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 more importantly they are a source of nitrite.

Nitrites cause respiratory distress due to interference with oxygenation of blood and 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, 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, especially in the week following rain. 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
• spraying plants with hormone-type herbicides (such as 2,4-D)

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.

Another potential source to consider is water, if soil water from soil containing high nitrogen levels is accessed by livestock

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 carbohydrates (e.g. good quality pasture, grain), which is needed to fuel the microbial activity in the rumen

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

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

• diarrhoea
• salivation
• abdominal pain

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

• difficulty breathing, with gasping, rapid breaths
• weakness
• trembling
• staggering
• blue/chocolate coloured mucous membranes
• collapsing
• convulsions
• death

In some nitrite poisonings of cattle, if the animal survives the initial exposure, abortion can occur 10-14daysafter exposure.

At post mortem, you might notice that the blood has a dark brown appearance however, 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 nitrate and nitrite poisoning

Diagnosis is based on:

• observing the clinical signs
• establishing exposure to a possible toxic source
• post-mortem findings:
  • stripping of the stomach and intestinal linings (nitrate poisoning)
  • dark brown blood and pin-point haemorrhages (nitrite poisoning)

potentially blood and rumen contents can be analysed for nitrite levels if collected shortly after death Testing 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 their intake is controlled. This means not grazing hungry stock on forages that are potentially dangerous. If a high-risk pasture or area is to be grazed, provide access to safe pastures or roughage to reduce their appetite before moving to the high-risk pasture.

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.

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

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  hay or other low-nitrate herbage to dilute the nitrate or nitrite in the stomach.