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
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.
There are 3 main N strategies that we see on farms:
Clover is more temperature sensitive than ryegrass and does not fix much N through the winter months. Once the soil temperature is below around 10°C (at 10cm) the supply of N from clover is limited, yet perennial ryegrass can continue to grow and respond to N down to around 4°C 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).
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 - 4º 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 2°C 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 APPLYNITROGEN 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:
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
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.
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.
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:
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.
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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