Nitrate Poisoning

Nitrate poisoning is caused by high nitrate levels in feed and it usually occurs in late autumn or winter, particularly during a flush of growth after a dry period.

• Nitrate levels build up in herbage when nitrate is taken up by the plant faster than it can be converted into protein.
• Nitrate converts to nitrite in the rumen, and then binds to haemoglobin in the blood stopping the haemoglobin from carrying oxygen
• Death can occur rapidly due to lack of oxygen.
• Nitrate levels in forages can be tested and critical levels are shown in Table 1. Units vary between laboratories and can be expressed as a percentage or ppm of either ‘nitrate’ or ‘nitrate-nitrogen’ as a proportion of forage dry matter.
• Toxicity risk progressively increases where the nitrate-N level is greater than 0.22% or 2200 mg/kg (or ppm), or 1% nitrate.

Table 1: Critical nitrate levels in forages for cattle.

Risk factors

• Animals are under some sort of physiological stress – for example, if they are sick, hungry, pregnant, or lactating.
• Hungry animals eating high rates of nitrate quickly.
• Nitrate accumulation is higher in young plant growth (physiologically immature plants).
• Stressed plants have higher nitrate levels.
• Conditions that reduce the rate of photosynthesis e.g. lack of sunshine (cloudy) or cold conditions (below 12ºC).
• Using high levels of nitrogen fertiliser late in the season can predispose plants to nitrate accumulation, particularly if grazing soon after application and before full dry matter response. Be aware that you can get high nitrate levels without using N fertiliser.

Reducing risk

• Forage samples can be tested via your vet or soil test lab (results back next day).
• Avoid putting hungry stock onto risk feed; give them some safe feed such as hay or silage first.
• Check the animals 1-2 hours after you put them on a new break.
• Check after each new break not just the first break of a paddock. Cases of poisoning have been reported on the 3rd or 4th break of a paddock after no sign of trouble on the earlier breaks.
• Acute nitrate poisoning symptoms will show within an hour or two of eating nitrates. Monitor stock and call a veterinarian at the first signs of trouble.
• Feed risk crop late in afternoon as sunshine will reduce nitrate levels.
• Wilting high nitrate pasture before grazing will not reduce nitrate levels. Making silage will not drop nitrate levels, but may be a way to manage high nitrate pasture that needs to be grazed.
• Don’t let animals graze kale, rape or ryegrass too hard: the plant parts closest to the soil (stem) contain the highest concentration of nitrate.

Symptoms

• Staggering (like they are drunk) due to lack of oxygen to the brain.
• Muscle tremors.
• Rapid fast breathing (basically a reflex to try and get more oxygen).
• Cows may salivate and froth at the mouth.
• Bluish/chocolate brown colour of the mucous membranes.
• Recumbent animals, can be large numbers, death can occur quickly.
• Eventually death through suffocation.
• A cow can consume a toxic amount of nitrate in one hour, and will start to show signs very soon after.

What to do if you see symptoms

• Call the vet immediately and outline how many animals are affected. The vet will get extra help if the outbreak is severe.
• Remove animals that can walk out of the paddock.
• When the vet arrives, they will start treating cows that are down, and move on to those less affected.
• The treatment is to inject methylene blue intravenously (into a vein).

What causes high nitrate levels?

Plants take up nitrate from the soil and then use energy from photosynthesis to convert nitrate to protein for growth. However, if nitrate uptake from the soil is greater than the conversion to protein, then nitrate can accumulate to abnormal levels. This can be caused by several factors:

• deficiency in plant energy due to:
  • low sunlight levels;
  • wilting or stunting from low moisture conditions; or
  • plant stress and loss of leaf area due to insect damage or frost
• low temperatures - most plants require moderate temperatures to grow. Nitrates can be absorbed quickly by plants when temperatures are low, but conversion to protein occurs very slowly during cold weather, thus nitrates accumulate
• rapid uptake of nitrate from the soil with the first rains following drought conditions, as the plant begins to grow again
• new grasses or crops – nitrate levels are reduced as the plant matures.

Soils high in nitrogen readily supply nitrate to plants, this can occur without the addition of nitrogen fertilisers.  Soils that have high acidity (lack of lime fertiliser), high N from white clover, sulfur or phosphorus deficiencies (lack of fertiliser); and low molybdenum also increase nitrate uptake by plants.

N fertiliser is only a very small piece of the puzzle and the other factors mentioned above (e.g. natural soil state, plant type and maturity, sunlight, rainfall, temperature, pests) play a much bigger role in the risk of nitrate poisoning. Nitrate poisoning was occurring on farms 60-70 years ago, when little N was used.

Spikes in nitrate levels can occur when seasonal conditions combine in the autumn and winter, particularly following summers and autumns that are drier than normal (see this graph of DairyNZ’s Waikato farmlets from winter of 2008 and the spike that occurred even for the Low N Farm, green line).