Multiple factors, including a mild winter and current farming practices, played a role in the not-yet-fully-understood deaths of hundreds of Southland dairy cows last year, DairyNZ says.
Dairy NZ Southland-South Otago regional leader Richard Kyte said animal tissue testing was done after the deaths and plant testing was expected to be completed at the end of July.
Poisoning probably occurred in the cows due to a high level of glucosinolates in the swedes they were fed.
A herbicide tolerant (HT) variety might have been a factor, but the warm winter in the south accentuated changes to farming practices over the past 10 years, which included using more swedes and feeding more mature swedes to dairy cows later in the season, Mr Kyte said.
''There was a higher incidence of cows with disease on the HT variety, especially in the mature plants, but overall, the higher the proportion of crop fed across all varieties, the higher the risk.''
The reason for the increased incidence of cows with disease was still unknown, he said.
''We're looking for the glucosinolates [which caused the disease], but we haven't done that and we don't actually know that's the actual cause; that's the theory,'' Mr Kyte said.
''I think we're on the right track, but whether we'll ever pin those glucosinolates down, that's a different story.''
Half the farmers who used HT swedes last year experienced no problems, he said.
''But again it comes back to proportion of dry matter fed. The higher the proportion of dry matter fed and the higher the amount of leaf, the bigger the issue.''
Many farmers had moved to feeding cows swedes twice in the season: when wintering cows off-farm and again when bringing cows back to the farm to calve and milk.
The message DairyNZ was sending to farmers was to put more supplement into their systems when feeding swedes.
''Back to basics in a sense; we haven't had problems - swedes have been fed for generations - and this year, bang.''
Mr Kyte said DairyNZ was confident farmers would ''pick up these messages we've sent them''.
For farmers to understand the higher toxicity of mature swedes was to be ''a long way ahead'' of where they were last year.
''Last year was one out of the bag,'' Mr Kyte said.
''You try things and they work and then suddenly last year it didn't.''
Plant testing when finished would probably not reveal a ''smoking gun'' in the deaths.
And there would be no quick test on plants to determine whether they had toxic concentrations of glucosinolates.
Blood testing of some cows to test their liver function and monitor their health would occur through this winter, he said.
