Nitrate poisoning is a rare but important cause of poisoning in cattle. It can occur as the result of eating crops such as Brassicas, green cereals or sweet clover that contain high levels of nitrate. However, the most common source is inorganic nitrate fertiliser, either directly (straight from an open bag), via grazing an over-fertilised field, or via water run-off from heavily fertilised fields. Spring is the most common season for nitrate poisoning.

Nitrate poisoning occurs because the nitrate is broken down to nitrite in the rumen. In normal circumstances this nitrite is further broken down to ammonia in the rumen and is then used by the rumen microbes to make protein. However, when large amounts of nitrate are eaten over a short period of time, the nitrite accumulates in the rumen and is absorbed. Once in the bloodstream it reacts with iron in the red blood cells so that they can no longer bind oxygen.

Clinical Signs

The signs are usually seen within a few hours of eating the nitrate. The higher the dose the faster the signs develop.

• Abdominal pain
• Scour
• Weakness
• Muscle tremors
• Drooling of saliva
• Blue discolouration of the mouth
• Mouth breathing
• Collapse
• Coma
• Death

Diagnosis

The clinical signs are vague, particularly early on. Early veterinary involvement is therefore important. If your cattle have had access to nitrate and start showing signs of illness, get them checked as soon as possible.

Nitrate poisoning can be identified using a blood test for either nitrate or methaemoglobin (the product formed when nitrite reacts with the red blood cells). In severe cases and at PM the methaemoglobin is visible as high levels in the blood result in the blood becoming chocolate in colour

Treatment

Remove the source of nitrate. Veterinary treatment with methylene blue can be very effective at reversing the changes in the blood, particularly in the early stages.

Prevention

Prevent access to nitrate sources. In particular, prevent cattle getting into fertiliser stores. Ensiling high nitrate pastures will usually reduce the nitrate levels to safe levels (as will allowing the pasture to set seed). Feeding a high grain diet alongside high nitrate forage has a protective effect because carbohydrates enhance the conversion process from nitrate to microbial protein

The animal can become conditioned to eat larger amounts of feed with a high nitrate content if the increase is gradual. Healthy animals are less likely to be adversely affected than animals in poor health. Adequate amounts of available carbohydrates (grain) allow the animal to consume more nitrate.

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