Be wary in winter with nitrogen
By Bala Tikkisetty
For a range of economic and environmental reasons, it is best to be cautious when applying nitrogen fertiliser to
pasture during winter.
Winter applications of nitrogen are generally least effective in promoting grass growth. Slow growth of pasture and high
rainfall in winter can result in nitrate leaching before plants can take it up. The leached nitrogen eventually makes
its way to waterways, which is both damaging to our rivers and streams, and also literally money down the drain for the
farmer.
Farmers should get clear information about the risks involved with winter nitrogen applications on their individual
properties.
A property’s nutrient budget, combined with a feed budget, helps farmers understand whether they are using too much or
too little fertiliser. From there, they can potentially manage costs better and reduce their impact on the environment
by working out a pragmatic nutrient management plan.
In this context the key term to understand is the “response rate”.
This response rate is the amount of pasture grown in terms of kilograms of dry matter (DM) per hectare (ha) per kilogram
of nitrogen (N) applied.
For example, when 20 kg N/ha is applied and an additional 200 kg DM/ha of pasture is grown the response rate is 10 kg
DM/kg N applied.
The response rate is dependent on a number of factors such as soil temperature, plant growth, soil moisture, the
deficiency of available N in the soil and the rate of N applied per application.
The timing of N fertiliser application is a key consideration when it comes to ensuring nutrient uptake. 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 impact on profitability of applying N is dependent on the utilisation of the extra feed. Therefore, N needs to be
applied to fill genuine feed deficits. Anticipation of feed deficits and application of N fertiliser four to six weeks
in advance is critical to filling these deficits with quality feed and getting the best economic response from
fertiliser use.
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 rates also depend on the season and on the N 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.
Nitrogen fertiliser reduces N fixation by clover by about one kg/ N/ha/year for every three kg N fertiliser applied. In
addition, clover content will be further reduced if nitrogen boosted pastures shade the clover. This effect is seen
during spring.
The amount of N cycling in pastoral systems is greater than other nutrients and it is also more mobile than most other
nutrients. This leads to the potential for significant losses of N into the environment through leaching to ground
water. Excess nitrate levels in groundwater can restrict the use of the water for drinking and can have other impacts on
water quality. Groundwater nitrate moves laterally into streams and lakes where it can affect algae and plant growth,
fish and other animal habitats.
Progressive farmers, irrespective of farming types, are focussing on achieving increased productivity with an aim of
minimising environmental impacts.
Farmers wanting to maximise their N use efficiency should talk to their nutrient management advisor to discuss the
options for their property.
Bala Tikkisetty is a sustainable agriculture co-ordinator at Waikato Regional Council.
ENDS