Cyanide (prussic acid) and nitrates in sorghum
crops
Risk management
John D. Bertram, Roger J. Sneath, Kay M. Taylor, Queensland Beef Industry Institute,
Bill D. Mills, Farming Systems Institute,
Ross A. McKenzie, Jim D. Taylor, DPI&F's Biosecurity Unit, and
Keith G Reichmann, Intensive Livestock Institute
Introduction
Both Cyanide (Prussic acid) and Nitrates can be present in various amounts in many grain and forage sorghum crops suffering under hot, dry conditions. Cyanide poisoning is the more likely risk, but nitrate-nitrite poisoning is also a possibility when grazing sorghum under drought. Hay made from sorghums containing toxic amounts of cyanide, nitrate or both will remain dangerously toxic. Ensiling will significantly reduce or eliminate these risks.
However, cyanide poisoning may be an overrated problem and the fear of it, through preventing the use of valuable crops, may cause more loss than actual poisoning would. The number of deaths due to poisoning is very small compared to the number of animals grazing sorghums. Nevertheless, it is always a threat , particularly under drought conditions. By observing a few simple rules, you can prevent serious losses.
The golden rules for using sorghum crops for fodder are:
- Avoid grazing stressed (droughted) plants and in particular, avoid regrowth such as crops ratooning as a result of storm rain.
- Delay grazing until plants are over 45 cm high for shorter varieties or over 75 cm high for tall varieties. This greatly reduces the risk of cyanide poisoning. Plants forming flowers or grain are least likely to poison stock.
- Don't graze hungry stock on sorghum crops, particularly if the crop is wilted or stressed. Fill them up with hay/roughage first.Hungry animals are the most likely to be poisoned because they eat large amounts in a short time.
- Watch your stock continuously for the first hour and then frequently for the first few days. Make a point of checking on them at least twice a day.
- Keep a supply of sodium thiosulphate on hand for emergency treatment of cyanide poisoning.
- Supplement stock on sorghum crops with sulphur (10% sulphur in a salt lick). Sulphur is required for detoxifying cyanide in the rumen and liver and sorghums are generally low in sulphur.
- For fodder conservation, don't make hay from sorghums that are considered unsafe to graze. Ensile them. Get hay or silage samples tested for cyanide potential and nitrate before feeding out.
Cyanide
Prussic acid (cyanide) is also known as hydrocyanic acid. It kills by preventing the use of oxygen by the animal's tissues.
The poison can be present in dangerous amounts at any growth stage of the crop, with the least risk of cyanide in flowering/seeding plants. However, experience has shown that the risk is highest when growth is checked by drought, frost, trampling or other damage (for example, hail or insect damage including grasshoppers and caterpillars). The cyanide risk is also higher with young plants or regrowth. Hence, stressed regrowth is considered the most dangerous situation. Taking particular care at this stage is important to preventing poisoning.
All sorghums have cyanide-producing potential, but there is variation. Sorghum almum and Johnson grass (Sorghum halepense) are considered to be the most dangerous, and silk sorghum is generally one of the safest. However every sorghum can cause poisoning. Some common sorghum varieties have cyanide potential between 400 and 900 ppm (parts per million) on a dry weight basis. Because there are several important factors leading to cyanide poisoning of cattle and sheep, it is difficult to state a realistic threshold for safe grazing that would apply to all circumstances. A cyanide potential above 600 ppm can be considered hazardous grazing, but potentials down to about 200 ppm could be dangerous for very hungry animals in drought.
Grain sorghums and the sweet sorghum and delayed-flowering varieties tend to have a much higher cyanide potential than other varieties. If you are worried about cyanide, millets can be fed instead of forage sorghums, because they pose no danger of cyanide poisoning.
Nitrate
Nitrate can accumulate in many weed, crop and pasture plants. Forage sorghum, grain sorghum, sudan grass, sudan grass hybrids and pearl millet are well-recognised nitrate accumulators. Plants accumulate nitrate when soil nitrate is high but conditions are not suitable for normal forage growth which would allow the nitrate to be converted to protein. This can occur during droughts, cloudy weather, cold weather, when plants are wilted, after herbicide application or with combinations of these factors. Nitrate will persist in hay. Nitrate is converted to nitrite in the rumen. The nitrite is absorbed and prevents the blood from carrying oxygen, turning it brown (the colour of methaemoglobin).
Plants containing more than 1.5% potassium nitrate (KNO3) on a dry weight basis are potentially dangerous for hungry stock. The nitrate content on a dry weight basis of sorghum crops in Queensland has been measured at up to 6.4% and that of sorghum hay in Queensland can reach 12%. Animals can become acclimatised to large concentrations if introduced to them gradually, but any sudden increase in feed intake or the feeding of supplements containing monensin can lead to poisoning because of swamping or modification of the rumen bacteria's capacity to deal with nitrate.
Treatment
Obviously, if you find a sick or dead animal, immediately remove the rest of the stock from the crop and consult your veterinarian. Affected stock often die within minutes of eating a lethal amount of dangerous forage. Most affected animals are found dead. If poisoned animals are seen alive, you may find increased rate and depth of breathing, frothing at the mouth, muscular twitching, staggering, collapse, convulsions and coma before death. However, many poisoned animals that are found alive can be saved.
Cyanide poisoning:
Affected cattle should be treated immediately by drenching with sodium thiosulphate (sometimes known as photographic "hypo". Note that some "hypo" does not contain sodium thiosulphate). Give 60 g in 600 ml of water. Repeat this drench hourly until the animal recovers. 'Normal" animals in an affected herd should also be dosed by drenching. But, the most effective treatment for cyanide poisoning is an intravenous injection of sodium thiosulphate. This is best administered by a veterinarian at a dose rate of 660mg/kg plus oral or intraruminal doses of 30g. Repeated intravenous doses may be required if relapses occur. Sodium thiosulphate in high dose can be effective when given up to 30 minutes after ingestion of a toxic dose of cyanide, but the sooner it is given, the better.
Nitrate-nitrite poisoning:
The best treatment is intravenous methylene blue, best given by a veterinarian at a dose rate of 2 mg/kg as a 2-4% solution in water (20 g in a litre).
Testing plants for toxins
The presence of these toxins in plants can be assessed by commercial agricultural laboratories (or, only as part of disease incident investigations and with prior arrangement, by DPI&F's Animal Disease Surveillance Laboratory, Toowoomba Phone: 4688 1352 or DPI&F's Biosecurity Sciences Laboratory, Yerongpilly Phone: 3362 9513). Samples for cyanide assay need to reach the lab fresh (unwilted and uncrushed) and as rapidly as possible to ensure that the results obtained reflect the field situation. Cyanide is highly volatile and its release from plants through wilting or physical damage to tissues in transit will give a falsely low result. It's a good idea to dig up a sample plant and pack the root ball in wet newspaper during transport to the lab. Nitrate assays are not quite so critical. In both cases, samples should not be submitted in plastic bags - use paper wrapping. Condensed moisture in plastic bags promotes bacterial breakdown of both cyanide and nitrate, producing falsely low results. Hays can also be tested, but do not need the special handling required for fresh plants.
Post mortem examinations
Contact your veterinarian. The blood can be bright red in carcases dead of cyanide poisoning. Chocolate-brown blood is characteristic of nitrate-nitrite poisoning, but it fades rapidly after death. Neither of these signs are reliable indicators, so post mortem examinations and the interpretation of carcase changes are best done by veterinarians who are also able to sample tissues appropriately for laboratory tests to help achieve a diagnosis.
To access DPI&F's information and services
- DPI&F Business Information Centre: Phone 13 25 23 (Queensland residents) between 8 am and 6 pm weekdays; non-Queensland residents phone +61 7 3404 6999; email callweb@dpi.qld.gov.au
- DPI&F's web site
This Note is also published on the DPI&F's PrimeNotes CD-ROM.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 10 June 2005