Questions are being asked about the accuracy of spot checks on water quality on farms after a South Otago trial showed variable results.
Clutha Agriculture Development Board project manager Malcolm Deverson said in a report on the project that sheep and beef farmers were more aware of their obligations to look after water flowing through their farms.
However, trial results raised questions about whether water quality spot checks by regulatory agencies were a reliable way of showing a farm's effect on water quality.
"We believe the readings on this site show that any individual spot check will not necessarily show accurately the health of the catchment's water," Mr Deverson said.
"Nor does a spot-check approach show any understanding of the complex chemistry involved and the breadth of natural variable such as the soil and water," he said.
The board's water filtration study was backed by the Sustainable Farming Fund.
It took place on a Te Houka property.
It tested for water-borne nutrients at three testing sites in gullies both before the water reached a wetland and when it exited.
Mr Deverson said results were variable.
There was difficulty identifying a direct cause for a particular reading, which was of concern given regulatory authorities' approach to environmental monitoring.
The Otago Regional Council tested for turbidity, e. coli, total phosphorous nitrite-nitrate nitrogen and ammoniacal nitrogen.
There was evidence the wetland was doing its job of filtering nutrients.
Tests showed significant improvement in nutrient filtration with additional planting.
Dairy farmers have long been urged to use riparian planting to capture nutrient runoff, but the Te Houka study did reveal that sheep and beef farmers may need to consider doing the same.
Tests showed additional planting helped nature by slowing the movement of water and nutrients across the wetland.
"We planted kinuyanagi willows and some natives in the half-hectare block and they appear to have helped nature do its filtration work," Mr Deverson said.
A small related trial looked at ewe fecundity on stock grazing the willows and the wetland.
The ewes lost weight on a diet of low-quality pasture, a little clover and willows, but scanning revealed pregnancy rates as high as those grazing regular pasture.