If their enterprise was affecting soil, farmers could change aspects like fertiliser application and stocking rates, he said.
Some farmers have had problems with old poplar plantings.
At the field day they were shown how to address the problems, and how to plant new trees.
The Ohura is a tributary to the Whanganui River, and contributes 30 per cent of the sediment that makes its way downstream past Wanganui to the Tasman Sea.
Coal mining in the area has been blamed for the extra sediment, but Mr Kirk said that was a thing of the past and the coal mining area was all vegetated now.
The memorandum of understanding signed was between Horizons, the Whanganui River Enhancement Trust and the Parkes.
Each is to contribute $12,000 a year to the property for the next three years to improve erosion prevention. Mr Kirk will provide advice and support, and publicise open days and field days, and Mr Parkes will contribute his labour and money.
The aim is to influence others and speed progress with Horizons' Whanganui Catchment Strategy.
The strategy aims to prevent erosion and has lagged behind the rest of the region, where the Sustainable Land Use Initiative has the same aim.
Mr Kirk said new owners in the Taumarunui area and new relationships would also help move it along.
"It's a bit slower, but it's gaining momentum."
The fertile soils of the Ohura area make it more productive than most hill country and also prone to heavier stocking and more erosion.
They are mudstone (papa) soils, rated as Land Use Capability Class VI.
They erode differently from soils with a sandstone base.
Instead of slipping they form earthflows, with the fertile slopes still covered with grass and generally carrying more stock.
Keeping those soils on the land and out of the river requires fencing of waterways, plantings of poplars on the slopes and detention dams in the gullies.
Picnic Point is a 376ha sheep, beef and dairy grazing property near Matiere.
It borders the Ohura River for 3.4km.
Mr Parkes has already done a lot to prevent his soil eroding into it.
He's put a two-wire electric fence along the river, and planted below it.
Some of the streams on his property are also fenced and planted where needed, and there are poplars on the slopes.
He's taken cattle off steep hillsides, where they were damaging pasture and potentially starting soil erosion.
He's a "corporate" farmer who accepts that he has responsibilities to the environment, Mr Kirk said.
"I went and talked to him and he said 'That's a good idea, and let's make it a demonstration farm that all farmers can aspire to'."
