Feedlots harm the environment unless on a sealed pad where waste is captured and spread where plants used all nutrients, says Massey University freshwater ecologist Mike Joy.

The common dairy-farm model of waste containment was acceptable but Hawke's Bay feedlot operations were very different, he said.

"It is all going down into the waterways," he said.

"It's shocking - it's about as bad as you can get."

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Nitrates leeching through soil was the biggest issue. Nitrous oxide, from waste exposed to air, was another issue because it was a greenhouse gas.

"The root zone is only about 100mm down at the most, not that there is any plants there anyway, so it is going straight down to groundwater or a stream further down."

He said it was difficult to know where nutrients leached to underground.

"It might go straight down to an aquifer and turn up 20km away."

Nitrates could stay in water "forever" and all water from productive land in New Zealand was increasing its nitrate level.

"There is no way around it, no matter what you feed them, on a concentrated feed pad nitrogen levels will be thousands of times more that plant roots could possibly absorb.

"If plants aren't taking it up it is going to the environment and doing damage."

Federated Farmers Hawke's Bay president Will Foley said existing feed pad operations were once best practice and nutrients were currently monitored through bores.

More science was needed to quantify any excess nitrogen and its environment impact, because monitoring showed no impact from specialised feedlots.

Any change needed to be measured and if there was excess nitrogen a cost-benefit analysis needed to be made.

"Is it a measurable and significant impact on the environment and if so, then the feedlot operator has to do something about it.

"We need to measure what that impact is," Mr Foley said.

If improvements were needed to avoid environmental impact then they should happen "but it must be economic and valid".

"As farmers we don't want to have to jump through hoops and spend lots of money unless it is based on science and evidence."