization appears to be less important than the conditions listed previously in causing high nitrate content in forages.

Table 2. Common crops and weeds that have potential to accumulate nitrate.

Harvest Method and Nitrate Levels

Harvest as Hay - If hay in the windrow remains damp for several days or is rained on, nitrate levels will decrease slightly. Hay dried quickly will lose very little nitrate. Nitrate concentration in hay bales does not change appreciably over time.

Cut hay when the weather is most favourable. Bad weather has destroyed more hay than nitrate. After the hay is baled, test for the presence and the level of nitrate. Forages with high levels of nitrate can be diluted with other feeds.

Harvest as Silage - In some cases, nitrate levels may be reduced by the ensiling process, however this comes with a significant decrease in quality of the silage and is a poor way to manage nitrates. It is much more important to ensure good quality silage is being made than to try and use this process to reduce nitrate content. Producing low quality silage creates more management problems than it solves. Harvest forages suitable for silage at the stage of optimal quality and quantity. If silage is suspected to be high in nitrate, analyze the feed for nitrates before feeding. The analysis will help in designing rations to prevent livestock losses from nitrate poisoning.

Harvest Near Maturity - Crops normally have lower nitrate levels at maturity, so for crops suspected of containing nitrates, silage should be harvested as near to maturity as possible. If the field being harvested as silage has been identified as or is suspected of having high nitrate concentrations, raise the cutter head to avoid stalk bases that have the highest nitrate concentration.

In crops such as oats for hay, it may be wise to harvest the crop at a more mature stage than desirable for maximum yield of digestible nutrients.

Harvest as Green Chop - Some forages are harvested in an immature stage and fed to cattle in the form of green chop. High nitrate feeds that are piled in mounds and allowed to heat before feeding are especially dangerous to livestock. Heating causes nitrate to convert to nitrite, making the feed about 10 times as deadly. An analysis of nitrate will help in designing feeding programs to prevent livestock losses from nitrate poisoning.

Nitrate in Water - Mature livestock can tolerate higher concentrations of nitrate in their water supply than young livestock. In the case of calves, however, much of their fluid intake is derived from nursing and very little nitrate is secreted in milk.

Livestock drinking well water are not likely to experience nitrate toxicity. Nitrate toxicity from water is more likely to occur when livestock drink water from ponds, road ditches or other surface depressions that collect drainage from feedlots, heavily fertilized fields, silos, septic tanks or manure disposal lagoons.

As with feed, frequent intake of water containing nitrates appears to increase the overall nitrate tolerance; however, when evaluating possible toxic situations, the nitrate in both the feed and water must be considered because they are cumulative.

Nitrate toxicity is not likely to occur from water containing less than 443 ppm nitrate NO₃, provided that animals are fed a balanced ration that is not high in nitrate, and that sound feeding, watering and management practices are followed.

Sampling and Testing Feeds for Nitrates

Feeds can be tested for nitrate levels. When sampling suspect silage or green chop for nitrates, take representative samples from six areas of the pit or mound. Ideally a forage probe or silage sampler would be used to obtain a representative sample of the feed. If this tool is unavailable, take grab-bag samples from six areas of the feeding face of the pit or mound. Mix the samples and sub-sample an amount to fill a plastic bag that can be sealed at the top. Compress the air out of the bag and seal. The sample is now ready to send to the laboratory for analysis.

For suspect forages being put into an upright silo, take grab-samples on three successive days, then sub-sample and transfer to a plastic bag as described above.

Baled hay and greenfeed should be sampled using a hay probe. Sample the bales or stacks that represent the suspect hay. For baled hay, probe about 20 different bales, and for hay stacks, sample each stack in six different areas to obtain a representative sample. Transfer the sample to a plastic bag, and compress the air out and seal before sending the sample to the laboratory.

It is difficult to obtain a representative sample from suspect pastures that cattle are actively grazing. Cattle are selective in the plants and plant parts they consume, so a clipped sample will not represent what is actually being consumed.

Laboratories that test for nitrates