Urea poisoning has been confirmed as the cause of last week's deaths of 122 cows on a South Taranaki dairy farm.
The cows were in a herd of 600 owned by 2007 Taranaki Sharemilkers of the Year Chris and Catherine Cook.
There were still some unknowns around the deaths, Taranaki Veterinary Centre chief executive Stephen Hopkinson, of Hawera, said.
"But urea is the cause."
The farmers were using a urea tank to fill their water troughs, he said. "We know it was urea and we know the level because it was shown in the testing.
"The Cooks say the tank was rinsed," Mr Hopkinson, who declined to reveal details of the tests, said.
He could not say whether the urea was absorbed by the plastic of the Donaghys container the Cooks were using, whether some had not dissolved, or whether it had settled out of the solution and remained after rinsing.
Mr Hopkinson said the cows would have absorbed the urea quickly because it was in a liquid.
Urea could be fed at a certain level to cows to convert it into protein, but there was no treatment for urea poisoning.
Farmers who use the Donaghys product LessN - a pasture boost added to a urea solution, a recent innovation in urea application - receive a plastic mixing tank, which is less absorbent than stainless steel.
Donaghys managing director Jeremy Silva said the confirmation of urea poisoning showed the deaths were not related to LessN itself.
The company's calculations showed almost 20 kilograms of urea residue would have had to have been in the 5000-litre tank to poison 122 cows . The cows would have had to drink the entire contents, or more than 40 litres each.
"This sort of amount does not seem likely if rinsing was reasonable and especially if the rinsing was followed by draining the rinsing water out," Mr Silva said.
If the cows had no access to water before the troughs were filled, sudden gorging might have exacerbated their reaction to urea.
"Every farmer will want to know what has happened on this farm."
Massey University toxicologist Kathy Parton said cows would show signs of toxicity 30 minutes to an hour after ingesting excess urea. Urea poisoning caused severe abdominal pain, shivering, staggering, rapid respiration and a marked jugular pulse.
Cows were likely to bellow, struggle violently and be bloated.
Urea was one of the world's most common fertilisers and a common feed supplement overseas, but it was toxic to cattle when the dose was excessive.
Meanwhile, Mr Hopkinson is warning farmers against multiple use of containers in the wake of the deaths.
For example, he said, teat spray in a detergent bottle would be neutralised.
- Taranaki Daily News
What's your expectation of former tropical cyclone Pam?Related story: Cyclone Pam hits New Zealand
Get Taranaki's frequent news and sport updates
Get your mid week news fix
Get your South Taranaki news online