Cyanide poisoning from sorghum
Animals, especially ruminants, can suffer cyanide poisoning from eating plants that contain high levels of cyanogenic glycosides.
All sorghums can accumulate cyanogenic glycosides and have the potential to poison stock, but some are safer than others.
Inside the animal's rumen the plant cyanogenic glycoside is hydrolysed to release hydrogen cyanide. Cyanide prevents the release of oxygen from the red blood cells, starving the tissues of oxygen.
Risk during the life cycle of the crop
Cyanide-producing potential is present during any stage of the crop's life cycle. The risk is higher when growing plants are stressed by:
- wilting
- crushing
- droughts
- frosts
- trampling
- hail damage
- insect damage (grasshoppers and caterpillars).
Therefore, stressed regrowth is considered the most dangerous—take care at this stage to avoid poisoning.
The lowest risk of cyanide accumulation is when plants are flowering and seeding.
Safe levels of cyanide for cattle and sheep
There is a wide difference in cyanide accumulation in different plant varieties, for example, grain sorghums, and the sweet sorghum and delayed-flowering varieties, tend to have much higher cyanide levels than other varieties. Sorghum almum and Johnson grass (Sorghum halepense) are the most dangerous, while silk sorghum is generally one of the safest.
The cyanide concentration of plants is measured as mg/kg or ppm (parts per million) of hydrogen cyanide on a dry weight basis. Some common sorghum varieties have relatively high cyanide concentrations of between 400–900mg/kg.
Many factors can lead to cyanide poisoning in cattle and sheep. For this reason it's difficult to apply a threshold for safe grazing to all circumstances. However, as a general rule-of-thumb cyanide levels above 600mg/kg are a risk to grazing stock but levels down to 200mg/kg can still be a threat to very hungry animals.
Hay made from hazardous sorghum forage will remain hazardous. Ensiling such material can decrease its cyanide potential, but actual levels in the end product should be checked before feeding it.
Signs of cyanide poisoning in livestock
Signs of cyanide poisoning in animals include:
- rapid laboured breathing
- frothing at the mouth
- mucous membranes that are bright red in colour
- muscle weakness or twitching
- staggering
- convulsing
- unconsciousness
- death.
Animals eating large amounts of toxic feed will die within 5–15 minutes of developing clinical signs of poisoning. Affected animals rarely survive more than 1–2 hours after consuming lethal quantities.
Diagnosis and treatment of cyanide poisoning
If you find a sick or dead animal, promptly remove all stock from the sorghum crop and seek advice from your veterinarian. A post-mortem examination will help identify the exact cause of death in your animal. Your veterinarian will collect the appropriate samples and submit these for laboratory testing.