NATURALLY occurring cyanide in stressed grass is believed to have caused the deaths of 22 horses on a Queensland property.
Agricultural scientist Kerry Marsh analysed the results of hair and tissue samples taken from two of the three surviving horses last October at Kooralbyn.
The results showed there were exceptionally high levels of manganese which is common in pasture grasses such as Johnson grass which grows on the property.
Ms Marsh then examined the symptoms of the dead horses and the Biosecurity Queensland environmental studies at the time, which ruled out contaminated water in a nearby stream.
She came to the conclusion the horses ate Johnson grass, which can be highly toxic and fatal to animals at certain stages of growth.
"Johnson grass needs to have gone through a stress period of drought or frost and when stressed the grass releases cyanide to cope," Ms Marsh, who is based in Melbourne and has been practising for 15 years, told AAP.
"Those horses had all the symptoms of cyanide poisoning."
Biosecurity Queensland last November concluded the most likely scenarios were either a scrub tick infestation or botulism.
Ms Marsh said ticks paralysis on horses were rarely fatal.
But their presence may have lowered the immunity levels of the Quarter and Thoroughbred horses and made them susceptible to cyanide poisoning.
"In defence of Biosecurity (Queensland), it is hard to pick Johnson grass when it is low, but that's when it is toxic," Ms Marsh said.
"I think it's a fairly cut and dried case of Johnson grass poisoning."
A Biosecurity Queensland spokesperson was not available.