Several cases of nitrate poisoning in livestock have prompted a warning from Oamaru veterinarians that recent rapid growth of winter crops has also elevated nitrate levels.
Oamaru Veterinary Services vet Kevin Kearney said in one case seven rising yearling heifers strip grazing short rotation rye grass were found dead, while tests on other winter crops in the region also revealed elevated nitrate levels.
"Veterinary laboratories in the South Island are also reporting some very high nitrate levels. Farmers should be aware of the potential of sudden death from nitrate toxicity," he said.
Mr Kearney said nitrates accumulated in plants during periods of rapid growth, especially after rain following extended dry periods, as had happened this autumn.
They could also accumulate in plants during flowering or regrowth, in overcast, dull conditions, frosty weather and when there was leaf damage from hail, herbicides or plant disease.
Death from nitrate poisoning was rapid with animals suffering from a lack of oxygen.
If they survived, pregnant animals could abort.
Intravenous treatment could reverse the symptoms.
Mr Kearney recommended introducing animals to winter crops slowly and not when they were hungry, suggesting they first be fed hay, silage or baleage.
He also suggested only half an animal's daily intake come from brassicas and, after a frost, farmers should let the crop thaw before letting stock have access.