Tree solution to nutrient discharges

The ideal riparian zone has a mixture of rank grass, native plants and exotic trees, DairyNZ freshwater ecologist Justin Kitto says. Photo from Allied Press files.
The ideal riparian zone has a mixture of rank grass, native plants and exotic trees, DairyNZ freshwater ecologist Justin Kitto says. Photo from Allied Press files.
Trees could help solve farmers' concerns with nutrient discharges.

The Trees on Farms workshop at Pleasant Point on April 30 outlined the uses of trees to improve the environment.

AgFirst consultant Nicola Chisholm said there had been a lot of investigation into trees' role in nutrient management. Riparian strips had been shown to stabilise stream banks, intercept groundwater, reduce nutrient input into streams and help control stream temperatures.

However, they had only a minimal impact on reducing nitrogen losses. Farm forestry blocks could reduce a farm's overall nitrogen losses by balancing the nutrient equation.

Trees took up effluent, cleaned up waterways, helped with drainage and helped to form litter layers to dam surface runoff. They could be useful at ''leaky'' sites, Ms Chisholm said.

Dairy farm effluent could be used to irrigate growing trees. A southern Wairarapa trial showed good uptake of nutrients in some cases, she said. That research had potential applications for poorly draining dairy farms.

Willows could be an option, because of their vigorous nature. In an Otago trial, they took up a lot of effluent as long as the application level was correct.

In Southland, willows had been successfully used to absorb effluent at truck wash sites.

A farm had a gully planted in willows to take up effluent running off a sloping paddock, Ms Chisholm said.

With climate change predicted to increase temperatures and droughts on the South Island's east coast, landowners should consider the role of trees, she said.

They would become more important as shade, because dairy cows had lower milk yield and quality in hot conditions. Cows with high genetic merit were the most susceptible to heat stress.

Lincoln University studies showed shelter removed the effects of heat stress.

And while people suspected cows offered shade would sit in it rather than grazing, the research revealed provision of shade actually increased grazing time, Ms Chisholm said. The cows also had a better feed conversion efficiency.

''Animals with shade do better.''

Shade and shelter trees could also produce fruit and nuts to supplement a farm's income. Examples already working included feijoas and chestnuts on a Waikato dairy farm and avocados in Northland.

An organic dairy farm in Britain was trialling willows that cows would graze before the trees were harvested for bioenergy.

DairyNZ freshwater ecologist Justin Kitto said about 80% of farm runoff came from about 20% of land - areas that leaked only during high rainfall. Grass filter strips were ''fantastic'' at reducing sediment, phosphorus and bacteria, he said.

Riparian planting could shade the waterway to prevent growth of unwanted duckweed, improve the reproductive habitat for invertebrates, and hold banks together to prevent sediment entering the water.

The ideal riparian zone had a mixture of rank grass, native plants and exotic trees, where appropriate, Mr Kitto said.

However, the whole river catchment needed to be planted or managed. He had seen ''loads of cases'' where the efforts of some farmers were undone by a person at the top of the catchment letting contaminants pour in.

Planting was not the whole solution, either - nutrient inputs and irrigation also had to be managed.

Meanwhile, the riparian zone was ''the picture postcard for your farm and the state of New Zealand farms in the public's eye'', Mr Kitto said.

- by Sally Brooker