Phosphorus loss to water was down an estimated 20 per cent to 25 per cent, but nitrogen loss grew by an estimated 25 per cent, and almost all the gain was the result of more dairying done in the study years. The national sediment load was estimated to have decreased by 29 per cent over the same 20 years.
During those years, many stock farmers undertook measures to lessen the amount of phosphorus, nitrogen and sediment leaving their land, said the study authors, led by Dr Ross Monaghan of AgResearch and the Our Land and Water National Science Challenge.
The farmers fenced streams, better managed fertilisers, reduced over-watering, excluded animals from some lands, planted trees and stream banks and undertook other measures – all of them at some cost.
The scientists at AgResearch and Landcare wanted to know how effective those measures were and what the picture would have been without those mitigations.
To do this, they didn’t sample waterways – an empirical approach – because of inconsistent or poor data dating back to the 1990s.
Instead, they combed databases and manipulated variables to estimate the phosphorus, nitrogen and sediment losses in 1995 and in 2015 at different types of sheep, beef and dairy farms all across the country. The goal was to produce national estimates rather than estimates for each watershed or province.
But stock farming didn’t stand still between 1995-2015. In addition to the fencing streams and such, farmers expanded the land area used by dairy by 40 per cent and increased total dairy production by about 160 per cent. The land area occupied by sheep and beef farms contracted, but the intensity of production per hectare increased.
The result was a “mixed bag”, said Monaghan in an interview. Phosphorus and sediment loss to water were both down by 20 per cent to 29 per cent, but nitrogen loss to water was 25 per cent higher. The community would have to decide if that was a good result, he said.
But these were better results than if no streams were fenced or trees plants and so forth.
“Significantly more nitrogen (45 per cent more) and phosphorus (98 per cent more) would have entered rivers from dairy-farmed land between 1995 and 2015 if farmers hadn’t changed their practices,” reads a plain language summary of the research.
On beef and sheep farms, nitrogen and phosphorus losses would have been somewhat higher if farmers had not adopted better practices. But sediment loss on beef and sheep farms would have been 30 per cent higher without farmer action.
The team also estimated the “best case scenario” if all the known and developing mitigation measures were fully implemented by stock farmers by 2035. It was predicted that nitrogen contamination would decrease by 34 per cent, phosphorus decrease by 36 per cent and sediment decrease by 66 per cent, compared to 2015.
It was noted that mitigation measures may not be the best way to protect fresh water, and that land-use change and intensification might have to be considered.