Farmers must be very cautious when applying nitrogen fertilisers to pasture during winter, says Environment Waikato sustainable agriculture coordinator Bala Tikkisetty.
"While fertiliser use is an important input for maintaining soil fertility, its inappropriate use can lead to environmental problems that, if not addressed, may pollute waterways," said Mr Tikkisetty.
"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. The nitrogen can make its way to waterways where it can stimulate nuisance algal growth."
Mr Tikkisetty said it was important for farmers to get clear advice about the risks involved with winter nitrogen applications.
He said nutrient budgeting, combined with feed budgeting, enables farmers to understand whether they are using too much or too little fertiliser and, from there, reduce their impact on the environment by working out a pragmatic nutrient management plan.
From a technical perspective, Mr Tikkisetty said all farmers need to understand the term "response rate".
The response rate is the amount of pasture grown in terms of kilograms of dry matter per hectare 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 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. The profitability of applying nitrogen is dependent on the utilisation of the extra feed. Therefore, nitrogen needs to be applied to fill genuine feed deficits. Anticipation of feed deficits and application of nitrogen fertiliser four to six weeks in advance is critical to filling these deficits with quality feed and getting the best economic response from nitrogen 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 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 is higher than 30 kg N/ha.
Further, 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.
The amount of nitrogen 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 nitrogen into the environment through leaching to ground water. Excess nitrate levels in groundwater will 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.
"The farming industry is focussing on achieving increased productivity with an aim of minimising environmental impacts," said Mr Tikkisetty.
"Efficient use of nitrogen from fertiliser and other sources is an important component of this focus. Avoiding or minimising nitrogen fertiliser application in late-autumn or winter reduces the likelihood of any direct leaching of fertiliser nitrogen."
Mr Tikkisetty is available for nutrient management advice on 0800 800 401.