Caution urged on winter nitrogen

By Bala Tikkisetty

A cautious approach is recommended when it comes to applying nitrogen fertilisers to pasture during winter.

This is because applications in winter in particular can contribute to reducing water quality in the region’s waterways.

Winter applications of nitrogen fertilisers are generally least effective for promoting grass growth. Slow growth of pasture and greater drainage in winter can result in nitrate leaching directly from fertiliser before plants can take it up. That’s when nitrogen can more easily make its way to waterways where it can stimulate nuisance algal growth.

I recommend that farmers get clear advice about the risks involved with winter nitrogen applications on their individual properties.

Whatever the season, nutrient budgeting, 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. This becomes all the more important in winter due to the increased risks of leaching.

From a technical perspective, one of the key things for farmers to understand when making winter application decisions is the “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, 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.

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.

“Nitrogen conversion efficiency” for any farm is another key point to be remembered. 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 farmer, around 40 per cent means efficiency is generally fine. However, this must be matched with low nitrogen leaching rates measured in kg of nitrogen lost per ha per year. This can be calculated by your nutrient advisor using the Overseer nutrient budgeting programme. All dairy farmers should be aiming for a leaching figure in the low 30s.

For those seeking such efficiency and increased productivity, whilst minimising the environmental impacts of their operations, a nutrient management plan prepared by a competent nutrient advisor is essential.

As suggested, avoiding or minimising nitrogen fertiliser application in late-autumn or winter helps achieve this goal by reducing the likelihood of any direct leaching of nitrogen.

Bala Tikkisetty is a sustainable agriculture coordinator at the Waikato Regional Council and is available for nutrient management advice on 0800 800 401.