Southern veterinarians believe they have linked the deaths of hundreds of dairy cows in Southland last winter to the use of herbicide tolerant (HT) swedes as feed.
VetSouth director Dr Mark Bryan said from accessing VetSouth clients' data, VetSouth had shown ''the risk of death on HT swedes was significantly higher than on any other form of crop''.
The key analysis was of more than 8000 cows on eight farms, measuring mortality rates over winter 2014, Dr Bryan said.
''We've pinpointed a very strong association,'' he said.
''Cause and effect is difficult to determine really. There could be a whole bunch of co-factors.''
While poisoning from swedes was not unheard of, the amount of liver disease, photosensitivity and death of dairy cows he saw last year was unprecedented.
''The question is: can you extrapolate beyond those farms into the wider area?'' Dr Bryan said.
However, he said, VetSouth results backed up a DairyNZ study that showed an increase risk of death and disease on HT swedes in spring last year.
Massey University veterinary pathologist Dr Mark Collett has been looking at the issue of animals becoming photosensitive while feeding on brassica crops for the past 15 years.
He said shortened transition periods would be as much to blame as any particular swede variety.
''I don't think there's any such thing as a safe brassica from the point of view of potential liver disease,'' he said.
''When animals are moved on to a crop you've got to keep an eye on their intake and start them on a very low intake and gradually increase it.
''It's a small area around Winton where this problem was the most intensive and I think it was possibly just by accident that HT formed a lot of the diet in that particular area - I wouldn't say that you should condemn the HT brand because cows have got sick and died. Cows get sick and die when they eat turnips, rape and other varieties of swedes.
''They all have their problems. There is no such thing as a perfect feed.''
Both vets are part of a multistakeholder group looking into the issue that plagued southern farmers last year.
DairyNZ regional team leader for Southland/South Otago Richard Kyte said he could not comment on the proposed strong association between illness and HT swedes found by VetSouth until he had seen Dr Bryan's report.
PGG Wrightson Seeds group general manager John McKenzie referred comment to Mr Kyte.
PGG Wrightson Seeds is also a member of the swedes working group.
Mr McKenzie said PGG Wrightson Seeds would make a statement at a later time.
