Farmers should be cautious when applying nitrogen in winter

Milking cows will excrete, in urine, about 70 per cent of the nitrogen (N) they consume. The chance of N leaching from urine patches is much higher in winter due to weather conditions.

Also, farmers should be particularly cautious when applying N based fertilisers to pasture or crops during winter because of the extra risks winter weather poses for nutrient loss.

Winter applications of N are generally least effective for promoting grass growth. Slow growth of pasture in winter and more drainage can result in nitrate leaching before plants can take it up.

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Nitrogen leaching phosphorous runoff not only contaminate the water bodies but also represent a loss of economically valuable nutrients.

Most N is leached during winter and early spring when rainfall exceeds evapotranspiration. Generally, the pasture species, particularly rye grass, are not active during low temperatures adding to the potential for N loss through leaching.

Some of the research on ways to mitigate the N losses has focussed on growing pasture with more rooting depth for intercepting nitrates, duration-controlled grazing for reducing the amount of time animals spend on pasture, and feeding high sugar grasses for reducing the dietary protein.

Recently, a dairy herd improvement company has announced that it is possible to breed cattle that will reduce ‘milk urea nitrogen’ (MUN) resulting in reduced amount of nitrogen leached from grazed pastures.

It is important for farmers to get clear advice from their nutrient advisor about the solutions that best fit their farm to get the best return on their nutrient dollar and the risks involved with winter N applications.

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.

From a technical perspective, farmers need to understand the term ‘response rate’, which is the amount of pasture grown in kilograms of dry matter (DM) per hectare per kilogram of N applied.

For example, when 20kg 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.

The timing of applying N fertiliser is paramount, both in terms of pasture cover and growth. It is good to apply nitrogenous fertiliser when the pasture cover is between 1500-1800 kg DM/ha. This ensures that there is sufficient leaf area for photosynthesis leading to good pasture growth.

The best response to N fertiliser occurs on fast growing pasture, when other factors such as moisture and soil temperature are not limiting growth.