Urine from New Zealand's 4.8 million milking cows is putting 200 tonnes of leached nitrogen into our groundwater each day, new research suggests.
The calculations come from herd improvement company CRV Ambreed, which has determined the daily numbers using existing information related to milk urea (MU) concentration in daily bulk milk reports.
The company's head of research and development, Phil Beatson, estimated the national dairy herd was excreting 1000 tonnes of nitrogen each day - of which 20 per cent was leached through our soils and ultimately ended up in ground water.
If too much nitrogen entered waterways it could cause aquatic weeds and algae to grow too fast, reducing oxygen and harming aquatic life.
Beatson said dairy farmers could make use of the MU value on their daily bulk milk reports to calculate the amount of nitrogen their herd is excreting in urine and take steps to address that.
Research from overseas and in New Zealand shows a direct relationship between MU and grams urinary nitrogen excreted per day per cow.
On average, the relationship between MU and urinary nitrogen was about seven grams urinary nitrogen per one unit of MU.
As the average MU in New Zealand was 30 units, the average cow was excreting 210 grams of urinary nitrogen a day.
With 4.8 million cows in milk, nationally there was 1000 tonnes of nitrogen hitting the ground in urine every day, he said.
"This is an issue we must address now to safeguard future generations of New Zealanders," he said.
"It is important to realise that not all farms are the same in terms of milk urea, nitrogen excretion and leaching from their cows. Reducing milk urea, nitrogen excretion and leaching is achievable – and quickly."
He believed the most the effective way to deal with the nitrogen leaching issues was to reduce the amount of nitrogen hitting the ground - and that meant changes in the way we breed and feed cows.