Cutting nitrogen loss in winter
Winter’s a much riskier season for nitrogen leaching from urine patches on pasture to waterways.
Milking cows will excrete, in urine, about 70 per cent of the nitrogen they consume. The chance of nitrogen leaching
from urine patches is much higher in winter due to weather conditions.
Also, farmers should be particularly cautious when applying nitrogen fertilisers to pasture or crops during winter due
to the extra risks winter weather poses for nutrient loss.
Winter applications of nitrogen fertilisers 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.
Leaching (nitrogen) and runoff (phosphorus) losses not only contaminate the water bodies but also represent a loss of
economically valuable nutrients.
Most nitrogen 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 nitrogen loss
Some of the research on ways to mitigate the nitrogen losses has focussed on growing pasture with more rooting depth for
interception of nitrate, 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 nitrogen applications.
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.
From a technical perspective, 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
Timing of nitrogen fertiliser application is paramount, both in terms of pasture cover and growth. 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 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 (single dose) is higher than 40 kg N/ha.
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.
Further, nitrogen fertiliser reduces nitrogen fixation by clover by about one kg N/ha/year for every three kilograms
nitrogen fertiliser applied. In addition, clover content will be further reduced if nitrogen boosted pastures shade the
clover. This effect is seen during spring.
“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 farm,
if it is around 40 per cent, probably the farmer is doing fine.
One ideal state to aim for is increased productivity by judicious use of nutrients that also minimises environmental