Cows' urine source of lake trouble

Leaching of cows' urine, not nitrogen fertiliser, is the main cause of nutrient enrichment affecting the health of Lakes Rotoiti and Rotorua in the Bay of Plenty, according to AgResearch scientists.

Ruakura-based scientist Anwar Ghani has reported that studies showed the main source of nitrogen was cows' urine, even though it had commonly been thought nitrogen fertiliser leaching was the main culprit.

But nitrogen fertilisers did increase grass production, allowing more animals to graze. That resulted in more urine leaching into waterways.

Phosphorus from fertilisers, meanwhile, also got into the lakes through sediment run-off from paddocks.

The regional council, Environment Bay of Plenty, is developing rules aimed at cutting the amount of phosphorus and nitrogen that gets into water from rural and urban uses.

AgResearch has been trialling removing cows from farms during winter and installing "stand-off pads" made of bark or other material that absorb nitrogen.

Research suggested that removing cows during winter could cut nitrogen leaching by 40-50 per cent.

Ghani also said AgResearch was testing an inhibitor that could be applied to paddocks, slowing the conversion of urine into easily leached nitrates.