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Southland farmers will not know until next autumn the full story about what killed hundreds of dairy cows this winter.
DairyNZ general manager for extension Craig McBeth said a wide variety of both plants and animals would be tested and farmers would undergo ''in-depth'' surveying to understand the issues that caused vets in the South to raise the alarm in September.
''As we find stuff out, we will share that with farmers, but in a way that doesn't confuse the issue,'' he said.
Analysis of animal blood and crop tissue samples would be required to determine what caused the problem, but crop tissue tests would not be completed until March.
Vets raised concerns a new variety of herbicide-tolerant swede fed to dairy cows was behind the significant liver damage, photosensitivity and deaths of cows, but seed provider PGG Wrightson said a mild winter and other factors could have played a role.
An industry-wide working group was formed last month to investigate.
While herbicide-tolerant swede ''hasn't been scrapped as a hypothesis, it hasn't been confirmed, either'', Mr McBeth said.
''A lot of cows have been fed HT swedes and exhibited no symptoms. It's not as simple as saying it's one variety of swede or another.''
DairyNZ chaired a meeting of the working group
last month and surveyed 400 farmers in the area and would now interview about 120 farmers ''face to face'' to try to understand what caused the deaths.
''What are the management practices that contribute to the risk factors, and what are the ones that mitigate the risk factors?'' Mr McBeth said.
Farmers would plant most of their winter feed in November and he wanted to be able to provide farmers with ''some clarity'', Mr McBeth said.
''Farmers are making those decisions at the moment and it is a challenge. With the exception of fodder feed, I think that farmers do have a reasonable range of choice as to what they can plant this year.
"They can make some informed decisions now, but certainly not with as much knowledge as we would like to have.''