Filling Feed Gaps with Strategic Fertiliser Use
Richard Eckard
Senior Research Fellow, ILFR, the University of Melbourne,
RMB 2460 Hazeldean Rd, Ellinbank, VIC, 3820
[email protected]
Summary
Nitrogen (N) fertiliser is a useful management tool for manipulating seasonal pasture growth rate.
During the cooler months pasture are commonly deficient of N, as both legume N2 fixation and soil
N mineralisation are limited by lower temperatures. Nitrogen fertiliser is most effectively utilized
when applied at rates between 25 and 50 kg N/ha in a single application. Through most of the
autumn, spring and some of the winter, this will result in a pasture growth response of between 5
and 12 kg additional dry matter for every kg of N applied. Assuming that urea costs $450/tonne
and we could achieve a 10:1 pasture response, the extra pasture produced would cost just under
$100/tonne for high quality forage ‘bought’ onto the farm; there are currently no purchased feeds of
this quality available that could compete with this price. Thus the strategic application of N fertiliser
can be an economic option for filling feed gaps on farm.
Introduction
Nitrogen (N) fertiliser is a useful management tool for manipulating seasonal pasture growth rate.
All plants need nitrogen (N) for growth and usually derive their N needs from either the soil or the
atmosphere. In most intensive grazing systems N is in short supply, as it is constantly removed from
the soil/plant system through animal products (meat, wool, milk). However, N is subject to many
loss processes and is thus not efficiently recycled back to the pasture, with only 30 to 40% of the N
applied to the soil (fertiliser plus dung and urine) eventually used to grow more pasture (Whitehead,
1995).
Sources of N for plants
The main sources of N in intensively grazed pasture systems would be from legumes (like white
and sub-clover) and from the soil (the breakdown of roots from previous seasons, plus dung and
urine recycling). In low stocked systems, the above sources of N are usually sufficient for the level
of grass production required. However, as the stocking rate is increased and more animal product is
removed, the pasture very quickly becomes N deficient at certain times of the year.
Clover
Legumes like white clover or sub-clover can fix anywhere between 0 and 250 kg N/ha per year. To
achieve N fixation anywhere near this upper limit, clover would have to comprise at least 30% of
the pasture and would also require a warm winter and wet summer. Surveys of dairy farms in
Victoria show average Gippsland farms have only 12% clover (Eckard, 1998), while farms in
Western Victoria have around 8-15 % (Quigley et al., 1992; Riffkin et al., 1997).
Clovers are also subject to a range of pests (nematodes and Lucerne flea), but are also sensitive to
grazing management, low temperatures and soil fertility. Therefore most of our pastures are
probably only fixing between 30 and 100 kg N/ha annually (Eckard, 1998).
Nitrogen fixation by clover becomes negligible as soil temperatures drop below around 10°C.
However, most of our temperate grass species can continue to grow at soil temperatures down to
around 2 or 3°C. This means that most of our pastures will be deficient of N from legume N2
fixation between May and early September.
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Soil N
Soil N is derived from a combination of decaying grass and legume roots from previous seasons and
dung and urine deposited from grazing. Of the urine deposited, a large proportion will be lost as
ammonia gas, some will be lost through nitrate leaching during rainfall, with only around 30 to 40%
eventually taken back up into the plant (Whitehead, 1995). Remember that the effective N
application rate within a urine patch is between 600 and 1600 kg N/ha (Eckard, 1998) – no wonder
the soil-plant system cannot efficiently store and release this N. While urinary N is mainly in the
form of urea, and thus readily available to plants, the spatial distribution of this N is such that a
paddock is only fully covered in this manner at least once every 3 to 4 years.
Soil N from decaying roots and other organic matter can often contribute as much as 100 – 250 kg
N/ha per year to the pasture. This process of mineralisation is also affected by soil temperatures,
being much slower as soil temperatures drop below 10°C. Again, this means that most of our
pastures will be deficient of soil N between May and early September.
‘Bag’ nitrogen
Nitrogen fertiliser is commonly supplied in the form of urea (which has 46% N) or as ammonium
nitrate (34.5% N). Urea is both the cheapest and the most effective source of N to use for most of
the cooler and wetter part of the year, as gaseous losses are minimal, while nitrate sources can leach
and denitrify more readily (Eckard et al., 2003; Eckard et al., 2004). Of the mixed N sources, DAP
(Di Ammonium Phosphate; 18% N and 20% P) is often the cheapest source of both N and P.
Rate of N fertiliser
Figure 1 shows a typical response pattern of pasture to a single application of N fertiliser. What is
important to note is that the response below 25 kg N/ha and above 50 kg N/ha are both less efficient
(not as steep) as the response between 25 and 50 kg N/ha. This shape of response curve is supported
by the work of Eckard and Franks (1998) in Tasmania, Jacobs et al. (1999) in Western Victoria and
Mundy (1993) in northern Victoria.
The dotted line in Figure 1 demonstrates this effect more clearly, showing the efficiency with which
N fertiliser was used, in terms of kg DM per kg N applied; N fertiliser was most efficient at the
highest points (10 kg DM per kg N) on this line. Applying rates of N above 50 kg N/ha will usually
exceed the capacity of the soil-plant system to utilise the extra N resulting in an increasing fraction
of the N being lost to the environment as a pollutant (Eckard and McCaskill, 1999). Likewise, N
fertiliser rates below 25 kg N/ha are highly variable and thus less predictable (Eckard, 1998).
Therefore, in order to maximize the response to a single application of N fertiliser, but also
minimise the risk of a poor response, the rate of application should be somewhere between 25 and
50 kg N/ha. The actual rate applied depends on the additional growth rate needed. For example,
applying 25 kg N in Figure 1 will result in 250 kg additional pasture growth per hectare, while
applying 50 kg N to the same area yield an extra 500 kg DM/ha.
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12
2600
10
2400
8
2200
6
2000
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DM Yield
N Efficiency
1800
2
1600
Nitrogen Efficiency (kg DM/kg N)
DM Yield (kg DM/ha) .
2800
0
0
10
20
30
40
50
60
70
80
N Fertiliser Rate (kg N/ha)
Figure 1. The dry matter yield response of a perennial ryegrass and white clover pasture to
increasing N fertiliser rate and the associated N efficiency in terms of kg additional pasture yield
per kg N applied.
Seasonal responses
We have already established that most of the ‘natural’ sources of N will be limited between May
and early September. Figure 2 shows the potential grass and white clover growth pattern over the
year. The grass growth provides an indication of the N demand of the pasture over the same period;
typically high in spring and low in summer and winter.
During the spring peak both legume and soil N are actively supplying N to the pasture. Unless
additional forage is required for silage production, pasture growth usually exceeds the demand from
the stock and no additional N is required.
Summer irrigated vs dryland
During the summer period lack of water and high temperatures are the factors first limiting grass
growth, not a lack of N. Applying fertiliser N under hot and dry conditions does not produce an
economical response to the N fertiliser unless there is sufficient soil moisture for active grass
growth.
However, if the pastures are summer-irrigated then additional inputs of N fertiliser would be
required to maximise water-water use efficiency and justify the capital expense of an irrigation
system. Under irrigation summer N fertiliser responses can range between 8 and 14 extra kg DM
per kg N fertiliser applied.
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Figure 2. The general pattern of grass and white clover growth in south eastern Australia. Dotted
lines indicate the potential growth pattern under irrigation or with summer rainfall. Arrows indicate
periods in the season when N supply from legumes and the soil are limited by low soil
temperatures.
Late autumn, winter and early spring
This is the period of the year when ‘natural’ sources of N are most limiting, yet the grass
component of the pasture can grow and thus respond to tactical applications of fertiliser N. Pasture
responses to N fertiliser at this time of the year range between 5 and 12 kg of additional grass
produced for every 1 kg N fertiliser applied. In other words, 46 kg N fertiliser (100 kg Urea) to 1
hectare at a 10:1 response (as in Figure 1) should yield an additional 460 kg DM/ha pasture
growth.
Economics
A simple means of evaluating the economics would be to compare the cost of the N fertiliser
response against the cost of purchasing alternative feed sources. Thus if urea cost $450/tonne and
we could achieve a 10:1 response, the extra feed would cost just under $100/tonne for high quality
forage ‘bought’ onto the farm; there are currently no purchased feeds of this quality available that
could compete with this.
At the same time, applying N fertiliser without irrigation in summer may result in responses of
between 2 and 5 kg extra DM per kg N applied. Using the above calculation show this would then
cost at least $200 to $500/tonne. This again emphasises the need to carefully consider the potential
response before applying N fertiliser. Lookup tables of likely responses to N in Victoria can be
found on the web at www.nitrogen.unimelb.edu.au.
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Responsible N use
Nitrogen is a very slippery element and can be lost from a soil-plant-animal system in many ways.
Often less than 10% of the total N in the system is finally used to produce animal product. Nitrogen
losses include volatilisation (ammonia loss from urine and urea), denitrification (nitrate converted
to N2 and N2O gasses, the latter being a very powerful greenhouse gas) and nitrate leaching (soil
nitrate draining with water).
Some simple management rules can minimise all these loss processes. They include only applying
N at the right rate (less than 50 kg N/ha), using the right source (mainly urea or DAP in the cooler
months of the year) and the timing (when pastures are actively growing and by avoiding
waterlogged soils and heavy rainfall events). If urea is used on an irrigated pasture in summer, then
a light watering will reduce losses as ammonia gas (volatilisation). However, as these losses are
usually less than 10% during the cooler and wetter months, N fertiliser will NOT need to be washed
in then.
A more comprehensive list of these best management practices can be found on the web at
www.nitrogen.unimelb.edu.au.
In Conclusion
Strategic or tactical N fertiliser applications, of between 25 and 50 kg N/ha per application, applied
as urea or DAP, can be an economic option for growing additional high quality pasture, if applied
during periods when natural sources of N are limited by low temperatures.
References:
Eckard R.J. (1998). A critical review of research on the nitrogen nutrition of dairy pastures in
Victoria. ILFR, The University of Melbourne and Agriculture Victoria, Ellinbank, DNRE.
October 1998. ISBN 0 7311 4270 5. pp 1-31.
Eckard R.J., Edis R., White R.E., Smith A. and Chapman D.F. (2004). Nitrate leaching from
temperate grass and clover dairy pastures in south eastern Australia. Australian Journal of
Agricultural Research (in review).
Eckard R.J., Chen D., White R.E. and Chapman D.F. (2003). Gaseous nitrogen loss from temperate
grass and clover dairy pastures in south eastern Australia. Australian Journal of Agricultural
Research 54 (6): 561-570.
Eckard R.J. and Franks D.R. (1998). Strategic nitrogen fertiliser use on perennial ryegrass and
white clover pasture in north-western Tasmania. Australian Journal of Experimental
Agriculture 38(2): 155-160.
Eckard R.J. and McCaskill M. (1999). The Environmental Impact of Nitrogen Fertiliser use on
Dairy Pasture. In: Using Nitrogen with Confidence. F.R. McKenzie (ed). April 1999.
Department of Natural Resources and Environment. pp 11-5.
Jacobs J. L., McKenzie F.R., Ryan M.J. and Kearney G. (1999). Effect of rate and time of nitrogen
application from autumn to midwinter on perennial ryegrass–white clover dairy pastures in
western Victoria. 1. Growth and composition. Australian Journal of Agricultural Research
50(6) 1059 – 1066.
Mundy G.N. (1993). Guidelines for the economic use of nitrogen on irrigated dairy farms. Final
report DAV 241. Dairy Research & Development Corporation.
Quigley P.E., Ward G.N. and Morgan T. (1992). Botanical composition of pastures in southwest
Victoria. Proc. 6th Australian Agronomy Conference p 533
Riffkin P., Quigley P. and Cameron F. (1997). Improving the White Clover Feedbase by Optimising
Nitrogen Fixation. Final Report DAV 313, Dairy Research & Development Corporation.
Whitehead D.C. (1995). ‘Grassland nitrogen.’ (CAB International: Wallingford, UK.)
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