Panic grasses — commonly known as fairy grass, witch grass and hairy panic — can cause photosensitisation and death when ingested, particularly in lambs.
Panic grasses are one of the common perennial grasses in pastures and crop stubbles in north-east Victoria that would have responded rapidly to the recent rains.
The young, rapidly growing grasses contain steroidal saponins, which when eaten can form crystals in the liver, damaging the liver cells and obstructing the outflow of bile.
The breakdown products of chlorophyll, which is found in green grass, are then no longer cleared by the liver and cause damage to skin tissues when exposed to light.
So, the thin-skinned, wool-free parts of the sheep which are exposed to sunlight get damaged and show signs like severe sunburn.
Typically affected areas are the ears, eyelids, nose, lips and vulva.
Affected lambs will seek shade and be reluctant to graze.
The liver damage may lead to jaundice, and with the swelling caused by photosensitisation the condition is sometimes known as ‘yellow bighead’.
This liver damage can kill the lambs before photosensitisation develops.
The only treatment is to remove affected stock from the toxic pastures and provide them access to shade.
If this happens promptly, affected stock can recover completely, as the liver has a remarkable ability to heal itself.
However, some livers may never recover totally, and this leads to ongoing poor metabolic processing of food consumed.
So, food conversion efficiency will be poor, and the animals may never thrive.
The best prevention is to avoid grazing risky paddocks.
If this is not possible, do not to put hungry sheep straight out onto risky pastures but give them a good feed of hay first, graze older sheep as they have better developed rumens and are more resistant to the effects of the toxin, and check the sheep twice daily until you are confident they have no ill effects.
For further advice, contact your local veterinarian or Agriculture Victoria veterinary or animal health officer, or in NSW, your Local Land Services.
By Agriculture Victoria district veterinary officer Dr Jeff Cave