Skilful use of nitrogen lifts output
It’s sensible to be cautious when applying nitrogen fertiliser to pasture during winter, for a range of economic and environmental reasons.
Winter applications of such fertiliser are generally least effective for promoting grass growth.
Slow growth of pasture in winter and excessive drainage can result in nitrate leaching directly from fertiliser before plants can take it up. And any ‘‘excess’’ nitrogen can make its way to waterways.
So it’s important that farmers have clear information about the risks involved with winter nitrogen applications on their individual properties.
A property’s nutrient budget, combined with a feed budget, helps farmers understand whether they are using too much or too little fertiliser.
From there, they can potentially manage costs better and reduce their impact on the environment by working out a pragmatic nutrient management plan.
From a technical perspective, the key term to understand is the ‘‘response rate’’. This 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 N in the soil and the rate of N applied each application.
The timing of N fertiliser application is a key consideration when it comes to ensuring nutrient uptake.
It is good to apply nitrogenous fertiliser when the pasture cover is between 1500 to 1800kg DM/ha.
This ensures that there is sufficient leaf area for photosynthesis leading to good pasture growth.
The impact on profitability of applying N is dependent on the utilisation of the extra feed.
Therefore, N needs to be applied to fill genuine feed deficits.
Anticipation of feed deficits and application of N fertiliser four to six weeks in advance is critical to filling these deficits with quality feed and getting the best economic response from fertiliser use.
The best response to N fertiliser occurs on fastgrowing pasture, when other factors such as moisture and soil temperature are not limiting growth. Response rates also depend on the season and on the N 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.
Nitrogen fertiliser reduces N fixation by clover by about one kg/ N/ha/year for every three kg N 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 N 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 N 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.
Overall ‘‘N conversion efficiency’’ for a farm is calculated as a percentage of the total N in farm product divided by the total N inputs into a farm.
For a dairy farm, around 40 per cent is probably a reasonable score.
The progressive farmers, irrespective of farming types, are focusing on achieving increased productivity with an aim of minimising environmental impacts.
Efficient use of nitrogen from fertiliser and other sources is an important component of this strategy. Avoiding or minimising N fertiliser application in late autumn or winter reduces the likelihood of any direct leaching to waterways.