Evaluating the optimal use of land

HortNZ has made it clear that elite soils – once market gardens – are disappearing under urban development. But would moving the market gardens to other regions cannibalise other sectors like kiwifruit or dairying? And what impact would result from changes in dairying or drystock farming?

The answers are complicated and require an understanding of the biological system. They depend not only on the geographical region under consideration (including infrastructure and market access), but also on the overall goal of NZ – economic, environment, employment, food security?

A Massey University professor of grassland science, Tony Parsons, has developed a map to help with understanding changes and make decisions.

“A map,” he says, “is a way of presenting the greatest amount of information in a single image. It shows how the system works and so allows one to weigh up objectively what the options for the future might be to achieve a desired outcome.”

Maps have been used to find optimal solutions to many problems in micro-economics and behavioural ecology. Professor Parsons has shown that they also have considerable benefits for the ‘wicked problems’ in agriculture that we are now facing.

Parsons’ research, published in international journals, at Australasian conferences and in newsletters, has shown that, for example, for a given land area with a total of 150kg/ha of nitrogen (N) inputs in total (from clover fixation, fertiliser and supplement), animal protein production (milk or meat) is substantially greater from dairy animals than from meat animals. At the same time, losses of methane are slightly higher with meat, and total nitrogen loss (greenhouse gases, nitrogen gas and nitrate), all per hectare, is also higher.

Doubling the N input to 300kg/ha in either system would increase products to a small degree, but hugely increase N losses. The use of high-metabolisable energy supplement increases production per hectare and increases methane but reduces N loss. (Note that this applies when the N in the supplement has been considered in the total N inputs and has allowed a reduction in the fertiliser N input.)

Parsons’ new research considers the outcome for changes in stocking rate and the effects of altering the land area allocated to dairy in comparison with meat.

The maps show that for the land area now under dairying in NZ, a reduction of inputs from a total of 300kg/ha nitrogen to 150kg/ha reduces production by about 30%, but total N emissions reduce by almost 70%.

Replacing the dairy area with meat production, still at 150 kg/ha of N (noting that this is common for clover fixation) would decrease food production by 65% but more than double the N loss.

Different options clearly have substantially different outcomes for the environment.

The economic outcome would depend on the milk payout and schedule prices, and on the costs of production and everything else that farmers have to calculate – including the current and future costs of compliance.

Employment and food security are also components important for society.

The mapping approach enables the evaluation of different farm systems and has considerable benefit, not least for policy, including considering the outcome of policies as a cap on inputs or on N losses.

Capping N inputs or loss at a specific value of, for instance, N per hectare, could be achieved in either farm system. However, neither of these capping mechanisms alone would ensure that the best combination of ‘most food production’ and ‘least environmental impact’ is achieved – onfarm, in catchment or for NZ as a whole.

The same approach is needed for evaluating the impact of other enterprises. More research is always necessary as technologies advance and biology changes.

• Dr Jacqueline Rowarth is chief scientist at the Environmental Protection Authority. She works with Professor Tony Parsons on nutrient cycling.