Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.4 Responsible management of N fertiliser

Responsible management of N fertiliser begins with the question – why should it be applied? It then comes down to the 4Rs: use the Right Source of fertiliser, at the Right Rate, in the Right Place and at the Right Time. Find out how to work through each of these steps, and how to work out the economics of N fertiliser use.

12.4.1 Why to apply?

The first question to ask before N is used should be why you want to use it, or what you expect it to do. As previously discussed, N stimulates growth and so its most frequent use is to increase pasture or crop growth rates or overall yield, to help fill expected feed gaps. This can be in a strategic context, where there is a budget for N use over the year, targeted at periods when it’s expected that it could have an impact, or in a tactical context, were N is used usually at shorter notice, often to address unexpected feed gaps. However, many farmers are regularly applying N after most grazings throughout most of the year, with variable results.

A less common, but still valid, use of N is to increase tillering at certain times of the year. Nitrogen might also help plants survive stress periods if applied up to a month prior to the stress – e.g. late autumn to better survive winter frosts, late spring to better survive summer heat/drought. However care must be taken in timing of N application, as if applied too close to the stress period (e.g. within 1-2 weeks), the stress effect could be worse.

Since stimulating growth is by far the most common reason for farmers to apply N, the following sections will focus more on this, with any additional benefits of stimulated tillering or healthier plants being taken as secondary effects which may occur.

Once the question of why N should be used has been addressed, the next steps should be to work out where and when it might best be used.

12.4.2 Right Place

Nitrogen fertiliser application will have greatest response when it is the major limiting factor to growth. What this important fact means is that in situations where plants are stressed or growth is being held back by lack of other nutrients, or low soil pH, dry/hot/cold conditions, or overgrazing, then applying N will have little or no effect, might even make the situation worse, and will result in a waste of the N applied.

So when determining where to apply N, the following key aspects should be taken into account to ensure the best result from N fertiliser application:

  • Apply N to pastures with a high density of desirable (i.e. sown) species. Applying N to pastures where weed species have invaded, will result in larger, healthier weeds and have no beneficial effect on feed supply for grazing cows.
  • Apply N to pastures with a good ground cover. Gaps or bare areas in pastures will result in more N lost through leaching and/or volatilisation.
  • Apply N to pastures that have no limitations to major soil nutrients. Regularly soil testing will establish the nutrient status of the soil and if other major nutrients or pH are limiting growth, these can be addressed before or at the same time as the N application.
  • Apply N to pastures that are not waterlogged, or in drought, or being overgrazed (grazed at less than the 2-leaf regrowth stage for most sown grass species).

The general, principles of using N fertiliser are not affected by differences between pasture types, or between pastures and crops. Remembering that N will have the greatest effect when it is the most limiting factor to growth, is important when planning to use N.

12.4.3 Right Time

In considering timing of N fertiliser to pastures or crops, the key considerations are in relation to season, stage of growth or regrowth, and soil conditions. These will be considered in the following sections, along with the economics of N application with season, which is a key consideration by many farmers.

12.4.3.1 Season

Timing in relation to season might use a feed budgeting approach, which is a tool that estimates likely pasture shortages and can indicate where N fertiliser might best be applied, as well as develops a strategy for supplementary feeding. A basic form of feed budgeting is to consider demand for feed from stock in relation to supply of feed from pasture and crops, and use this to indicate when N is best applied.

For example, depending on region, soil type, irrigation availability and climatic conditions, pasture shortages can occur at any time of the year – in autumn as daylength and temperature decrease, over winter as overcast and cold conditions limit growth, in dry spring conditions, and over summer under dryland conditions. Pasture types can have a further effect: in subtropical and tropical regions when tropical grasses such as kikuyu stop growing over cooler winters, and in areas where hot summers cause ryegrass to become dormant and species such as paspalum or kikuyu may form the basis of summer pasture production (parts of northern Victoria and Western Australia).

As previously explained, using N when growth is mainly limited by dry soil conditions for example, is a waste of time, however N can successfully be used in the majority of the above situations, either before, or even during, the period of pasture shortage, to stimulate growth to reduce the shortage.

12.4.3.1.1 Economics of seasonal N fertiliser application

The cost of alternative feed sources can influence the cost effectiveness of applying N fertiliser. Research conducted in the 3030 Project in western Victoria found that assuming an N response efficiency of 10 kg DM per kg of N applied and a cost of 8.7 cents per extra kg of DM grown, application of N in autumn was highly profitable, but only marginally profitable in spring. This was due to the high cost of alternative feed in autumn. If there is enough soil moisture, application of N fertiliser in summer may also be cost effective, due to the shortage of other feed products. In comparison, additional pasture grown in spring is often more than the milking herd needs and therefore needs to be conserved as hay and silage, which increases the cost of growing the grass in the first place. Figure 12.3 highlights the important things to consider when assessing the economics of N fertiliser application.

Figure 12.3 Things to consider when assessing the economics of N fertiliser application.
Figure 12.3 Things to consider when assessing the economics of N fertiliser application.
Source: 3030 Project (Perennial ryegrass management: V. Use of N fertiliser).
12.4.3.2 Stage of growth/regrowth

When applying N to pastures, key considerations are when to apply in relation to grazing, and how long to graze again after N has been applied.

Nitrogen is most often applied in the first few days after a pasture has been grazed, when plants are beginning to regrow, and this is a successful strategy as long as pastures are actively regrowing, with no other major limitations to growth, as mentioned previously. When regrowth is fast, as in spring or summer under good irrigation, best results are obtained from applying N within the first 1-3 days after grazing. When growth slows, as in winter, the timing can be extended to the first week after grazing. Nitrogen is far less efficient when applied partway through regrowth.

Urea can also be applied to pastures up to 2-3 days prior to grazing, as long as there is adequate soil moisture to allow the N fertiliser to dissolve and move into the soil, to reduce the risk of fertiliser being ingested by cows and causing ammonia toxicity (see Section 12.6.1). This is based on the understanding that it takes a few days in total for N to dissolve from the fertiliser granules, move into soil solution, be taken up by plants, and end up in leaves. It is very important to realise however, that in periods of rapid growth, fertiliser N might start appearing in plant leaves 3-4 days after being applied. If N is applied more than 2 days before plants are grazed, potentially a large amount of the N can be grazed off while it’s still in a soluble form in the plant and before it has contributed to plant growth. As well as being a waste of N in terms of not stimulating plant growth, it can also cause animal health issues as discussed in Section 12.1.4 and Section 12.6.1.

After N has been applied, it is important to remember the dilution effect discussed in Section 12.1.3. If pastures are grazed too soon after N application, then a high proportion of the N applied will still be in a soluble form in the younger leaves of the plant, will have contributed only marginally to extra growth, and will be very likely to be grazed off by stock, causing an upset to the rumen and resulting in more N being excreted in the urine. Applying the dilution effect means waiting for 2 leaves/tiller to regrow after applying N to ryegrass and tall fescue pastures, and waiting for 3 leaves/tiller to regrow after applying N to brome (prairie grass), cocksfoot and kikuyu pastures. This may be between 18-25 days at the earliest when growth is fast (e.g. spring), and 30-40 days at the earliest when growth is slow (e.g. winter), but will vary between regions, within seasons and years. The most accurate measurement is to not assume leaf growth, but to measure it.

It is important to wait for 2 leaves/tiller to regrow after applying N to ryegrass and tall fescue pastures, and waiting for 3 leaves/tiller to regrow after applying N to brome (prairie grass), cocksfoot and kikuyu pastures to allow pasture N concentrations to decrease before grazing.

So the key in applying N to pastures is to firstly add the N when plants are responsive, either 1-2 days before grazing or within 3 days after grazing when growth is fast (e.g. spring), or up to 5-7 days after grazing when growth is slow (e.g. winter). Then leave a long enough period after applying N, to allow the N to be taken up by the plant and result in extra growth, and not nitrate toxicity (see Section 12.6.1) or excess protein issues with grazing stock.

When applying N to crops, the key timing consideration is when to apply in relation to crop growth and harvest. Nitrogen is often applied to ensure high yields from pasture conservation (silage and hay) as well as from fodder crops (e.g. brassicas, maize, cereals, chicory).

In the case of pasture conservation, it is recommended to apply N at a higher rate (up to 50 kg of N/ha, see Section 12.4.4) after grazing when the pasture is closed up for conservation. This strategy is most efficient when there is a genuine surplus of pasture, so that the pasture receiving higher rates of N doesn’t have to be grazed off prematurely, as might be the case when growth rates temporarily drop in a period when pasture isn’t in surplus. Results from using split applications of N, for example after grazing and again partway through regrowth, are more variable, and this practice isn’t recommended.

In the case of crops, especially those with a long period prior to harvest (e.g. 10-20 weeks), it is recommended to apply a smaller amount of N early in growth (20-30 kg N/ha soon after crop emergence, when plants are actively growing), followed by several applications (40-50 kg N/ha) as the crop develops, but prior to maturity and harvest.

In all cases, in deciding how much N to apply, applications should be based on a budget and appropriate soil or plant testing (see Section 12.1.2.1 and Section 12.1.3.1). For example, harvesting a 20 t DM/ha crop of maize removes around 200-240 kg N/ha (Kaiser et al. 2003), and a 10 t DM/ha crop of turnips can contain up to 320 kg N/ha (DairyNZ, 2008). The N fertiliser inputs required to achieve these high yields will largely depend on the existing soil N pool at each site, and so it is recommended to undertake a soil test prior to a crop being established, followed by tissue testing of the crop as it develops, to best match N demand with N fertiliser supply.

12.4.3.3 Soil conditions

Timing of N in relation to soil conditions relies on appropriate temperature and moisture levels.

Temperate pasture grasses (e.g. ryegrass, cocksfoot, tall fescue, phalaris, brome (prairie grass)) generally respond to N fertiliser when soil temperatures are above 4°C, and subtropical pasture grasses (e.g. kikuyu) respond to N fertiliser when soil temperatures are above 10°C.

If pastures are no longer growing due to low soil temperatures, then pastures are unlikely to respond to N fertiliser applications.

In cold conditions, plants tend to take up N as ammonium in preference to the nitrate. This is because less ammonium is being nitrified to nitrate. Under warm, aerobic soil conditions, more nitrate will be available for pasture uptake compared to ammonium. However, under wet and cold soil conditions, ammonium is the dominant source of N for growing pasture.

12.4.4 Right Rate

Australian dairy industry research has found that the most efficient pasture growth responses occur when N fertiliser is applied at rates of between 25-50 kg N/ha at any one time. Rates below or above these recommended rates produce pasture responses which are unlikely to be economic.

The most efficient pasture growth responses occur when N fertiliser is applied at rates of between 25-50 kg N/ha at any one time.

Effective use of N fertiliser involves good grazing management to utilise the extra feed grown. This improves the economics of applying N fertiliser, as strategic use of N fertiliser can be effective at filling feed gaps. High pasture utilisation from each grazing must be achieved to maximise production and to minimise the cost of applying the N fertiliser. Table 12.3 shows the cost of extra pasture grown at various levels of pasture utilisation using urea (at $500/t ex Geelong, June 2013) over a range of application rates.

Table 12.3 Effect of pasture utilisation on cost of N-fertilised pastures over a range of application rates. Based on average 2013 urea prices.
Table 12.3 Effect of pasture utilisation on cost of N-fertilised pastures over a range of application rates. Based on average 2013 urea prices.

There is great debate over how much N fertiliser can be used in one year and its effects on N losses to the environment. There are no clear recommendations for Australian dairy systems, however New Zealand researchers suggest that N fertiliser rates above 200 kg/ha/year, substantially increase the risk of N leaching losses (Cameron et al. 2002; Meneer et al. 2004).

12.4.5 Right Source

The type of N fertiliser to be applied will depend on the cost per unit of elemental N and whether other nutrients are being applied (see Section 12.2.6). For example, if soil P is low, it may be more cost-effective to use DAP instead of urea and an additional fertiliser product containing P. When choosing an N fertiliser product it is important to select a product which will minimise environmental losses (see Section 12.3). For example, it is better to apply urea or an ammonium based fertiliser on waterlogged soils rather than nitrate based fertilisers, in order to reduce denitrification and greenhouse gas emissions.

Examples of comparing fertiliser costs are given in Chapter 14 in this manual.

The amount of fertiliser to apply (the fertiliser application rate) can be calculated from the desired nutrient application rate and the per cent of nutrient in the fertiliser, using the formula below:

Nutrient application rate (kg/ha) = % of nutrient in the fertiliser x 100 = Fertiliser application rate (kg/ha)