Pasture warning on photosensitisation

The cause of secondary photosensitisation has been investigated by the Department of Agriculture and Food and ChemCentre.

Secondary photosensitisation occasionally occurs in stock grazing sub-tropical, grass-based pastures.

Department research officer Geoff Moore said the investigation aimed to determine the prevalence of sub-tropical grasses containing concentrations of saponins which could cause liver damage in livestock, resulting in photosensitisation.

"The investigation focused on signal grass and panic grass which are reported to contain steroidal saponins," Mr Moore said.

"Rhodes grass does not contain saponins."

Mr Moore said saponins damaged the liver, reducing its ability to metabolise and safely excrete certain by-products of chlorophyll in green feed.

"These then accumulate in the body and make the skin extremely sensitive to sunlight," he said.

"Affected animals develop signs similar to a bad case of sunburn, called photosensitisation.

"If intake of saponins is not reduced promptly, the liver damage can result in the animal rapidly losing condition or, in severe cases, death."

Mr Moore said the results of regular sampling from six commercial paddocks between Gingin and Binnu indicated signal grass had high to very high concentrations of saponins year-round, at seven to 30 times higher than the critical value.

"This finding shows there is always a potential health risk to livestock grazing pastures containing signal grass, so we recommend not sowing signal grass alone," he said.

Department principal veterinary officer Jeremy Allen said producers needed to be vigilant when grazing livestock on pastures containing significant amounts of both signal and panic grasses.

"The risk of secondary photosensitisation is heightened when these perennial grasses represent all or most of the palatable green feed on offer," Dr Allen said.

"Just three of 100 panic grass samples collected from the field in WA had saponin concentrations above the critical threshold, and these results were all lower than for signal grass.

"The risk of photosensitisation from grazing paddocks containing panic grass, but not signal grass, is minimal, but producers are advised to monitor stock regularly."