Nitrate poisoning

Nitrate accumulation in plants is a potential danger to grazing animals.

It can cause two different disorders - nitrate poisoning and 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; death may follow.

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

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

Normally plants contain relatively small amounts of nitrate as such, because the conversions take place fairly rapidly inside the leaf.

However, during periods of drought the amount of nitrate in the soil can increase greatly because of lack of leaching, reduced uptake by plants, and decomposition of organic matter.

After the drought breaks nitrate uptake by plants may be high.

While high concentrations of nitrate are not 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), and 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.

Many of the major crop plants have been implicated, including maize, rape, soybean, linseed, sorghum, millet, wheat, oats and barley.

Lucerne, subterranean clover, and Tama ryegrass have also been reported to be capable of accumulating large amounts of nitrate in their leaves and stems.

Nitrates have a direct caustic action on the lining of the gut.

The main signs are diarrhoea, salivation and abdominal pain.

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.

Difficulty in breathing, with gasping, rapid breaths is the most obvious sign of nitrite poisoning.

Affected animals are weak and will tremble and stagger.

Severely affected animals will go down, convulse and die.

The best policy is to prevent poisoning.

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 nitrate or nitrite poisoning is suspected, animals should be removed from the suspect paddock and placed onto feed containing less toxic herbage.

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