DairyNZ has released new test results confirming high levels of toxins in herbicide-tolerant swedes that killed hundreds of cows in Southland last spring.
DairyNZ is urging farmers not to feed the swedes to cows in spring when the animals are in late pregnancy or early lactation but said it did not have a view on whether the swedes should be taken off the market altogether.
However, the Green Party said the swedes need to be removed from shelves now.
150 plants from three different swede varieties were analysed from 11 different farms.
DairyNZ Southland-South Otago leader Richard Kyte said results showed the HT swedes had higher levels of of glucosinolates, which can be toxic to livestock, compared with non HT swedes.
Mr Kyte said the risk to cows became much higher when the swedes flowered, got seed heads or grew an elongated stem, because the glucosynilate levels increased.
"The range in the HTs, and we're talking about the upper leaf and the upper stem and the flower, where the main issues are, the average was around 40 micromoles per gram, which is a measure of the concentration of the chemical within the dry matter plant sample.
He said some of the higher levels in the HT swedes were between 50 and 60 micromoles per gram.
"Basically the more the cow ingests, the higher those levels, so the more problems it's causing."
Mr Kyte said he was concerned about farmers feeding the swedes to cattle at this time of year.
"When these swedes start going reproductive, we're telling farmers not to feed them because of the risk. This time of year, all the risk factors come together: the weather becomes warmer, the plants start bolting, and there's a lot more new leaf growth as well."
Green Party genetic engineering spokesperson Steffan Browning reissued his call for the HT Swedes to be taken off the shelves immediately.
He said the swedes were developed by chemical mutagenesis which he said was similar to genetic engineering and that the Government had been negligent in not carrying out the investigation itself.
"The Government should have stayed involved very closely all the way. It should have been in charge of all testing. It should have insisted on intensive animal and plant testing.
"That hasn't happened, we have had no tissue results, no milk results, we haven't had results for chlorsulfuron, the herbicide involved.
"But we do have, of course, is some compelling evidence around glucosinolates and the fact there's been an unexpected outcome from the use of chemical mutagenesis in this instance."
Mr Browning said the Government was in damage control and that it and PGG Wrightson should be compensating farmers for their stock losses.