Stock exclusion and good-practice irrigation and effluent on dairy farms have done most to lower nitrogen and phosphorus levels over the past 20 years, Our Land and Water national science challenge team leader Dr David Houlbrooke said.
Without that mitigation today’s losses would be 45% greater for nitrogen and 98% for phosphorus, Houlbrooke told an Institute of Agricultural and Horticultural Science forum at Lincoln.
“That’s a good news story but, unfortunately, during those 20 years that farmers have been adopting a lot of good management practices we’ve also been intensifying and the mitigation hasn’t been enough to keep up with the intensification.”
Looking ahead, Houlbrooke pointed to a heat map showing nitrogen concentrations around the regions now, compared to forecasts for the next 20 years.
“We get a reduction of nearly 20% of nitrogen and if we take a broader view, let’s look at all the mitigations that are being worked on, we get a 34% reduction.”
In many places that sort of reduction would be enough to meet water quality standards but in other cases “we don’t believe that will be enough given the pressure we’re under in some environments”.
It will continue to be important for farmers to match the right enterprise to the right place for the right result, Houlbrooke said.
Researchers involved in Our Land and Water science had come up with three indicators for the most suitable land use: production potential, relative contribution (measuring loss of nutrients from pasture to land) and impact on downstream water quality.
The information is part of a land-use capability study of high-intensity farming areas, like parts of Southland. A lot of work is being done on a proof of concept to show how the approach can be used more widely, Houlbrooke said.
Another study, led by Massey University, called Variable Attenuation Capacity, is looking at the flow of nitrogen from farms to rivers and lakes across catchments in Manawatu.
Some areas can be intensified successfully because of high attenuation in gravel but farmers on that land will still need to be sure they offset their losses. Even those farmers might need to realign the intensity of their land use.
It is clear from social research for the science challenge that farmers weigh up an array of factors before changing land use, from economic and social considerations to environmental and cultural influences. It is generally a question of relative risk, Houlbrooke said.
As an example, scientists are working with state-owned Pamu to potentially diversify land-use at the company’s Eyrewell dairy farm in North Canterbury. The property irrigates pasture for a relatively conventional milking system.
However, if it is to build an integrated agri-aquaculture system, Pamu could use its clean, reliable water to produce high-value food like salmon then get a secondary use of the water by growing koura (freshwater crayfish). By-products from the new industry could create opportunities for fertigation, either on the existing farm system or as a fertiliser option for arable or horticultural crops.
A feasibility study showed it is possible to use existing water resources to generate heaps of revenue and decrease land-use intensity, Houlbrooke said.
