It can also occur with new growth in brassica crops.
Farmers need to be aware that it can be a problem, Verkerk says. "There are test kits available from veterinarians to help monitor levels of nitrate.
"If nitrate levels are high you need to feed a safe feed first, such as silage or baleage, to slow the rate of intake. And then you can graze the affected crop with less risk. But farmers should talk to their vets."
The situation doesn't mean there will be less grass available, but farmers will need to set priorities for what they feed.
Ballance science extension manager Aaron Stafford says he is not aware of any cases of nitrate poisoning.
"I haven't heard anything. I would have thought it could have been a problem a couple of months ago when it was bone dry and there was heaps of nitrogen in the soil and heaps of nitrate.
"You've got that wet period, lots of rain and good growth that could have been the prime time for nitrate poisoning."
Farmers can't force down levels of nitrate in the soil, he says.
"Really it's just a process that will happen over time.
"So as the plants grow they will naturally convert that nitrate in the plant tissue to amino acids and protein and use it that way.
"There is nothing you can do to bring it down," Stafford says. "You just need to manage it in the short term.
"Avoiding hard grazing in the morning is another way to combat the problem.
"Nitrate seems to be higher first thing in the morning compared to late afternoon so it's advisable to avoid hard grazing in the morning."
Stafford says that if farmers are concerned about plants, getting them tested for nitrate is probably a good starting point.
