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Study results bring more clarity for lakes

Study results bring more clarity for lakes

Results from a study aimed at reducing nutrient loss from farms into Rotorua lakes show that mitigation efforts can make a real difference, and can also increase farm profitability.

Water quality is seriously declining in several Rotorua lakes, largely due to nutrient overloading from excess nitrogen (N) and phosphorus (P). Nutrient runoff from farming is just one of a range of sources, and though efforts such as fencing streams and using nutrient budgets have helped, they have not fully addressed the lakes’ complex and persistent problems.

The NZ Landcare Trust has led research on the effectiveness of wintering-off and nitrification inhibitors to reduce N and P losses from Ngati Whakaue Tribal Lands farms in the Lake Rotorua catchment at Wharenui. The work was done in partnership with Federated Farmers of New Zealand, Environment Bay of Plenty and AgResearch.

Dairy farm results

Project leader, NZ Landcare Trust Research Manager Dr Nick Edgar, says on the Ngati Whakaue dairy farm, stopping N use reduced N leaching by 50%, but also reduced profitability.

Options with maize silage, winter stand-off and winter feed pads also reduced both N leaching and profitability. Normal grazing and fertilising using a nitrification inhibitor was cost-neutral.

However, an optimised scenario (no winter N, greater wintering off, less maize, earlier calving) was calculated to reduce N leaching by 15% and increased profitability by 19%.

Sheep and beef farm results

Meanwhile, on the Ngati Whakaue Tribal Lands sheep and beef farm, more P was lost from grazed paddocks than non-grazed paddocks.

Also, a trial of straw bales as dams to block P loss showed they are not a good mitigation option. As the straw bales broke down, P loss actually increased.

The use of nitrification inhibitors reduced N leaching but also reduced profitability, even when growth and stocking rates were increased to cover nitrification inhibitor application costs.

Putting steep land in forestry reduced whole-farm N leaching. However, to be cost-neutral, remaining pasture land would have to be placed in a more profitable cattle-only system.

Increasing the lambing percentage of sheep increased profitability with no change in N leaching.

Benefits for all

Dr Edgar says for farmers in the Rotorua area, the research is a good indication that through working cooperatively with NGOs, research institutes and the Regional Council, farming practises can be improved to the benefit of all parties.

‘Some gains may not be as big as some might want, but they are steps in the right direction, and a big improvement on the status quo.’

Dr Edgar says though the research results are specific to the study farms, they are potentially transferable.

‘Much depends on the topography, soil and farm management on a particular farm, and adjustments would need to be made for differences.

‘However, any of the mitigation options trialled have potential to reduce N or P entering waterways.’

The research was funded by MAF's Sustainable Farming Fund. MAF has agreed to fund a new 3­–year project that will further refine the N and P mitigation options, and continue to look at N-inhibiting options and P runoff, including new P sorption materials. The work will continue with the same partners.


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