Reduce winter nitrogen loss
By Bala Tikkisetty
Winter is a time when farmers should take special care to protect both profits and the environment from the effects of
increased nitrogen leaching at this time of year.
Applications of nitrogen fertilisers in winter are generally least effective for promoting grass growth.
That’s because slow growth of pasture and drainage from increased seasonal rainfall can result in nitrate leaching
directly from fertiliser before plants can take it up. The nitrogen can then make its way to waterways where it can
stimulate nuisance algal growth.
At the same time, the risk of nitrogen leaching from animal urine patches is much higher in winter.
So what can farmers do to help counter these impacts?
Some of the research on mitigating nitrogen losses has focussed on growing pasture with more rooting depth for better
take up of nitrates. Other ideas include reducing the amount of time animals spend on pasture and feeding high sugar
grasses for reducing the amount of nitrogen excreted by grazing animals.
It is also a good idea for farmers to get clear advice about the risks involved with winter nitrogen applications.
Nutrient budgeting using computer models such as Overseer, combined with feed budgeting, enables farmers to understand
whether they are using too much or too little fertiliser. From there, they can reduce their impact on the environment by
working out a pragmatic nutrient management plan.
To help make judgments on what’s the right amount of fertiliser to use, it’s important to understand the term “response
rate”.
This refers to the amount of pasture grown in terms of kilograms of dry matter per hectare per kilogram of nitrogen (N)
applied.
For example, when 20kg N/ha is applied and an additional 200kg DM/ha of pasture is grown the response rate is 10kg DM/kg
N applied. The response is dependent on several factors such as soil temperature, plant growth, soil moisture, the
deficiency of available nitrogen in the soil and the rate of nitrogen applied per application.
Timing of nitrogen fertiliser application is paramount. It is good to apply nitrogenous fertiliser when the pasture
cover is between 1,500 to 1,800 kg DM/ha. This ensures that there is sufficient leaf area for photosynthesis leading to
good pasture growth.
The profitability of applying nitrogen is dependent on the utilisation of the extra feed. Therefore, nitrogen needs to
be strategically applied to fill genuine feed deficits.
The best response to N fertiliser occurs on fast growing pasture, when other factors such as moisture and soil
temperature are not limiting growth. Response rate variation also depends on the season and on nitrogen application
rate.
In winter, at the same application rate, responses are lower and slower than other times of the year. The response rate
also declines when the application rate (single dose) is higher than 40 kg N/ha.
Also, nitrogen fertiliser reduces nitrogen fixation by clover by about one kg N/ha/year for every three kg nitrogen
fertiliser applied. In addition, clover content will be further reduced if nitrogen boosted pastures shade the clover.
This effect is seen during spring.
“Nitrogen conversion efficiency” for any farm is another factor to bear in mind. It is calculated from the total
nitrogen in product leaving the farm (such as milk or meat) divided by the total nitrogen inputs into a farm and is
expressed in percentage. A dairy farm with a figure of around 40 per cent is probably doing fine in terms of nitrogen
conversion efficiency.
Bala Tikkisetty is a sustainable agriculture coordinator at Waikato Regional Council. Contact him at
bala.tikkisetty@waikatoregion.govt.nz or 0800 800 401.
ends