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Nitrate poisoning in sorghum

Nitrate can accumulate in many weed, crop and pasture plants. Plants accumulate nitrate when soil nitrate is high but conditions are not favourable for normal plant growth, which would normally allow the nitrate to be converted to plant protein.

Nitrate in plants is converted to nitrite by bacteria present in the animal's rumen. The excess nitrate and nitrite is absorbed through the rumen wall into the bloodstream. Nitrite changes the normal red haemoglobin in blood cells to brown methaemoglobin that cannot carry oxygen. If more than 75% of haemoglobin is converted this prevents the blood from carrying enough oxygen, starving the tissues of oxygen, causing the animal to die.

Plants that accumulate nitrate

Nitrate can accumulate in many weed, crop and pasture plants. Forage sorghum, grain sorghum, sudan grass, sudan grass hybrids and pearl millet are well recognised nitrate accumulators.

Factors that influence nitrate build-up

Nitrate build-up in plants depends on many factors but generally occurs where soil nitrate is high and the plants are not able to convert the nitrate to protein (reduced photosynthesis).

High soil nitrate concentration can be due to:

  • nitrogen fertiliser application
  • high manure deposition
  • fallowing
  • drought

Nitrate build-up in plants can occur during:

  • drought and rain post drought
  • cloudy weather
  • cold weather
  • wilting
  • herbicide application
  • insect or fungal damage

Most nitrate in plants is concentrated in stems and stalks. Toxic amounts of nitrate will persist in hay.

Safe levels of nitrate for cattle and sheep

The nitrate concentration of plants is generally measured as per cent (%w/w) nitrate, expressed as potassium nitrate (KNO3) equivalent on a dry weight basis. The nitrate content of sorghum crops in Queensland has been recorded as high as 6.4% KNO3. Sorghum hay in Queensland has reached levels of 12% KNO3.

Plants are regarded as hazardous to ruminants if they contain 1.5% or greater KNO3. Animals may become acclimatised to concentrations up to 3%, but any sudden increase in feed intake or the addition of supplements containing monensin can lead to poisoning.

Signs of nitrate poisoning

Signs of nitrate poisoning in livestock are very similar to that of cyanide poisoning and include:

  • increased heart rate
  • rapid laboured breathing
  • muscle weakness or tremors
  • blue-grey or brown* mucous membranes,
  • frothing at the mouth
  • abortion
  • staggering gait
  • unconsciousness
  • death.

*When nitrite oxidises haemoglobin the blood colour changes to chocolate-brown (the colour of methaemoglobin). This discoloration fades with time and is generally not observed in an animal that has been dead for several hours.

Diagnosis and treatment of nitrate poisoning in animals

If you find a sick or dead animal, promptly remove all stock from the suspected feed source and seek advice from your veterinarian.

A post-mortem examination will help identify the exact cause of death in your animal. Your veterinarian will collect the appropriate samples and submit these for laboratory testing.

Adapting animals to higher nitrate levels

Animals can adapt to higher (but not very toxic) nitrate levels, if introduced slowly. This allows time for bacterial populations in their rumen to change. The extra bacteria help the animal break down the nitrite.

Monensin supplementation can increase deaths from nitrate-nitrite poisoning because it breaks down nitrate, contributing to the build of toxic nitrite.