Farmers are being warned to watch for high nitrate levels in new grass after 35 dairy cows died in Taranaki earlier this week.
Local vets rushed to save the herd after they were grazed on a new paddock of grass for too long.
Last week, five cows in other parts of Taranaki also died from nitrate poisoning and there have been reports of nitrate deaths in dairy herds in Canterbury.
Dangerous levels of nitrate occurs in mainly new grass and on cloudy days - when there isn't enough light to photosynthesise the toxin away.
It can poison animals by stripping the blood of its oxygen-carrying capacity.
A Taranaki vet, Dr Neil Chesterton, says nitrate poisoning is an issue every season, but it is particularly bad this year due to warm weather and cloudy days.
Symptoms of nitrate poisoning in livestock include darkening of the gums and mucous membranes, diarrhoea and vomiting, and death can also follow.