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Reduce winter nitrogen loss

21 May 2018

The coldest months of the year are just around the corner, so farmers should be particularly cautious when applying nitrogen fertilisers to pasture or crops.

Winter applications of nitrogen fertilisers are generally the least effective for promoting grass growth. That’s because slow growth of pasture in winter and greater drainage can result in nitrate leaching before plants can take it up. The nitrogen may make its way to waterways where it can stimulate nuisance algal growth.

Lactating cows excrete, in urine, about 70 per cent of the nitrogen they consume. Again, the risk of this nitrogen leaching from urine patches is much higher in winter.

Some of the research to mitigate nitrogen losses has focused on:

• growing pasture with more rooting depth for interception of nitrate

• reducing the amount of time animals spend on pasture

• feeding high sugar grasses to reduce the amount of nitrogen lost in urine.

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. By doing this, farmers can optimise the use of nutrients and reduce the impact on the environment by working out a pragmatic nutrient management plan.

Understanding the term “response rate” helps farmers when it comes to implementing these plans. The response rate is the amount of pasture grown in terms of kilograms of dry matter (DM) 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.

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.

It is better to apply nitrogenous fertiliser when the pasture cover is 1500-1800kg DM/ha. This ensures that there is sufficient leaf area for photosynthesis leading to good pasture growth.

Also, nitrogen fertiliser reduces nitrogen fixation by clover by about 1kg N/ha/year for every 3kg of N fertiliser applied. In addition, clover content will be further reduced if nitrogen boosted pastures shade the clover. This effect is seen during spring.

Remember that 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.

Nitrogen conversion efficiency for any farm is another key point to be remembered. This is measured by calculating total nitrogen in product divided by the total nitrogen inputs into a farm and is expressed as a percentage. A dairy farm, for example, is probably doing fine with about 40 per cent.

Reducing nitrate leaching by adapting grazing systems to alternative plant species and cultivars has been a major focus of research in the recent past. Plantain emerged as an important pasture herb that has a proven ability to reduce nitrate losses. Plantain reduces the concentration of N in the animal urine and allows plants to take up a greater proportion of the N, which in turn reduces leaching.

I am happy to say that a number of farmers, as well as industry organisations, are already doing a great job trying to increase productivity and reduce environmental impacts through more careful use of nutrients.

• Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council. Contact him on 0800 800 401 or at bala.tikkisetty@waikatoregion.govt.nz

ends

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