Home > QLD and north NSW farmers urged to test summer forage for toxicity

QLD and north NSW farmers urged to test summer forage for toxicity

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article image Forage business manager Maree Crawford with Wyreema grower Warren Folker, who bales forage sorghum at his property 'Kilowen'

Farmers in Queensland and northern New South Wales are being urged to test any stressed summer forage for toxic substances such as nitrates and prussic acid, which can kill livestock.

According to Pacific Seeds forage business manager Maree Crawford, these poisonings occur when cattle eat forages stressed from severe environmental conditions such as the current drought. She explained that the stress disrupts normal plant growth and may cause the plants to accumulate high levels of toxins in the form of nitrate or prussic acid; knowing the causes, symptoms and treatments for these diseases can help producers prevent losses.

She said plants need nitrogen for growth and development; however, nitrate levels may rise when drought prevents them from converting the nitrogen they absorb into new growth. Nitrates do not accumulate when there is normal rainfall or irrigation. Under those conditions, nitrogen is absorbed by roots and moved into the plant and rapidly transformed into plant proteins. 

According to Ms Crawford, plants in the sorghum family, including both forage and grain sorghum and wild species including Johnsons Grass and Columbia Grass, can accumulate high levels of nitrates. 

The major difference between the two poisons is that prussic acid generally dissipates from plants if material is properly processed as hay, whereas nitrate levels remain constant in hay. However, in hay baled prematurely at high moisture the prussic acid may not have had a chance to dissipate. 

She advises farmers to have the hay tested before feeding if they suspect that it is high in nitrate. If high in nitrate, hay should be given to cattle with an energy supplement or in combination with low protein forages, or other hay low in nitrates.

Ms Crawford said nitrate poisoning acts on the oxygen-carrying capacity of blood in animals. Nitrite is absorbed into the bloodstream directly through the rumen wall and converts hemoglobin in the blood to methemoglobin, which cannot carry oxygen. An animal dying from nitrate (nitrite) poisoning actually dies from asphyxiation, or a lack of oxygen.

Most vets advise that, in general, all ruminants can safely eat forages that contain up to 1 per cent nitrates on a dry weight basis. Monogastrics (horses and pigs) are less sensitive to nitrate intoxication. Nitrate poisoning usually does not occur rapidly, but over time, depending on how high the nitrate level is in the forage.

Acute nitrate toxicity symptoms generally include death, blue mucous membranes (lack of oxygen), fast breathing, high pulse rate, weakness, uneasiness, excessive salivation, frequent urination and dilated and bloodshot eyes. 

She said animals treated with methylene blue may recover, but by the time an animal ‘goes down’, it is often too late to treat. 

Ms Crawford said producers can take steps to help prevent nitrate poisoning, and should refer to their local vets or DPI for information.

Discussing the other threat, prussic acid poisoning, Ms Crawford said it is one of the most toxic and rapidly acting poisons. Cyogenic compounds can develop in plants that are stressed and in the rumen the compounds are converted to cyanide, which can kill livestock.

Prussic acid can accumulate and fluctuate in the plant; it may be present for a short time and then dissipate. Severe drought stress can also cause formation of prussic acid. High concentrations of prussic acid may be associated with rapid cell division or rapid growth, such as shortly after a rain or irrigation on previously drought-stressed paddocks. A reading in excess of 600ppm is considered unsafe and 1000 ppm is lethal.

Hungry livestock are at high risk and can show symptoms within five minutes of eating plants with a high level of HCN (hydrogen cyanide), and may die within 15 minutes. Indications include salivation and laboured breathing, followed by muscular tremors, uncoordinated movements, bloating, convulsions and death from respiratory failure.

To prevent prussic acid poisoning, Ms Crawford suggests talking with a local consultant for the best advice.

When it comes to testing forages, the plants can be tested for both nitrate and prussic acid as standing forage or stubble, and as hay.

Initial testing can be carried out by the farmer using a diphenylamine test for nitrates and the picrate paper test for prussic acid presence; however, to obtain definitive results they should send plant samples to a registered animal feed testing laboratory such as SGS or local DPI.

Pacific Seeds is committed to bringing the best plant genetics and seed technology to Australia’s farming community.


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