Vets believe a new variety of swede being fed to Southland dairy cows is behind the deaths of more than 200 animals in the last two months.
Southern vets first started seeing liver-damaged cows dying in mid-July and identified the swede variety HT Swede as a likely culprit shortly afterwards.
The managing director of VetSouth Mark Bryan said it had completed ''a number'' of postmortems across about 40 case study farms that had significant problems this winter in the area surrounding his Winton base.
''We know that swedes, especially at this time of year, can be quite toxic,'' he said.
''We've never seen reactions like we have this year. They're almost all associated with this new variety.
"It would appear this new variety is the issue, I should think.''
More than 200 cows are believed to have died this year and others are suffering from liver damage and sensitive and peeling skin.
DairyNZ's regional leader for Southland and Otago, Richard Kyte, said it had been aware of the issue for two weeks and it was too early to determine the exact cause of the problem.
''We don't know - and that's Dairy NZ's role, to find the science.''
He said time was running out to investigate the plants believed to be causing the problems this year, as there was very little of the swede variety left.
Dr Bryan said it was believed the mild winter in Southland caused the plants to retain high levels of glucosinolate, which was beneficial in small quantities but became toxic in larger doses.
''We're really late in the season . . . and that's our main theory around it.''
The problem could be very significant for farmers and could extend beyond this winter, Dr Bryan said.
He said he believed some cows are affected that were not sick.
''We think there's a sub-clinical risk so there might be some chronic animal health problems or some chronic productivity losses.
"This is going to cost farmers a significant amount of money in treatment and investigation in the next few months to possibly six months,'' he said.
''We're seeing significant liver damage and we know that from other diseases where you get significant liver damage the ongoing effects are significant for a number of months or even years.''
Seed supplier PGG Wrightson Seeds' general manager, seeds, David Green, said the HT Swede variety had become very popular with farmers since it was introduced three years ago.
''In a lot of cases for farmers who have struggled to establish brassica crops because of weed burden, this has been a great solution.''
He said it was important to get answers for farmers.
''We're as anxious as everybody else to find out what's gone on here and what's driven this.''
