Two to three hundred dairy cows may have died after grazing on swede crops in Southland.
Many others have become ill and ironically, Southland's unusually mild winter is partly to blame.
An estimated 30 to 50 farms are affected, mainly in central and lowland parts of the province.
Swedes are a common winter feed crop in the south, but the mild winter has caused them to produce a lot more leaf than usual, which is the root of the problem.
Vets are advising Southland farmers who have been feeding their cows on swedes during the late winter to report any signs of illness in their stock.
David Green of PGG Wrightson Seeds, which supplies most of the seed for swede crops, said the symptoms were consistent with cows ingesting higher-than-normal levels of glucosinolates.
"The cows were exhibiting a range of issues that varied from moderate levels of photo-sensitivity right through to death in extreme examples.
"Glucosinolates are present in all brassica species and crops and sometimes, given environmental conditions, these can get to a level where they can become harmful to grazing animals.
"What we know is a lot of these compounds actually accumulate at much higher rates in the leaves than they do in the bulbs.
"So, when you're faced with a situation of mild conditions which accentuate leaf growth, then we are getting a greater accumulation of these compounds in the leaf."
Dairy cows die of lead poisoning
In July this year a veterinary clinic in Otautau, north of Invercargill, notified the Ministry for Primary Industries that dairy cattle on a farm nearby were being poisoned.
The ministry said the animals ingested high levels of lead while grazing on fodder beet grown on a gun club property. It said another 80 cows had to be put down and crop at the Nightcaps Clay Target Club has been destroyed.
The farmer who owned the cows has been leasing the grazing paddock from the club for the past 10 years. He has since been given a surveillance notice by the ministry so none of his cattle can be traded, slaughtered or moved out of his control until further notice.
The president of the Nightcaps Clay Target Club said fodder beet is more likely to pick up traces of lead, but the farmer chose to grow it for the first time this year.
Steve Diack said the farmer usually grows swedes and has never had any problems. He said the farmer was well aware the field is used the 8 hectare paddock as a shooting range.