Guidelines for the use of nitrogen fertiliser on rain-fed dairy pasture in South Eastern Australia
Dr Richard Eckard, The Institute of Land and Food Resources,
University of Melbourne & Agriculture Victoria, Ellinbank

Summary
Introduction
Before You Consider Nitrogen 1. Basal Fertility 2. Soil Temperature 3. Pasture Species 4. Soil Moisture
When to Apply Nitrogen - T Sum 200
How Much Nitrogen to Apply
What Sources of Nitrogen to use
Negative Effects of Nitrogen use 1. Environmental Implications 2. Animal Health
In Conclusion

Summary

    1. No N: Rely on clover for N – use no N fertiliser (cool season production will be restricted, suited to low stocking rates);
    2. High N: Apply N after every grazing as long as moisture allows pasture growth (may result in unacceptable N loss and inefficiency, suited to very high stocking rates);
    3. Strategic N: Apply N strategically (N used as a supplement not a fertiliser), as and when additional grass is required and when clover growth is limited.
    1. Basal fertility is high;
    2. There is adequate soil moisture for growth;
    3. Soil temperatures are not too low for grass growth (below 4C), and
    4. The pasture is of the best species composition available (mostly ryegrass).
    1. If there is sufficient moisture and temperature for grass growth N fertiliser can accelerate that growth;
    2. 50 kg N on your best half of the farm is better than 25 kg N over the whole farm applied to both good and poor areas.
    3. Where growth rate is low (15 kg DM/ha/day), the application of N fertiliser can potentially double it. Where growth rate is high (60 kg DM/ha/day), N fertiliser may increase it by one-quarter.

 

INTRODUCTION

Nitrogen (N) fertiliser is a management tool for manipulating seasonal pasture production, as all plants need N for growth. In recent years we have seen an exponential increase in the use of N fertiliser on pasture in Victoria (Figure 1). This article aims to provide some practical management guidelines for the use of N fertilisers on intensive dairy pasture.

Figure 1. Trends in N fertiliser sales to pastoral farmers in Victoria over the past 15 years.

  1. Three N strategies

There are 3 main N strategies that we see on farms:

  1. Temperature effects

Clover is more temperature sensitive than ryegrass and does not fix much N through the winter months. Once the soil temperature is below around 10C (at 10cm) the supply of N from clover is limited, yet perennial ryegrass can continue to grow and respond to N down to around 4C soil temperature. This means that the grass component may be short of N during the cooler months of the year (refer to the arrows in Figure 2). In South Eastern Australia, clover will only fix adequate quantities of N between September and March, depending on the season and assuming that clover makes up at least 25 to 30% of the pasture. Unfortunately, it is during the cooler period of restricted pasture growth when farmers require more pasture.

Rule-of-Thumb
If the pasture is growing, N fertiliser can increase that growth rate.

Another rough guide is that if growth rate is low, N fertiliser can potentially increase growth rate more (80 - 100%) than at higher growth rate (15 - 20%) (i.e. 10 kg DM/ha/day to 20 kg DM/ha/day, but only 60 kg DM/ha/day to 72 kg DM/ ha/ day).

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Figure 2. The relationship between potential grass yield and the ability of the clover to supply the total N needs of the pasture. Arrows and question marks indicate periods of the year when the N demand of the grass exceeds the clovers ability to supply. Dotted lines indicate potential summer yields if moisture is not limiting.

 

BEFORE YOU CONSIDER NITROGEN

The following factors all play a role in the pasture response to N fertiliser and should be considered before N fertiliser is applied to a paddock.

1. The Effect of Basal Fertility

If any other nutrients (P, K, lime) are limiting this will limit the response to N. Therefore, target high fertility paddocks with N applications for the best response. Remember 50 kg N on your best half of the farm will grow more grass than 25 kg N over the whole farm if this includes poor fertility paddocks.

Figure 3. An illustration of the effect of basal fertility of the N responsiveness of a pasture (Frank McKenzie, Western Victoria). 

 

2. Soil Temperature

Below 3 - 4 C soil temperature the grass response to N will be limited and N may be susceptible to leaching due to winter rain (see Figure 2). A north facing slope may be as much as 2C warmer than a south facing slope in winter. Figure 5 shows the effect of soil temperature on pasture growth rates.

3. Species Composition

Annual, short rotation and perennial ryegrasses respond most efficiently to N fertiliser in that order. Therefore it would make sense to target pastures high in these species. Generally, a pasture with poor species present (i.e. fog grass, brown top/ bent grass, sweet vernal) is a sure indicator of poor fertility as well; N fertiliser applied to such a pasture would be wasted. The primary investment here would be other basic fertilisers and renovation.

Figure 4. An illustration of the relative responsiveness of different pasture species to N fertiliser.

4. Soil Moisture

If the pasture is even slightly moisture stressed the response to N fertiliser will be restricted. At least 8 to 10 mm of rain is required in order to dissolve N fertiliser into the soil. Ideally N fertiliser should be applied to a dry soil surface just before rain or spray irrigation. If the root zone is not totally dry, usually 20 to 25 mm of rain in the days before N application will leave sufficient moisture is the soil surface to for N to be utilised by the pasture. If N is applied to a damp soil during a period of high evaporation, about 10 to 15% of N may be lost (volatilised) from urea.

Under waterlogged conditions, commonly encountered in the low lying pastures in winter or flood irrigation areas, Urea is the best source of N fertiliser to use, as waterlogged pastures tend to take up more ammonia from the soil than nitrate. In addition, nitrate is highly mobile in soil water and could leach more readily as the water drains away.

 

WHEN TO APPLY NITROGEN FERTILISER

"Nitrogen fertiliser should be viewed as a supplement and not a fertiliser". This statement will help farmers think of N in the right context. For example, you estimate that you will be short of feed in the mid June. You can either buy in grain at 35c/kg DM, or buy silage at 27c/kg DM, or you could apply 50 kg N/ha to 15 of your best paddocks, knowing that you will get a return of 10:1 or at a cost of between 11 c/kg DM.

It is important to note that there are times of the year when N is a very useful pasture management tool, and other times when its efficiency is greatly reduced. The following guidelines apply:

  1. During the SPRING growth peak, pasture response to N fertiliser is at its highest (in the region of 20 to 25 kg DM/kg N applied or more), but growth usually exceeds requirements at this stage. Nitrogen may be used at this time to get a higher yield of silage, or to get a silage crop quicker than usual.
  2. In the SUMMER soil temperatures favor clover N fixation, with soil moisture being the factor limiting pasture production. As long as soil moisture is not limiting clover growth, and there is at least 25% clover in the pasture, adding N fertiliser at this stage would be competing with a 'free' source of N from clover. However, a significant response to nitrogen fertiliser can be obtained if applied straight after a summer storm (at least 20 mm rain), but remember that losses of urea, as ammonia gas, can be as high as 15% at this time of the year.
  3. With the onset of AUTUMN, clover growth slows down as soil temperatures decline and the grass becomes N deficient. However, this is a time when additional pasture growth is required for both those farmers extending the lactation period, and those hoping to build up a winter feed wedge. Note: Responses to lower rates of N after the autumn break may be variable as some clover N may be left in the soil from the summer period. A strategic top-dressing of 30 to 50 kg N/ha at this stage should provide at least 12 to 15 kg DM/kg N applied, assuming that soil moisture, soil temperature and other nutrients are adequate.
  4. In WINTER lower soil temperatures reduce growth rates substantially and, therefore, also reduce the response to N fertiliser. Pastures in colder areas may stop growing altogether for six to eight weeks. When soil temperatures (at 10 cm) are below 4C the response to N fertiliser will be negligible. Added to this, rainfall is relatively high at this time of the year and N fertiliser will be highly susceptible to leaching. Urea is, therefore, a safer source to use than ammonium nitrate at this time.
  5. In the warmer, coastal areas, however, pasture growth could be as high as 10 to 15 kg DM/ha/day in winter. If additional growth is required, a top-dressing of 30 to 50 kg N/ha should result in a approximately 8 to 10 kg DM/kg N applied.

    Rule-of-Thumb:
    If the pasture is growing it will respond to N fertiliser
  6. LATE-WINTER/EARLY SPRING is a period of peak grass demand, as pasture growth rates are low and cows begin to calve. This is the period that most justifies a top-dressing of N fertiliser, as the grass is able to grow long before the clover or soil can provide N for growth. However, the timing of this application greatly affects the response. Apply the N too early and a high proportion could leach. Apply it too late and valuable early spring growth could be lost. As a broad guide, a top-dressing of about 30 to 65 kg N/ha, may be applied from the third week of July to early August.

A second top-dressing?

If additional pasture is required during the responsive periods discussed above, a second top-dressing of N fertiliser may be applied. However, for maximum efficiency of each application, top dressings should be no closer than four weeks apart.

T-Sum 200 - For those wanting to get more technical

The T-Sum 200 technique is useful to achieve a more precise timing of the first spring N application, but only in areas where pasture growth stops altogether through the winter. The method involves adding the average daily air temperatures (average of minimum and maximum) from the 1st July. These are called Cumulative Heat Units (CHU). When the total CHU's reaches 200 perennial ryegrass is able to respond to N fertiliser optimally.

 

Length of pasture

Nitrogen fertiliser is most effective when applied straight after grazing (1500 but not 1300 kg DM/ha) and is less efficient when applied to a pasture that has more than 2 weeks of active regrowth (above 1800 kg DM/ha). The N requirements of a pasture are highest during the active growth that takes place in the 2 weeks after grazing. Applying N to the taller regrowth cannot compensate any restriction on N at this stage. In addition, grazing the pasture too early after N application (less than 18 to 21 days) will result in a loss of potential yield.

Rule of Thumb: Delaying the application of N fertiliser reduces the potential response by about 1 % per day post grazing

 

HOW MUCH NITROGEN FERTILISER TO APPLY

Local research indicates that top dressings of between 30 to 50 kg N/ha at any one application are most efficient, with early spring responses best at the upper limit (i.e 50 kg N/ha on half the farm is better than 25 kg N/ha on the whole farm). Referring to the shape of the response curve in Figure 5, N fertiliser is most efficiently applied where the response is steepest (the linear portion between the 30 and 45 kg N/ha as indicated by the arrows in Figure 5). Applications below 30 kg N/ha appear inefficient with responses unpredictable, due to the unpredictable contribution of clover (see the difference in yield between 3 sites at no N applied in Figure 5). Likewise, applications above 50 kg N/ha appear beyond the plants ability to utilise immediately and are thus subject to greater losses.

The three sites in Figure 5 show a clear effect of temperature between a cold farm and a warm coastal farm.

Figure 5. A typical pattern of response of a perennial ryegrass/clover pasture to N fertiliser. Nitrogen fertiliser was applied in June 1995 on a warm coastal site, a cold back country site and an intermediate site. The figures on the graph indicate the efficiency on N fertiliser use in terms of kg DM/kg N applied for the total response up to 45 kg N/ha.

Costs

Assuming that the response to N fertiliser is 10 kg DM for every 1 kg N applied, the additional forage produced will cost less than 11c/kg DM.

 

WHAT SOURCES OF NITROGEN FERTILISER TO USE

Rule of Thumb: As long as the different sources of N fertiliser are applied at the same N rate (i.e. Urea =46% N, DAP=18% N) there is no difference in quantity or quality of pasture produced.

1) Ammonium Sulphate should only be used as a ‘once-off’ source of N if sulphur (S @ 23.5%) is required and no other source of S has been applied. However, even if the sulphur content is costed, it is a more expensive source of N than urea or DAP. Ammonium Sulphate Nitrate (ASN) is also available.

2) Urea and Ammonium Nitrate are examples of pure N fertiliser sources (no P, K or S). The choice between these sources should be based on cost per unit of N, as urea is currently almost 50% cheaper per unit N than ammonium nitrate and over 30% cheaper than ammonium sulphate.

3) Di-Ammonium Phosphate (DAP), is an excellent, low-cost source of both N and phosphate (P) IF both are required at the same time. For example, a top-dressing of 40 kg N/ha would require either 87 kg Urea/hectare, or 222 kg DAP/hectare. The DAP would also apply 44 kg P/ha, equivalent to about 500 kg Superphosphate.

Urea has often been labelled as inefficient due to the potential losses from the volatilisation of ammonia. However, two points should be remembered here:

  1. Urea is approximately 30% cheaper than the next nearest source of N, and
  2. Volatilisation losses from urea from grazed pastures are usually so small (6 – 12% maximum) that they are not measurable in practice i.e you would have to lose more than 30% of the urea to justify any other source.

The only difference between the sources of N fertiliser would be in terms of soil acidification. Regular use of Ammonium Sulphate acidifies the soil substantially more than Urea, which in turn acidifies marginally more than Calcium Ammonium Nitrate. However, the application of one or two strategic applications in a year will not acidify the soil notably and certainly not as much as the acidifying effect of a high clover pasture.

 

THE NEGATIVE EFFECTS OF NITROGEN FERTILISER USE

1. Environmental Implications

In recent years it has become a requirement to consider the environmental impact of fertiliser recommendations. The regular use of high rates of N fertiliser have been criticised world wide due to their nitrate leaching potential and inefficiency.

To minimise nitrate leaching from pasture:

Past research has estimated that, in a pasture with at least 30% clover and high rainfall, temperate climate, the clover may fix between 180 and 250 kg N/ha/year for the overall pasture. Most of this N fixation occurs between October and March, with the clover supplying roughly 40 kg N/ha/month during this time. As ryegrass grows at a lower temperature than clover, current recommendations merely aim to sustain the level of N supply to the pasture, when the clover N supply is limited. In addition, research in the UK has shown grass/clover pastures can be more "nitrate leaky" than pure grass pastures with N fertiliser applied strategically at no greater than 50 kg N/ha in any single application

2. Animal Health

The excessive use of N fertiliser on annual ryegrass pasture has been implicated in reduced fertility, appetite suppression, grass tetany, lower dietary fibre and isolated cases of nitrate or ammonia toxicity (ammonia or free gas bloat). Most of these cases are linked to high rates of N fertiliser (in excess of those advocated), season of the year (spring or autumn), abnormal weather conditions (long periods of warm, yet overcast weather in spring or autumn) and, worst of all, hungry animals. It is important to note that it is seldom nitrate toxicity, but more commonly a chronic or sub-clinical ammonia toxicity or even ammonia bloat (free gas bloat) that may affect cows on highly N fertilised pasture. The obvious problem here is that high quantities of N are being released in the rumen while the energy required to utilise this N is in short supply. The obvious solution is to provide the cows with a high energy supplement and not feed any protein supplements.

Cows that are suffering as a result of excess N in their diet tend to select for a lower quality roughage. A bale of low quality ‘bedding’ hay in the corner of the paddock can be used as an indicator of N stress.

These problems are also more common in annual and short rotation ryegrass, than in perennial ryegrass pastures, as perennial ryegrass accumulates less nitrate and protein. For more information for temperate and sub-tropical climates try the links from the Articles on Line.

 

IN CONCLUSION

Some guidelines to minimise the negative effects of N fertiliser include:

Nitrogen fertiliser is not a quick-fix solution to correct poor planning. If you plan to use N fertiliser, your management must be geared to utilise the additional forage produced.

Data from a completed project funded by DPI and the Dairy Research & Development Corporation

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Last modified:30 October, 2007                         Please Note: Disclaimer             Authorised and maintained by: Richard Eckard
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