Southland dairy farmers are anxious about cows falling ill and dying from grazing swede crops and are looking to vets and other authorities for more information.
Reports so far indicate that 200 to 300 dairy cows may have died after grazing on swedes, a common winter feed crop in the south, and that 30 to 50 farms may be affected - mainly in central and lowland Southland.
Vets say autopsies are showing signs of significant liver damage and in some cases kidney damage in the cows.
PGG Wrightson supplies most of the swede seed and said there appears to be a connection with a higher level of leaf growth in swede crops due to the unusually mild winter in the region.
Southland vet Dr Mark Bryan said they are looking into that link.
"We don't know for sure, but we think swedes can accumulate a thing called glucosinolates in the leaves and they tend to do this depending on the stage of growth.
"So typically, swedes in our area would experience a lot of frost and they get a bit of leaf drop and the leaves stop growing, but we haven't had that this year.
"We've had a very mild winter, we haven't had much leaf drop and we've got some swede crops with some significant amounts of leaf and we think that for whatever reason, the glucosinolates are accumulating in the leaves way more than they would normally do, so that's causing this toxicity in some cows, not all cows.
"The other interesting thing is, you get a mob of cows on swedes and the vast majority of them are fine, but a small, wee number will be sick."
However, Federated Farmers' Southland dairy chair Allan Baird says there are still questions about the leaf theory.
"It's still to be determined, in my mind. The farmer I spoke to, he grew quite a successful crop - a 16-tonne crop which would have quite a lot of bulb and he did indicate it wasn't highly dominated by leaf, so certainly I think we haven't yet worked out what is causing it and why it's affected some farms but not others."
Mr Baird said most of the cases appear to be associated with a new HT or herbicide tolerant swede that PGG Wrightson is supplying.
"It is a new variety, it's a new concept and it had been working well, particularly on farms where there were other weed related issues. They couldn't grow a successful brassica crop and these HTs were providing a real opening, so from an industry or farming perspective they seemed to be quite a good concept, but these health issues need to be followed through very carefully."
Vets and DairyNZ have formed a working group over the issue.
