Four herds of dairy cows in Otago and Southland have been affected by ergot toxicity, resulting in hundreds of the animals being euthanised.
VetSouth Winton veterinarian Hayden Dore knew of over 100 cows which had been put down in cases where two herds were affected.
He was unsure of the numbers in the other two cases.
A herd of 900 cows in Southland had at least 120 cows infected by the naturally occurring fungus.
About 60 of the infected had to be put down.
In another case, about 40 were put down after 56 were infected at another farm.
Dr Dore said it was an unusual event and the ergot fungus only grew into a seed in the right environmental conditions.
''The environmental conditions that allow that fungus to grow is a dry summer and a mild, wet autumn.
''It's in that autumn seed that's the problem,'' he said.
Ergot typically grows on ryegrass and other similar plants cows can feed on.
The fungus produces an alkaloid which can cut off blood supplies to the animal's extremities which can result in gangrene.
''You can have a reasonably large number affected and then not all of them will progress to that point but as soon as the limb goes cold then they're quite lame.''
Once the limb went gangrenous the only option was to euthanise the animal.
There was very little that could be done for the animals once they were infected other than pain management.
The Ministry of Primary Industries called a meeting with stakeholders to warn to farmers and veterinarians about the fungus.
Ergot could be identified as small black or purple structures in among the seeds on the rye grass.
Dr Dore said it could resemble mouse droppings.
In silage, the same black seeds or spores could be seen, although it was churned within the bale.
Feed with the spores or seeds should not be fed to animals.
Farmers with affected herds and land had a responsibility to prevent access to affected feed, provide treatment to animals and were restricted from transporting affected animals.
Dr Dore said infected feed should be buried 4cm underground to kill the spores and prevent them from spreading.
Suspected cases of ergot should be reported to MPI at ACVM-AdverseEvents@mpi.
govt.nz.