A Taranaki farming couple are "shellshocked" after 120 of their cows dropped dead one by one in their paddock.
Around 20 vets who rushed to the Oeo farm of Chris and Catherine Cook on Tuesday could not save the animals, part of a herd of 600.
Mrs Cook's brother, John Murphy, speaking for the family, said the loss of the cows was a devastating blow.
"The farmer was out there topping up the water troughs and minutes later the cows were falling to the ground," he said.
It was heart-breaking for the Cooks.
"They're gutted. Massively tearful. In disbelief and shellshocked."
An investigation under way by the Ministry of Business, Innovation and Employment should determine the cause of death, he said.
"They'll do all the tests and find out what happened and go from there, but until then it's all just speculation," he said.
He said the cows were worth around $400,000, and their deaths would probably mean around another $300,000 loss of profit for the season.
'It's like losing a loved one'
"Cows are just so important and so close to farmers. It's like losing a loved one. In this case it's like losing multiple loved ones," Mr Murphy said.
The vets were called immediately and arrived quickly.
"Just the massive scale of it, they all dropped everything and came straight out because they knew how serious it was."
He said the family wished to thank the vets who attended as well as Fonterra, Federated Farmers and Dairy NZ who had shown their support.
In May last year, the Taranaki Daily News reported the deaths of 35 cows from nitrate poisoning on a Tikorangi dairy farm.
The cows died after eating new pasture on a cloudy day. Similar cases were reported in South Taranaki.
Three months ago Taranaki Veterinary Centre vet Stacey Bateman, of Manaia, warned a seminar that cows were at risk of the illness in spring if carbohydrates were introduced too quickly into their diet.
Nitrate poisoning occurred when nitrates in forage was converted to nitrite, which starved animals of oxygen, causing them to collapse and possibly die. Those who recovered were likely to abort if pregnant.
On cloudy days photosynthesis may not occur, so plants do not convert nitrogen in the soil into protein.