WESTERN District sheep producers have seen an outbreak of phalaris sudden death since the rain started to fall earlier this month and mobs have been moved onto rapidly growing pasture.
Livestock Logic veterinary consultant Andrew Whale, Hamilton, said phalaris sudden death was always more prominent after a prolonged dry period and this year was proving worse for the illness than most.
"The first six weeks post autumn break after a dry season are the most critical, so the next three weeks is crucial – come July the risk is significantly diminished," he said.
Mr Whale said he had diagnosed phalaris toxicity sudden deaths on about six properties, with another dozen having reported deaths.
"When sheep go out onto a phalaris paddock, the deaths will occur within 48 hours and if there are no others looking crook, that is enough evidence to be confident phalaris sudden death is the cause," he said.
Phalaris sudden death occurs in animals introduced onto new phalaris growth, with the toxins in the grass resulting in sheep presenting high levels of ammonia in their system.
Little more is known about the illness, and producers and animal health specialists have flagged a research gap in the area.
Mr Whale said some producers may choose not to stock phalaris paddocks until July, which would be an effective decision from an animal health perspective, but the lack of feed across the region means many would still have to use whatever pasture they had.
"Taking steps to limit the intake of phalaris in the first 24-48 hours greatly reduces the risk of deaths," he said.
"Filling up animals by feeding them prior to introducing them to the new pasture will limit deaths, but it is probably impossible to eliminate deaths altogether if you have phalaris paddocks."
Set stocking of high risk paddocks at the autumn break and not introducing sheep to the pasture in the six weeks after the break can also be an effective method of managing the risk, he said.
Phalaris sudden death has an average mortality rate of three per cent, varying from one to 10pc, and Mr Whale said they had seen as high as 90pc. However figures had not been as high as that in these instances.
"From a pasture production point of view it is our most productive perennial in this climate and part of a lot of people's pastures – to go away from that solely may not be the right thing," he said.