The company which sells herbicide tolerant swedes linked to the death of several hunded cows in the lower South Island is advising farmers not to let pregnant cows feed on them as an investigation continues.
Up to 500 cows have died after eating swedes in Southland and South Otago and a working group has been set up to try and figure out why.
PGG Wrightson Seeds general manager David Green said it is far from certain that HT Swedes are the culprit - as normal swede varieties and other brassicas have been linked to some cattle deaths also.
However, Mr Green said the company is urging farmers to be cautious until the reason for the fatalities is known.
"We've adopted what we believe is a very prudent approach, which is to recommend that HT Swedes will not be fed to pregnant dairy cows.
"That's where the vast majority of cases have been reported and I think the advice would be that they represent the most volatile stock class that is likely to be fed on brassicas."
He said the company did not believe it appropriate to take the product off the market completely as 95 percent of the people who had used these products have done so with good results.
"So we thought that a warning on those areas where we have seen the issues is appropriate but continue to give those people who are willing and happy to continue using the product the opportunity to do so."