Farmers should soon know more about what caused Southland dairy herds to become ill after feeding on swedes.

More than 200 cows died from a mystery illness last year.

DairyNZ has carried out an in-depth survey to get to the root of the toxic swede issue and find out whether farm management practices may have been a contributing factor.

134 farmers and 34 graziers have been interviewed, with more than 400 people responding to a short survey of all dairy farmers.

The results of the survey are expected to be available by the end of May.

The results of the analysis of blood and autopsy samples have already been released.

They showed the animals suffered liver damage, which can happen when grazing brassica forage crops, but the visible signs of illness seemed to be more severe than usual cases.

DairyNZ is still waiting on tests of plant material samples, with those results expected by mid-July.

DairyNZ is urging farmers to be cautious when transitioning cows onto brassicas and fodder beet.

The industry body is advising farmer to be cautious if they see swede crops showing signs of developing flowerhead or if leaf regrowth is occurring.