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 are exacerbated by drought and heat and can kill livestock.
Pacific Seeds forage business manager, Maree Crawford, said although uncommon in normal years, these poisonings occur when cattle eat forages stressed from severe environmental conditions such as the current drought being experienced.
"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, but knowing the causes, symptoms and treatments for these diseases can help producers prevent losses," Ms Crawford said.
She said plants need nitrogen for growth and development, however, when drought prevents them from converting the nitrogen they absorb into new growth, nitrate levels may rise.
"Nitrates do not accumulate when there is normal rainfall or irrigation. Under those conditions, nitrate nitrogen absorbed by roots and moved into the plant is rapidly transformed into plant proteins," she said.
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.
"Although the growing conditions causing them are similar, the diseases differ greatly, and sampling and testing can indicate when forages pose a danger to livestock," she said.
"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.
"Have hay tested before feeding if you suspect that it is high in nitrate. If hay is high in nitrate, feed carefully with an energy supplement or in combination with low protein forages, or other hay low in nitrates.
"Ensile forages high in nitrate. When forage is properly fermented, nitrate levels can be reduced significantly."
Ms Crawford said nitrate poisoning acts on the oxygen-carrying capacity of blood in animals.
"When forages have a high concentration of nitrate, the animal cannot complete the conversion and nitrite accumulates. Nitrite is absorbed into the bloodstream directly through the rumen wall and converts hemoglobin (the oxygen carrying molecule) 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," Ms Crawford said.
"Most vets advise that, in general, all ruminants can safely eat forages that contain up to 1 percent 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 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," she said.
"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 prussic acid to form. 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.
She said 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.
Salivation and labored breathing occur first, 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 either 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.