The poisoning is caused by high nitrate levels in feed, usually in autumn or winter, and occurs in rapid growth after a dry period.
Excessive nitrates can kill cattle by suffocation because they stop haemoglobins in the blood carrying oxygen, DairyNZ says.
Federated Farmers dairy vice-chairman Ben Moore said there have been huge problems with nitrate poisoning as dairy farmers look to recover from the drought and put condition on their cows before calving.
Waikato farmers including himself have had abortions in their herds, which he puts down to nitrate poisoning. He has also heard of cases in Coromandel and on the Hauraki Plains.
“A guy rang me from Coromandel and he was testing for nitrates in his grass because he had issues with cows slipping and it was coming from permanent pasture.
“I think what is happening is that there is such a large natural build-up of nitrogen in the ground and then there’s some rain, especially in those drought-stricken areas, and it just shoots off and it’s so fast growing and so full of nitrates that it’s unexpected.”
He heard of one bad case that killed 30 cows on a south Waikato farm and another where 130 cows had to be treated by a vet with methylene blue, which helps a cow’s haemoglobin carry oxygen.
In Hauraki many farmers have resorted to large amounts of undersowing pastures to repair insect pest damage following the drought.
“In doing that and not having feed on hand to dilute the young grass they’ve run into big problems.”
Farmers have changed from calling him to help coordinate feed because of drought to calling him for feed to substitute for the young grass.
Moore on-off grazes paddocks on his farm with new grass by giving his herd food from another source before putting them onto the paddock further into the day. The delay allows the sun to burn off some of the nitrates through photosynthesis and makes it safer to eat.
But it is tricky on farms recovering from drought with a large proportion of new grass.
“There’s no easy answer.”
It affects farmers who have applied nitrogen fertiliser during autumn and those with naturally accumulating nitrates in the soil.
“A lot of the guys who I have spoken to who have had problems haven’t put on urea.”
He urges farmers to test their paddocks and do what they can to minimise the risk.
DairyNZ Waikato regional team leader Wilma Foster said there have been a number of cases in Waikato and Northland.
“But on some farms where there haven’t been cow deaths there have been unexplained abortions in cows.”
It is suspected they were caused by high nitrate levels in the grass, Foster said.
“It means there has not being enough oxygen in the cow to support the calf and so, pretty sadly, that’s one of the outcomes that we’re seeing.”
She urges farmers to keep testing their pasture if they suspect it contains high levels of nitrates. That will give farmers confidence about what they are feeding their animals.
“If there are any concerns contact your local vets. They’ve got a very good range of ways to be able to help someone work through what the problems might be.”
In Hawke’s Bay an estimated 120 ewes died from nitrate poisoning on one farm, which Vet Services Hawke’s Bay managing director Richard Hilson believes is a record.
However, he believes it is an isolated case with the bulk of farmers accustomed to dealing with nitrate poisoning after recurring droughts.
Most took preventive measures and are not taking any risks despite being desperate to put stock on feed crops to maintain cow and ewe condition before calving and lambing.
“It’s really common. You can just put money on it that it’s going to happen after a drought.”
On his own farm he lost 17 ewes in 2007. He recently tested his annual ryegrass and found the nitrate levels to be sky high.
“There’s a hell of a lot of crops and a hell of a lot of grass that’s at risk.
“There’s no coming back from it. There’s a treatment you can use but it all happens so quick that you’re not fixing a lot. You’re saving a few of the animals that haven’t yet hit the ground.”
