Applying N fertiliser is best when other factors are not limiting pasture growth and it is economically sound to do so based on the cost of growing and harvesting the extra pasture relative to milk price.
It is best not to apply more than 50 kg N/ha at any one application and the need for application should be assessed against;
- The need for extra pasture growth and how well the additional pasture can be used.
- Soil Conditions – temperature and moisture (i.e. soil temperature more than 6oC and less than 16oC).
- The ratio of milk price to the cost of N fertiliser. Occasionally this ratio results in milk revenue insufficient to pay for the cost of N.
Profitable use of nitrogen fertiliser
The profitability of applying N is dependent on the extra feed grown (the response) being utilised for animal production. Therefore, N needs to either be applied to fill genuine feed deficits, or any surplus feed needs to be identified and harvested to make quality supplement.
As the response to N occurs over 3-14 weeks (refer to table below) N should be applied in anticipation of a feed deficit. Feed budgeting and monitoring of actual pasture cover against target cover will help ensure N is applied early enough to fill a feed deficit.
What influences the response rate to nitrogen?
The amount of pasture grown in kg DM per kg N/ha applied is the ‘response rate’. For example where 30 kg N/ha is applied and an additional 300 kg DM/ha of pasture is grown the response rate is 10 kg DM/kg N fertiliser applied.
The best response to N fertiliser occurs on fast growing pasture (N fertiliser is a growth multiplier), when other factors such as moisture and soil temperature are not limiting growth and when all other soil nutrient levels are satisfactory. A summary of N responses over 40 days from 400 trials was 4, 9 and 15 kg DM/kg N for winter, early spring and late spring applied N.
Impact of pasture growth rate on response rates to N fertiliser (N applied at optimum rates i.e. < 50 kg N/ha)
Pasture growth rate |
Pasture growth (kg DM/ha/day) |
Response (kg DM/ kg N) |
Time to full response (weeks) |
Slow |
10 |
5 |
10-14 |
Moderate |
20-40 |
10 |
6-8 |
Fast |
50-70 |
15 |
5-6 |
Rapid |
>80 |
20 |
3-4 |
The response rate is dependent on:
- Amount of available mineral N in the soil – the greater the deficit, the higher the response
- Soil temperature – the warmer the soil, the greater and more immediate the response
- Plant growth – the faster the growth, the greater and more immediate the response
- Moisture – too much or too little water will lower the response
- Rate of N applied per application – there is a diminishing response at high application rates (>50 kg N/ha).
Response rate variation depends on the season and on N application rate as illustrated in Graph 1 below. At the same application rate, responses are lower and slower in winter than in spring.
For example, if 25 kg N/ha is applied in the spring the expected response is 12 kg DM/kg N compared to an expected response of only 7 kg DM/kg N at the same application rate in the winter. If the spring application rate is increased to 100 kg N/ha, then the expected response rate falls to 7 kg DM/kg N.
Graph 1: Effect of season on response rate (adapted from Cameron et.al, 2005)


Seasonal nitrogen response and recommended management
Plant uptake of nitrogen
Ryegrass/white clover pastures do not take up N from the soil in the first four days after grazing. All the N required for the first four days after defoliation comes from remobilisation of reserves, mainly from stubble (80%) and roots. After this initial period, pastures rapidly take up mineral N from the soil (including N added in fertiliser) driven by pasture growth rate and dry matter accumulation.
Graph 3: N uptake (day 0-20 after grazing) and time of regrowth (adapted from Ourry et.al., 1990)

From day 7 to 14 after grazing the percentage of N in pasture is high as the N taken up by the plants has not all been converted to plant protein. After 14 days when growth rates increase and dry matter (DM) starts to significantly accumulate the percentage of N in pastures is diluted. Grazing pastures at this early stage (before 2 leaves) will not only forgo significant amounts of potential DM growth but also this DM will have a significantly higher N content. Grazing ryegrass pastures between 2½ and 3 leaves ensures optimal accumulation of DM and dilution of N concentration.
N fertiliser application increases the mineral N (mostly nitrate) available in the soil for plant uptake. This affects potential pasture growth and N concentration depending on the rate of N application, the N already available in the soil (from previous N fertiliser applications, clover fixation and mineralisation of soil organic matter), and when N is applied in relation to the grazing cycle.
Perennial ryegrass and white clover pastures are not known to accumulate excessive nitrates (luxury uptake). However, other plant species that can be present in the paddock e.g. weeds, and annual grasses may have luxury uptake which can result in animal health issues due to high nitrate content. For annual grasses as well as some brassica crops, timing of N application in relation to when grazing will happen is an important factor to consider.
Timing of N fertiliser application within the grazing round
The length of time between N applications and grazing is a balancing act; on one hand, allowing enough time for pasture to use the fertiliser and on the other, maintaining quality and minimising clover suppression by shading. Some of the N fertiliser applied that is not taken up by the plants before grazing will be available for the next regrowth, especially if no drainage occurs that results in the surplus N being lost beyond the root zone.
It is common to apply N fertiliser within 3-4 days before or after a grazing event for maximum response. Practically this would imply applying N once a week to all paddocks that will be grazed within 3-4 days or have been grazed in the last 3-4 days. This strategy ensures that N from fertiliser will be available and ready for plant uptake from the start of the pasture growing cycle (from day 5 after grazing). If the rate is adequate for the growing conditions (temperature and moisture), most N applied is likely to be used in this grazing round. A common rule of thumb to decide an adequate rate is to apply ~0.8 - 1 kg N/ha/day of the grazing round, i.e. 25-day round = 20-25 kg N/ha.
Applying N less frequently, i.e. twice or once a month, is a strategy that suits some systems, especially under irrigation when drainage is managed and N is not leached, as it is easy to implement and easy to stay within a monthly N budget. Under this system paddocks receive N fertiliser at different stages of the pasture growth cycle. The paddocks that cannot get a full response in this growing cycle because they were grazed too soon could get a deferred response at the next grazing round if N is not lost due to drainage.
In many situations i.e. non -irrigated pastures and/or rolling land, rainfall and/or ground conditions after the rain usually dictates timing of N applications (i.e. not applying immediately prior to a heavy rainfall event when N can be washed away or drains out of the soil). In this situation dealing with these factors are a bigger driver in deciding when to apply N fertiliser than the stage of the grazing round.
Deciding individual nitrogen applications?
There are several factors to consider when deciding individual N fertiliser applications. These factors will affect the potential response to N fertiliser (kg DM/kg N applied), and how economically sound it will be based on the cost of growing the extra pasture relative to milk price.
Key decision points for individual N applications
How N efficient is your farm?
The indicators below can help assess if N fertiliser use can be expected to provide sufficient pasture and milk production responses, or if the amount of N in the system poses a risk to the environment.
1. Farm N surplus or Purchased N surplus
This is the difference between N inputs (N in fertiliser and supplements = purchased N) and N outputs in products (milk, meat, crops) and is related to the risk of loss to the environment. Efficiency gains in N use are possible when a farm’s surplus of purchased N is high, for example greater than 130kg N/ha.
Note: Overseer’s N surplus includes N inputs from biological fixation and irrigation water and is therefore higher than purchased N surplus.
2. Soil organic matter or soil total N
Soils with a high organic matter or total N content have relatively high soil mineral N and mineralisable N available for plant growth. This reduces the need for N fertiliser7 when environmental conditions are right for mineralisation of this organic matter. Soil tests and associated recommendations are available commercially to identify these areas.
What is the cost of N in fertiliser?
All the common N fertilisers (urea, ammonium sulphate and DAP) produce the same amount of DM per kg N applied, i.e. a kg N is a kg N regardless of the product. Therefore, choose the cheapest form based on the cost per unit N, after considering the value of the other nutrients. As prices regularly change it will pay to update these calculations frequently.
Cost per unit for different N fertilisers*
Fertiliser (N-P-K-S) |
Cost ($ tonne) |
Value of companion nutrient ($/tonne) |
Cost of N ($/tonne) |
Amount of N (kg/tonne) |
Cost of N ($/kg N) |
Urea (46-0-0-0) |
$580 |
Nil |
$580 |
460 |
$1.26 |
Sustain N1* |
$637 |
Nil |
$637 |
459 |
$1.382* |
Sulphate of Ammonia 20-0-0-22 |
$409 |
$134 (220 kg S at $0.61/kg) |
$275 |
205 |
$1.34 |
DAP 18-20-0-2 |
$751 |
$654 (200 kg of P @$3.27/kg) |
$97 |
180 |
$0.53 |
Phased N 25-0-0-29 |
$605 |
$177 (290kg@$0.61/kg) |
$428 |
250 |
$1.71 |
*As at August 2020: All costs excl GST and do not include delivery and storage
1 Sustain N is urea coated with Agrotain, which reduces field losses of N due to volatilisation (gas loss). This is helpful when applying N in dry conditions, e.g. early summer.
2 The extra cost of Sustain will be recouped if 2.8 kg N/ha of volatilisation is prevented.
The cost of fertiliser, transport and application should be added. These costs will vary for each farm. For example, if the cost is $100/tonne of urea applied this adds another 22 cents/kg N to the cost of applying N fertiliser. For urea this will bring the current cost up to $1.48/kg N ($1.26+$0.22).
What is the estimated cost of the extra pasture grown?
The expected cost of the additional pasture grown from N fertiliser can be calculated as the cost per kg N applied divided by the predicted pasture response to the N fertiliser applied.
Example:
- Cost per kg N applied = $1.48
- Predicted pasture response = 10 kg DM per kg N applied
- Expected cost of pasture from N fertiliser = $0.148 /kg DM (14.8 cents/kg DM)
Farm systems response (MS per ha and operating profit) to N fertiliser
Farmlet studies in Hamilton showed that response efficiency expressed as kg milksolids per kg N differed between application rates. A response efficiency of 0.8 kg milksolids/kg N occurred when N was applied at 200 kg N/ha/year, compared to 0.6 kg milksolids per kg N applied at 400 kg N/ha/year.
An economic analysis of the study showed marginal profitability for the 400 kg N/ha/year farmlet due to the large amount of silage harvested in the spring and fed out in the autumn with extra costs for harvesting and labour. The profitability depended on the milk price, costs of N fertiliser and whether stocking rate was increased to harvest the extra feed grown. Farm system trials around the country have found that as the amount of N applied per year increases so does the amount of N leached, and that there are large increases in N leached at rates above 200 kg N/ha.