AgResearch scientists Mark Shepherd and Diana Selbie presented their research on nitrate leaching at the DairyNZ Farmers Forum in May.
Pastoral 21 is a collaborative venture managed by AgResearch aiming to provide systems to lift production and reduce nutrient loss.
The salt theory assumes the more water a cow drinks the more she will urinate, effectively spreading the concentration of urine patches.
“We are feeding standard table salt with a little bit of supplementary feed at key nitrogen loss risk periods and it’s resulting in greater water intake, more peeing which means a lower loading rate at each urine patch, so effectively spreading that nitrogen out over a greater area,” Selbie said.
In initial plot trials it decreased nitrate leaching from urine patches by 50%, which indicated at a farm scale it had potential to reduce nitrate leaching by 20%.
The theory was in the second year of being tested at a grazing stage. The salt was administered to cows during late summer through to autumn. In the first year they only administered salt in autumn, but that was too late.
“We were only capturing the reduction in nitrate leaching towards the end of the draining period. We weren’t capturing the benefit we thought we could.”
In the early studies they administered 150g salt/cow/day, but that has been increased to 200g salt/cow/day.
There was a plan to look at different dose rates of salt and its other impacts in the farm system, she said.
When it came to nitrate leaching in dairy farm systems, the big challenge was urine, Shepherd said.
“We can control fertiliser application rates, we can control effluent application and minimise those direct losses.
“But it’s really how we manage that urine, where it’s deposited and at what rate.”
In the New Zealand grazed pasture system grass contained large amounts of crude protein or nitrogen. Most of what was eaten by cows was excreted back to pasture by urine and faeces with any excess going either to the atmosphere or the waterways.
Nitrogen fertiliser and effluent could have a direct affect on nitrate leaching if applied at high rates, when a crop didn’t need it, or at the wrong time and it resulted in ponding or runoff. But most of that could be managed by good farming practices.
The challenge was the indirect nitrogen sources, mainly from urine, Shepherd said.
The four goals Pastoral 21 was looking at to reduce nitrate leaching were excreting less urine, capturing urine, recycling urine and treating urine.
One solution being researched to treat urine was the idea of applying sugar to lock up nitrogen from urine patches. It was a concept that had been looked at in the past, but now there was technology available to detect urine patches in the paddock which made it more applicable.
“It’s opened the opportunity that we could apply a formulation to the patch to try and change some of the processes happening in the urine patch rather than blanket applying sugar,” Selbie said.
Sugar was a source of carbon that when added to the soil acted as an energy source for microbes which retained and locked up nitrogen.
The plot trials had showed a potential reduction of nitrate leaching of up to 66%.
However, an unintended consequence was reduced yield because the sugar locked up the nitrogen and didn’t release it for growth later in the season.
There were preliminary plot trials looking at the use of zeolite instead, because zeolite was able to capture the urine, retain it for a short time and release it for uptake later.
