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Nitrate Poisoning Causes Stock Losses In The Mid

One farm in the Macalister Irrigation District (MID) recently had fourteen dairy cows die in a twenty minute period.

By Department of Primary Industries - 24th May 2007 - Back to News

The cause was nitrate poisoning. The source of the nitrate was newly sown and fertilised annual ryegrass. Another farm also had deaths from nitrate poisoning at the same time.

What is nitrate poisoning?

Nitrate poisoning occurs when ruminants (including cattle and sheep) eat crops or fodder with high nitrate levels. Nitrates are converted into nitrites in the rumen and then into ammonia which is belched out or detoxified by the liver. If there is a high level of nitrate intake and the resultant nitrites are not converted to ammonia rapidly enough, the nitrites are absorbed into the bloodstream where they bond with haemoglobin in the blood. The haemoglobin loses its ability to transport oxygen and affected animals die from oxygen starvation.

What are the signs of nitrate poisoning?

Nitrate poisoning occurs very quickly. Symptoms are often not observed and animals are found dead. If symptoms are seen, affected animals stand apart, have an unsteady gait, have rapid and shallow breathing and eventually collapse. The whites of the eyes and membranes in the vagina, mouth and nostrils will appear initially bluish in colour and then muddy brown. Following exposure to abnormal nitrates or nitrites, a cow that survives may abort her calf that died because of oxygen starvation.

What is the treatment?

Death usually occurs suddenly and treatment is not possible. If cattle are observed in time intravenous administration of methylene blue can save cattle.

Why has the drought made the situation potentially more lethal than usual?

Some farmers have utilised the drought and lack of irrigation water allocation to renovate pastures on their farms. Annual ryegrass has been a favourite species to use. Risk factors for nitrate poisoning include:

• Lack of grazing over summer of paddocks that are not irrigated, allowing the nitrogen-fixing bacteria to maximise their effect and raise nitrogen levels in the soil.
• Reseeding of paddocks with annual pastures.
• Adequate rainfall to allow maximum grass growth.
• Not allowing a minimum of 30 days between nitrogen application and grazing in the period of May to August – longer than a spring strategy due to slower grass growth and longer leaf appearance rate.
• Grazing young, rapidly growing plants before the 3-leaf stage that have taken up a lot of nitrogen.
• Allowing hungry cows onto a risky pasture. Make sure that they are relatively filled with hay/straw before being allowed onto a high-risk pasture for a short time period or a small area of pasture.
• Not keeping a close eye on stock that are on a risk pasture.

Plants containing more than 1.5 % nitrate are dangerous. Nitrate concentration in forages increases greatly during drought conditions. Other conditions frequently associated with losses are cool temperatures, cloudy days and heavy or recent applications of nitrate fertilisers. As well as annual ryegrass, some plant species are more likely to accumulate toxic concentrations of nitrate. These include forage crops such as millet, corn, rape, sudan, oats, weeds such as cape weed and some pasture species.

Management practices can reduce and prevent losses from nitrate poisoning. Awareness of high-risk pastures or crops in these drought conditions is important. High risk pastures or crops can be measured for nitrate levels via laboratory analysis or quick paddock side nitrate test strips. These strips can give an indication of nitrate levels in plants but your pasture sample must accurately reflect what the cows are eating. Cattle grazing a high risk crop should be observed carefully and quietly removed at the first sign of toxicity.

For further information contact your veterinarian or Department of Primary Industries office.

Source: http://gippsland.com/

Published by: news@gippsland.com

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