South Perth, Western Australia
July 29, 2011
The Department of Agriculture and Food WA and the ChemCentre WA are investigating the cause of secondary photosensitisation that sometimes occurs in stock grazing sub-tropical grass-based pastures.
Department pasture research officer Geoff Moore said the focus had been on signal grass and panic grass, which were reported in the scientific literature to contain steroidal saponins that are capable of causing this condition.
“Rhodes grass does not contain saponins and is considered safe in this respect,” he said.
“Preliminary results from regular sampling of field trials in WA show that signal grass contains high concentrations of steroidal saponins and appears to be the main contributor to the saponin content in perennial grass-based pastures. It seems likely that it is the pasture species responsible for the sporadic cases of photosensitisation that may occur.”
The saponins damage 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. Affected animals develop signs similar to a bad case of sunburn, called photosensitisation. If access to the saponins is not removed promptly, the liver damage can result in ill-thrift, or in severe cases death.
Department principal veterinary officer Dr Jeremy Allen said it was important to note that the majority of pastures containing a mix of perennial grasses, including signal grass, have not caused any issues in stock that have grazed them.
“It seems that a particular interaction of weather, pasture and grazing factors is required before the potential for photosensitisation occurs,” he said. “This occurs infrequently and only for short periods of time, explaining why these pastures are usually very productive.”
A number of commercial sub-tropical grass seed mixes contain signal grass along with panic grass and Rhodes grass. Signal grass has often been included in the mix for non-wetting sands, as its larger seeds and ability to germinate from depth act as an insurance against establishment failure from sowing too deep, or following sand in-fill from strong winds after seeding.
“Producers need to be vigilant when grazing perennial grass-based pastures containing signal grass and panic grass,” Dr Allen said. “The risk of photosensitisation is heightened when the perennial grasses represent all or most of the palatable green feed-on-offer.
“Young animals are most susceptible, so short and hard grazing periods with adult animals may help to avoid this problem. Also graze before the Rhodes grass is tall and rank. Early detection of the signs of photosensitisation and removal of the stock from the pasture will significantly minimise any reduced production that may be caused by this condition.”
Mr Moore said further details would be released when investigations were completed.
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