It has become so well-known that farmers can reduce stocking rates and increase or maintain profitability that journalists and society in general have stopped asking for the evidence.

They have also stopped asking why, when it is so well-known, farmers aren’t doing more of it.

Maybe the evidence, or rather, the lack of it, has something to do with the answer.

Certainly, there have been specific studies, notably in the Upper Waikato catchment, and in Canterbury where farmers reducing their stock numbers have also reduced their nitrate leaching. The leaching was estimated using the Overseer nutrient model, which changes when significant updates can be implemented – achieving greater accuracy.

Changes in Overseer have led to major frustration for farmers in the past when a new version applied to their farms has resulted in an apparently increased N-loss, even though the input numbers (stocking rate, fertiliser application, soil type, topography and annual rainfall, for instance) haven’t changed.

The reverse can also occur. In the Upper Waikato study almost 30 per cent of the decrease in nitrate leaching was accorded to the use of the latest version of Overseer. The new version was ‘more accurate’ – but the bottom line was that the new model made a significant difference; the farmers’ changes might also have been important, but the result using the old model was not reported.

Reduction of stocking rate, nitrogen application and nitrate leaching in Canterbury has been associated with decrease in flexibility to respond to environmental challenges such as slow pasture growth.

It has also been associated with a significant decrease in income. In a high milk payout year, the loss to the regional economy if all farms had decreased their stocking rate was assessed in the tens of millions.

Theories that land currently under dairy can be used more profitably with reduced environmental impact by changing to cropping and horticulture are ‘interesting’.

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