Helicopter cropping has added flexibility and profitability to Justin and Mary Vennell's farm near Hunterville.

Spraying and sowing from the air costs $950 a hectare but farm advisers say it is well worth it.

"It's hugely important to try and finish your lambs - hugely important to have that high-performance feed. It can add 12kg to a lamb in summer and easily double your money."

The Vennells farm at Rewa, on the south side of the Rangitikei River near the Vinegar Hill Domain. Their farm was the supreme winner at the region's Ballance Farm Environment Awards, and people flocked to a field day there on Thursday.

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Their 489ha includes 60ha of river flats, with the rest hill country ranging from easy to steep.

Mr Vennell has been helicopter cropping since 2007. He said it was a technique pioneered by local man Jim Bull. In it, initial crops sown were followed by modern grasses more productive than native ones.

He can have choppers spray hillsides with herbicide and seed on the same day if rain is predicted.

Resource consent is not needed, as it would be for ploughing the same areas, because the soil is not disturbed.

The Horizons Regional Council did not monitor the practice because it was not "on the radar" when the One Plan was being written. One woman at the field day said the council should pay attention to it now.

"The land is bare for a period of time. I've seen things that weren't a good idea," she said.

On gentler slopes, Mr Vennell can disc before sowing a crop. He has to get council consent to disturb soil on slopes of more than 20 degrees.

One of his advisers gave the crop pictured - a mix of Spitfire rape and plantain - a mere four out of 10 - but said there was still good value in it.

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Grazing the mixed crop proved to be a problem, Mr Vennell said: "To graze one right you are not grazing the other one correctly."

The Spitfire rape was quick-maturing and good for lambs. Having it, meant he could take on 600 replacement ewe lambs last summer. Another crop on the flats enabled him to get some rangy ewes into better condition.

"It's nice to have options on this country that, historically, can dry up quite quickly. You don't have to hit the store market in a panic."

Goliath rape, which he grew elsewhere, was bigger and coarser and better for cattle.

Plantain, in a red and white clover mix, was highly recommended as a crop. It was hardy on all classes of country and kept growing in winter.

"Hill country plantain is a very, very serious option nowadays."

Mr Vennell also grows leafy turnips in summer.

Environmental issues existed with hill-country cropping, his adviser said. The herbicides and insecticides used had to be kept out of water bodies and soil had to be kept on the hills.

Insecticide was needed, to keep down the slugs, thrips and springtails that could devastate a crop. Seeds sown were often coated with insecticide as well.

The initial herbicide spray created a mulch of dead vegetation for the seeds to germinate in - but, if it was too thick, it could prevent them reaching the soil and growing.

Horizons' land manager Grant Cooper said tracking and vegetation clearance on land with a slope of over 20 degrees also required consent under the One Plan. It's a new rule. The free consents can be signed off in five days.

Crops on slopes of 20 degrees or more have to have 10m buffer zones along water bodies. For lower slopes, the buffer is 5m.