Nitrate accumulation in plants is a potential danger to grazing animals. It can cause 2 different disorders:

• nitrate poisoning
• nitrite poisoning

Nitrates may cause inflammation of the gut when eaten in large quantities, but their main importance is as a source of nitrite. Nitrites cause respiratory distress due to interference with oxygenation of blood. It can result in death.

Pigs are the species most susceptible to nitrite poisoning, followed by cattle, sheep and horses.

What causes nitrogen build-up in plants

Plants absorb nitrogen from the soil in the form of nitrates, which are then converted into proteins and other nitrogen-containing substances.

Normally plants contain relatively small amounts of nitrate as such, because the conversions take place fairly rapidly inside the leaf. But during periods of drought, the amount of nitrate in the soil can increase greatly because of:

• lack of leaching
• reduced uptake by plants
• decomposition of organic matter

After the drought breaks, nitrate uptake by plants may be high. While high concentrations of nitrate aren't toxic to plants, animals grazing on such plants may suffer from poisoning. Major outbreaks of nitrate and nitrite poisoning have occurred after prolonged dry periods in Victoria.

Apart from high natural availability of soil nitrogen, various other factors promote high concentrations of nitrate in plants. These include:

• moisture stress
• decreased light (cloudiness, short day length)
• low temperatures

The use of nitrogenous fertilisers, and spraying plants with hormone-type herbicides (such as 2,4-D) can also cause a build-up of nitrate levels in plants.

Plants that cause nitrate poisoning

Many weeds, crop and pasture plants have been reported as capable of causing nitrate and/or nitrite poisoning.

The following weeds are well-known accumulators of nitrate:

• capeweed
• pigweed (Portulaca oleracea)
• variegated thistle

Many of the major crop plants have been implicated, including:

• barley
• linseed
• lucerne
• maize
• millet
• oats
• rape
• sorghum
• soybean
• subterranean clover
• Tama ryegrass
• wheat

Conserved fodder such as hay and silage can also contain high levels of nitrates.

Nitrate tolerance in ruminants

Nitrate in the diet of ruminants is normally broken down by microbial action in the rumen, first to nitrite and then to ammonia.

Ruminants can tolerate fairly high levels of nitrate in their diet if:

• the intake is spread over the whole of the feeding day
• their diet is also high in readily available carbohydrate, which is needed to fuel the microbial activity in the rumen

If these conditions are not met, the nitrate can accumulate, resulting in poisoning.

Signs of nitrate poisoning

Nitrates have a direct caustic action on the lining of the gut. The main signs are:

• diarrhoea
• salivation
• abdominal pain

How nitrate can cause nitrite poisoning

If an animal's nitrate intake is too high, or if conditions are not right for the conversion of nitrite to ammonia in the rumen, nitrite will accumulate and may be absorbed into the blood.

Nitrite may also be absorbed direct from fodder such as hay if it becomes wet or mouldy. Microbes on the fodder convert nitrate to nitrite under these conditions.

Nitrite reduces the ability of the blood to transport oxygen throughout the body of the animal. If the nitrite level is high enough, death can occur through oxygen starvation.

Non-ruminants, such as horses and pigs, have no mechanism for converting nitrate to nitrite in their digestive tracts and so are not in danger of getting nitrite poisoning from an excessive intake of nitrate. However, they are highly susceptible to oral intake of nitrite (for example, in mouldy hay) because they cannot convert the nitrite to ammonia.

Signs of nitrite poisoning

Depending on the severity, the signs of nitrite poisoning might include:

• difficulty breathing, with gasping, rapid breaths
• weakness
• trembling
• staggering
• collapsing
• convulsions
• death

In some nitrite poisonings of cattle, the main problem is abortion a few days after exposure.

At post mortem, you might notice that the blood has a dark brown appearance. Though, the blood returns to its normal colour a few hours after death.

Pin-point haemorrhages may be present in the heart and trachea along with general congestion of the blood vessels.

Diagnosing nitrite poisoning

The best way to diagnose nitrite poisoning is through the animal. As well as looking for the signs described earlier, blood and rumen contents can be analysed for nitrite levels.

Specimens must be collected within 1 to 2 hours of death to be of any value. Keep the samples cool and send them to the laboratory as soon as possible.

Analysing plant material from the pasture on which animals have been grazing can also be a useful guide to diagnosis, but shouldn't be the only method. The accuracy of the testing depends on the thoroughness of sampling, as plants growing near each other can have large variations in their nitrate content. There can also be different concentrations of nitrate in different parts of the same plant.

Plants containing more than 1.5% nitrate (expressed as potassium nitrate) on a dry weight basis are considered to be potentially dangerous.

Preventing nitrate and nitrite poisoning

Animals can usually cope with grazing pastures that are high in nitrate as long as you control their intake. This means not grazing hungry stock on forages that are potentially dangerous, such as lush capeweed, or fodder crops such as oats or millet.

Holding paddocks around shearing sheds often contain lush capeweed because of the high fertility of the soil and infrequent heavy grazing. In these areas where grazing management is fairly intense (or on dairy farms) animals should be initially given access to safe pastures. When their appetite has been reduced they can, if it's necessary, be moved to high-risk pastures later in the day. They are then less likely to gorge themselves on the high-nitrate fodder.

You could also feed hungry stock hay that you know is safe. This will reduce their appetite before they are given access to high-risk fodder.

Conserved forages such as hay and silage can be high in nitrate. Testing feed will identify the level of risk and help guide feeding strategies.

Get veterinary advice if you suspect poisoning

Nitrite poisoning requires urgent veterinary attention.

If you suspect either nitrate or nitrite poisoning:

1. seek immediate veterinary advice
2. remove the animals from the suspect paddock and place them onto feed containing less toxic herbage
3. feed them hay or some other low-nitrate herbage so as to dilute the nitrate or nitrite in the stomach

If capeweed is the dominant species on your property (this often occurs after drought years), consider methods of eliminating it or reducing it, at least from some paddocks.

Be careful of feeding stock on fodder crops, particularly if the crop has been fertilised with nitrogen.