Adjunct Professor Lincoln University, farmer-elected director on the boards of DairyNZ and Ravensdown, and a member of the Scientific Council of the World Farmers’ Organisation.
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It is the very ludicrousness of the concept of a high-speed elephant that givespause for thought … and the same should apply to the notion of a dairy cow urinating nitrogen at the rate of “up to” 1000kg per hectare.
She doesn’t – in fact, she can’t.
The data that have misinformed the reality came from grain-fed cows in the US housed on porous bedding.
A 1993 review in which the unfortunate figure appeared relied on data from a decade earlier and included research from the 1950s and 1960s.
Cows, technology and understanding, including the fact that data from other parts of the world might be interesting but not relevant, have changed since then.
In the research that followed the review, the authors used real New Zealand dairy cow urine samples.
The urine had half the nitrogen concentration of the American cows.
This has been supported by research in the last 10 years at Lincoln and Massey Universities and in the Waikato.
The “tonne” load became 500.
Researchers measured real urine patches on pasture and found that the wetted area covered 0.38-0.42sq m – twice the area used to calculate the “tonne per hectare”.
The “tonne” load has reduced to 250kg nitrogen per hectare.
This amount of nitrogen, arriving in a warm, dilute nutrient solution, can be taken up by newly grazed pasture immediately.
Research at Lincoln University using isotopic nitrogen has shown that uptake capacity is over 300kg within two grazing rounds, even in winter.
Further, and again at Lincoln University, research with GPS has shown that at any single grazing, only 2-3% of the area receives urine.
Within a year, only approximately a quarter of the grazing area receives urine.
Urine patches, observable as darker green and higher-yielding areas within a paddock, are the clue – plants are limited by nitrogen availability. Add urine, and the growth response is rapid.
The question then becomes “if not cows, what?”
The answer is in the records and in research.
On the Canterbury plains, nitrogen has been apparent in groundwater since records began.
A science fair project published by the NZ Journal of Hydrology in 2019 highlighted that “there is evidence of groundwater being impacted, in terms of elevated nitrate-nitrogen concentrations, before dairy farming expansion occurred in Canterbury”.
Historical land use and local contaminant sources were indicated.
Dr Jacqueline Rowarth.
In 2024, an Aqualinc report, Nitrates in Groundwater and Impacts of Climate Change, explained that “The main inputs of nitrate-N to groundwater are from agricultural sources. Whilst ‘dirty dairying’ is typically seen as the driver, other forms of agriculture are likely to contribute even higher concentrations in infiltrating water.”
The “other forms” were discussed at the NZ Institute of Agricultural and Horticultural Science Forum in October 2024.
Further sources were outlined in the institute’s March 2024 Hot Topic #8 – Reconsidering the role of the bovine urine patch in the groundwater quality debate, and included the role of organic matter turnover releasing nitrogen (the more organic matter, the more potential nitrogen loss), the influx of human housing and hence septic tanks, and the mostly overlooked role of gorse, broom and other nitrogen-fixing plants.
Taken together, these sources help to explain why dairy catchments tend to have higher nitrate in the water than drystock catchments – they were initially chosen for dairying because the soils were fertile (generally associated with high organic matter).
Listen to Hamish McKay’s interview with Dr Jacqueline Rowarth on The Country below:
Then more people moved in (septic tanks and their influence on water quality is shown by the presence of sunblock, caffeine and the human contraceptive pill in well water) with their dogs and cats.
The use of superphosphate to address phosphate limitations for legume growth allowed nitrogen-fixing clover to flourish, and the higher nitrogen environment, sometimes supplemented with nitrogen fertiliser, increased grass growth.
The combination allowed organic matter to increase.
Land use change, including decreasing fertiliser, will have the unintended consequence of eroding that organic matter and allowing nitrogen release.
Removing dairying from the Canterbury plains would not change water quality and would severely deplete the economy.
If the Government’s goal of doubling the export value of product is to be realised, so that New Zealand can pay for more bowel cancer screening, better health services and support systems, improved education and upgrading of infrastructure, the dairy cow must be involved.
Minimising the role that she can play dooms New Zealand to borrowing to pay for what society needs.
Dairy feeds more people their essential protein requirements for the fewest GHG, the least nitrogen loss and the highest water use efficiency than other land uses.
A New Year’s resolution should be to re-examine thinking on the role of the dairy cow in New Zealand.
