Many Southland and Otago farmers were sold the wrong seed and have inadvertently grown crops linked to the deaths of hundreds of dairy cows in 2014.
PGG Wrightson Seeds general manager New Zealand David Green said swedes sown two or three months ago, which had started to mature, were last week seen in a paddock in Southland and "we noticed that the bulb colour was, in fact, white as opposed to yellow''.
That prompted an investigation by the company that showed "we had made an error in our supply chain and the wrong line of seed had been processed''.
"Straight human error" led to 556 farmers with the wrong crops in the ground. Farmers who assumed they had bought a new seed variety, Hawkestone yellow-fleshed Cleancrop swede, were sold a white-fleshed swede, HT-S57, that was linked to dairy cows suffering from liver disease, photosensitivity and dying three winters ago.
The HT-S57 swede variety was discontinued last year.
"This is very disappointing,'' Mr Green said. "As an organisation we handle thousands upon thousands of grain and seed every year and we pride ourselves on our quality systems. This is nothing short of embarrassing.''
VetSouth managing director Dr Mark Bryan said he was fielding calls yesterday from "many, many" farmers who were "a mixture between incredulous and angry".
"I guess the issue is, we thought we were over this problem and we thought we wouldn't have to deal with it again," he said.
A lack of frosts during the plant's growth had been a major contributing factor in 2014 and farmers would be watching the weather and other environmental factors, but at this stage he did not know if farmers faced a risk from the mistake.
"The only positive in this whole situation is that they [affected farmers] know ahead of time, so they've got two or three months now to plan for how they manage it. But the unknowns now are the weather conditions and the environmental factors that we think make the crop more toxic."
Federated Farmers arable industry group chairman Guy Wigley said the federation would watch whether PGG Wrightson followed through with affected farmers.
He applauded the response so far, saying the company had done the right thing once it had identified the problem.
DairyNZ regional team leader for Southland-South Otago Richard Kyte said he was "extremely disappointed".
and urged concerned farmers to contact the company immediately.
DairyNZ had provided advice to farmers about managing HT swedes in the past, he said.