Dr Evans said risk periods arose when "we've had a prolonged period of drought, followed by growth".
"There is lots of nitrogen sitting in the soil and when the plant grows really rapidly, the plant can't convert those nitrogen compounds into protein. The chemical process requires daylight and photosynthesis," Dr Evans said.
"It [the plant] draws up this nitrogen and can't convert it."
When an animal eats this grass, "those free nitrogen compounds combine with haemoglobin" which carries oxygen in the blood.
The animal then "suffocates internally at a cellular level". It becomes slow and sleepy and will die without treatment.
"We've had calves dying this year - a few odds and sods," Dr Evans said.
"What we are looking for is hot, sunny days so the plant can photosynthesise properly and turn a lot of that nitrogen into plant protein."
Dr Evans said farmers should feed hungry animals safe feed and only graze newer pasture for two hours at a time.
DairyNZ's weekly Farmwatch report published recently also warned of the risk of nitrate poisoning.
The report cites new grass and forage crops such as kale as producing the highest risk and suggests monitoring stock one or two hours after releasing them on to these pastures.
Veterinarians can also provide spot tests of pasture.
"Contrary to popular opinion, using Roundup spray, chopping or a frost does not reduce the nitrate content of the crop," DairyNZ consulting officer Leo Hendrikse said.
"The problem may not go away over winter and farmers need to be thinking about nitrate levels in pasture until July and August."
