Winter is around the corner and currently most farmers are focusing on achieving increased productivity by judicious use
of nutrients with an aim of minimising environmental impacts.
Cooler months can pose a significant challenge for nutrient management, some simple actions can reduce winter nutrient
loss. Caution must be taken when applying nitrogen fertilisers to pasture or crops during winter.
Winter applications of nitrogen fertilisers are generally least effective for promoting grass growth. Slow growth of
pasture in winter and drainage can result in nitrate leaching directly from fertiliser before plants can take it up.
Milking cows will excrete, in urine, about 70 per cent of the nitrogen consumed. The risk of nitrogen leaching from
urine patches is much higher in winter.
Leaching (nitrogen) and run off (phosphorus) losses not only contaminate the water bodies but also represent a loss of
economically valuable nutrients. Most nitrogen is leached during winter and early spring when rainfall exceeds
evapo-transpiration. Generally, the pasture species are not active during low temperatures adding to the potential for
nitrogen loss through leaching.
Some of the research to mitigate the nitrogen losses has focused on growing pasture with more rooting depth for
interception of nitrate, duration controlled grazing for reducing the amount of time animals spend on pasture, and
feeding high sugar grasses for reducing the dietary protein.
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.
It is important that all farmers to understand the technical term, “response rate”.
The response rate is the amount of pasture grown in terms of kilograms of dry matter (DM) per hectare per kilogram of
nitrogen (N) applied. For example, when 30kg N/ha is applied and an additional 300kg 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.
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 30kg N/ha.
Nitrogen fertiliser reduces nitrogen fixation by clover by about 1kg N/ha/year for every 3kg 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 key point to remember. It is calculated from the total nitrogen
in product divided by the total nitrogen inputs into a farm and is expressed in percentage. For a dairy farm, if it is
around 40 per cent, probably the farmer is doing fine.
Further, owing to the prolonged dry spell, nitrate poisoning can result from high nitrate levels in pasture, usually
occur in late autumn. This is particularly during a flush of growth where– nitrate levels build up in herbage as nitrate
is taken by the plant faster than it can be converted into protein. Toxicity risk progressively increases with high soil
nitrogen from various sources.
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.
Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council. Contact him on 0800 800 401 or email bala.tikkisetty@waikatoregion.govt.nz