Managing nitrogen fertiliser use

When to use nitrogen fertiliser

The decision to apply nitrogen fertiliser should take into account pasture growth needs, soil conditions, and the cost of growing and harvesting the extra pasture relative to milk price. The profitability of applying nitrogen depends on the extra feed grown (the response) being utilised for animal production. This means that nitrogen either needs to be applied to fill genuine feed deficits, or any surplus feed needs to be identified and harvested to make quality supplement.

It is important to remember that a response from pasture to a nitrogen fertiliser application occurs over 3-14 weeks and therefore should be applied in anticipation of a feed deficit.

Nitrogen fertiliser response is highly dependent on several environmental factors:

• Amount of available mineral nitrogen in the soil – the greater the deficit, the higher the response.
• Soil temperature – the warmer the soil, the greater and more immediate the response.
• Pasture height and growth – a minimum height of around 1500 kgDM/ha (or 3.5cm) and the faster the growth, the greater and more immediate the response.
• Moisture – too much or too little water will lower the response.
• Rate of nitrogen applied per application – there is a diminishing response at high application rates (>50 kg N/ha).
• Pasture sward health – ryegrass dominant swards will have a greater response than a poor performing pasture which is due for renewal.
• Seasonal change – response rate depends on the season and on the application rate (see below for more).

Getting the timing and application rate of nitrogen fertiliser right will help increase efficiency and minimise nitrogen loss from your farm. It is important to follow the ‘4 Rs’ of good fertiliser management:

Right place

• Ensure fertiliser is placed where the crop or pasture can use it most efficiently. Avoid areas of high fertility such as stock camps, effluent paddocks, or areas that may have a low response to nitrogen.

Right time

• Environmental factors such as soil temperature and soil moisture can affect response.
• Check the weather to avoid applying nitrogen before significant rainfall or drainage events to avoid direct losses of nitrogen fertiliser to waterways.
• Use tools such as feed budgets and feed wedges to identify when a potential feed deficit could be coming, and how best to manage it.

Right rate

• Seek advice from a fertiliser rep or farm consultant alongside regular soil and plant testing to inform what rate of fertiliser is needed.

Right product

• There are several different types and forms of fertilisers that all have different percentages of nitrogen. Seek advice from your local sales or fertiliser rep to know which product is the right one for your situation.
• Urease-coated fertilisers reduce gaseous losses to the atmosphere from nitrogen use and maximise the nitrogen available for plant uptake. This means less nitrogen fertiliser needs to be used. For more on the nitrogen cycle, see the reducing nitrogen loss page.
• Fertigation is fertiliser in liquid form, distributed through an irrigation unit. Trials have shown that there is not a higher response rate to applying nitrogen in a liquid form compared to nitrogen in solid form. However, it allows lower and more precise applications for farms that are suited to the set up.

Planning, recording, and reporting

Having an annual plan for nitrogen fertiliser use can help maximise the response rate and improve efficiency of use. Contact your local fertiliser rep or farm advisor to help you set up a plan.

Recording and reporting your annual nitrogen use across your farm(s) is also required to ensure you are meeting the nitrogen cap regulations.

Pasture response rate

The amount of pasture grown in kg DM per kg N/ha applied is the nitrogen ‘response rate’. For example, where 30kg N/ha is applied, and an additional 300kg DM/ha of pasture is grown, the response rate is 10kg DM/kg N fertiliser applied.

The best response to nitrogen fertiliser occurs on fast-growing pasture, when other factors such as moisture and soil temperatures are not limiting growth, and when all other soil nutrient levels are satisfactory.

Results from research projects, including the Forages for Reduced Nitrate Leaching (FRNL) trials, showed that a maximum of 25 to 40kg N/ha are the best application rates. 40kg N/ha application rate was only useful when conditions for pasture growth were optimal.

Managing applications over the seasons

Late winter/early spring (July-September)

Profitable milk production responses are commonly achieved from nitrogen application in this period where there is a genuine feed deficit. That is when it is applied between the planned start of calving and before balance date – i.e. when feed demand is greater than pasture growth.

The reasons why this is a great time to apply nitrogen fertiliser include:

• From late autumn to early spring, lower temperatures restrict clover growth and soil microbial activity, and therefore lower nitrogen fixation and soil nitrogen mineralisation, resulting in less nitrogen available for the pasture.
• Nitrate is also leached from the soil over the late autumn/winter period, reducing plant available nitrogen in the soil.
• In districts with warmer winters, soil microbial activity is higher for longer and this, combined with higher pasture growth, depletes the available soil mineral nitrogen. In these districts, spring nitrogen deficiency can become evident ahead of the onset of rising temperatures that trigger increased rates of soil nitrogen mineralisation.

Note: it takes about 20 days to see a significant pasture yield response to a nitrogen fertiliser application during late winter/early spring.

Spring (October-November)

In spring, temperature and moisture usually don’t limit nitrogen uptake for grass growth, so rapid responses to nitrogen fertiliser can be expected (about 20 days).

There is an increased risk of wastage of the extra pasture generated, reducing the profitability of nitrogen use. Have a plan as to how surplus pasture can be best utilised and practice good pasture management to ensure clovers and spring tillers aren’t affected by canopy closure.

When nitrogen applications are done in early summer, it can stimulate the development and growth of new ryegrass tillers. Nitrogen applications boosted tillering by 37 percent, with an extra 800 kg DM/ha and 66 kg MS/ha produced from December to April after split applications of 100kg N/ha was applied.

Summer (December-February)

At this time of year, clover is fixing nitrogen in the soil, which means mineral nitrogen is likely available. Nitrogen is therefore unlikely to be a limiting factor in pasture growth. If soil tests show nitrogen to be a limiting factor, nitrogen fertiliser applications need to be made before moisture and temperatures also become limiting factors. For farms that have irrigation available, using urease-coated fertiliser can reduce the risk of volatilisation to the atmosphere which occurs in hot and windy conditions.

Autumn (March-April)

Nitrogen has a role in restoring pasture cover after summer or autumn dry periods. After a dry summer, there is normally a considerable pool of nitrogen already in the soil, although sometimes a lot of this nitrogen is unavailable for plant uptake due to immobilisation. In other years, soil moisture is quickly restored through significant rain events that can wash available soil nitrogen below the plant root zone, especially in free-draining soils. It is therefore important to time fertiliser applications well so that plants receive nitrogen while the soil processes recover, and nitrogen doesn’t become a limiting factor for pasture growth. Responses to nitrogen applied in autumn are variable from 4:1 to 10:1, or about 40 days.

There are potential risks when applying nitrogen during this period, including:

• Adding more nitrogen to the pool of nitrogen that is subject to drainage and leaching during winter. Nitrogen fertiliser should be applied before the soil temperature drops below 7°C so that plant uptake occurs.
• Pasture growth rates going into winter exceeding normal levels once the pool of soil nitrogen accumulated during the dry period is mobilised.
• Adding to the large pool of soil nitrogen in the autumn increases the risk of nitrate poisoning in late autumn and winter, especially when grazing new grass and annuals.

Winter (June-July)

Avoid nitrogen applications during this time as the response rate is limited by soil temperature and slow plant uptake and there is a much higher risk of leaching due to high soil moisture.