The sprayed-off Parapara hills that shocked visitor Graham Gibbons are permitted - but with conditions, Horizons Regional Council environmental manager Grant Cooper says.

When he drove along SH4 before Christmas, Mr Gibbons was shocked to see large areas of aerial spraying near Raetihi. It's the practice known as "spray and pray", because it relies on rain to make seeds germinate.

Mr Gibbons owns a landscaping business in England. He said glyphosate herbicide shouldn't be used near water or used to kill native bush.

Horizons land manager Grant Cooper said his council has rules about clearing native woody vegetation.

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The "spray and pray" practice is common, but not extensive, in hill country around the Whanganui, Whangaehu, Turakina and Rangitikei rivers, he said. It's less common in the upper Whanganui and Tararua.

In it, glyphosate herbicide is sprayed from the air to kill vegetation. Then fertiliser and slug bait are applied, and the area is oversown - usually with a feed crop such as rape or turnips, followed by pasture species such as plantain.

It's considered safe for stock to eat pasture after it has been sprayed with glyphosate, Mr Cooper understands, though that is not his area of expertise.

Whatever crop or pasture is used will establish best on the sprayed land if good management practices are used, he said.

Woody vegetation - especially poplars or willows used to stabilise slopes - should not be sprayed, and slopes of over 20 degrees should not be sprayed. Vegetation in gullies and swales (not just watercourses) should be maintained, and winter grazing should not be allowed to denude the soil or compact it.

Poor management will cause erosion and topsoil loss, especially on steep slopes. It will remove vegetation from gullies, swales and watercourses.

Horizons has supported an application to the Sustainable Farming Fund to get more research carried out on "spray and pray".

Aerial spraying is expensive, and national-level research has been proposed on its costs and benefits.

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The council will consider imposing more restrictions if the practice leads to significant impacts, Mr Cooper said - impacts on water quality for example.