Managing N fertiliser
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Nitrogen (N) fertiliser timing and rate are crucial for efficient pasture growth and reducing N leakage to waterways. Apply N when other growth factors aren't limiting and it's financially viable, considering the milk price and costs of growing extra pasture. Don't exceed 50 kg N/ha per application. The decision to apply should account for pasture growth needs, soil conditions (temperature between 6C and 16C), and the milk price to N fertiliser cost ratio. Profit from N depends on using the extra feed for animals or harvesting it as a quality supplement. Track pasture cover to anticipate feed deficits. Your response rate, which is the pasture growth against N applied, is best when conditions are ideal. All N fertilisers have the same effect, so choose the cheapest per unit N. Regularly recalculate costs as prices change. Note, increasing N application can lead to more leaching, especially above 200 kg N/ha annually.
Getting the timing and application rate of nitrogen fertiliser right will help increase efficiency and minimise N leaching to waterways.
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 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.
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)
(kg DM/ kg N)
|Time to full
The response rate is dependent on:
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)
Profitable milk production responses are commonly achieved from late winter/early spring applications of 30-50 kg N/ha to pastures with a cover of 1200-2200 kg DM/ha where there is a genuine feed deficit (e.g. applied between planned start of calving and before balance date i.e. when feed demand is greater than pasture growth). Good responses in late winter/early spring are due to plants being N deficient because:
In districts with warmer winters soil microbial activity is higher for longer and this combined with higher pasture growth depletes the available soil mineral N. In these districts spring N deficiency can become evident ahead of the onset of rising temperatures that trigger increased rates of soil N mineralisation. i.e. a slower transition from winter to spring growth rates compared with colder winter districts.
In spring, temperature and moisture usually don’t limit N uptake for grass growth so rapid responses to fertiliser N can be expected5.
There is increased risk of wastage of the extra pasture generated, reducing the profitability of N use. Thought needs to be given to how surplus pasture can be best utilised, i.e. using N fertiliser so that other parts of the farm can be removed from the grazing round for re-grassing, summer crop preparation, or conserving surplus pasture for silage.
Care needs to be taken when using N in spring to avoid long-lasting shading of clover stolons (runners) from prolonged canopy closure resulting from high pasture yields and heavy silage cuts. This shading of clover stolons reduces branching. This reduces clover production and, hence, N fixation later in the year, risking lower summer pasture yields.
N applications in early summer stimulate the development and growth of new ryegrass tillers. DairyNZ research shows N applications boosted tillering by 37%, with an extra 800 kg DM/ha and 66 kg MS/ha produced from December to April after 100kg N/ha was applied, split between December and January, i.e. an 8:1 kg DM response per kg of nitrogen.
Nitrogen applications should only be made if soil moisture levels are adequate for good pasture growth. Mid- to late-summer applications of N fertiliser are not recommended when low soil moisture limits growth.
On summer dry farms, N can be applied in late November/early December to increase pasture cover going into dry summer. N needs to be applied before moisture is limiting growth.
When moisture is not limiting (i.e. irrigated farms or regions with reliable summer rainfall), pasture responses are similar to those in spring when soil temperatures are below 16oC.
When soil temperatures are above 16oC response can be lower as the temperature is above the optimal range for ryegrass growth and there is increased available N in the soil from greater mineralisation and N from fertiliser is not required.
Graph 2: (Pastoral21 Lincoln Farmlet trial N Use)
On irrigated land to reduce the risk of volatilisation apply N near to the time the paddock will be irrigated.
Urea coated with a urease inhibitor can also be used for this purpose if N volatilisation is a risk.
Nitrogen has a role for restoring pasture cover after summer or autumn dry periods. After a dry summer there is a considerable pool of N already in the soil. Despite this, responses to fertiliser N still occur.
In some years soil moisture is restored with significant downpour events which wash some of the available soil N below the plant root zone in free-draining soils. In other years the soil processes making N plant available recover slowly with a slow improvement in moisture levels. Fertiliser N gives pasture plants immediate access to some N, while the soil processes recover. Responses to N applied in autumn are variable from 4:1 to 10:1.
While autumn applications of 20-40 kg N/ha can help achieve desired pasture cover targets going into winter and increase days in milk, care should be taken not to overdo this as there are risks. These risks include:
Avoid winter N applications (June/ July) as the response rate will be limited by soil temperature and slow plant uptake and risk of leaching is highest. In warmer, northern areas soil temperatures will usually be greater than 7ºC from mid to late July.
Farmers starting to apply N at this time are encouraged to first assess pasture cover levels against their targets and quantify potential feed deficits, while ensuring soil temperatures are sufficient for plant uptake. For August applications (see late winter/early spring section above).
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.
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.
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.
Identify deficit using:
It is important to identify the potential feed deficit early so N fertiliser can be applied with sufficient time for a response when it is needed.
A tactical approach to N fertiliser use, for supporting the overall farm strategy is to use fertiliser to boost pasture covers to fill anticipated feed shortages. Variations in weather during the year requires anticipating potential feed shortages (estimating depth and expected duration of a deficit), and where it is possible to manage through a short-term deficit. If not, it is a choice between imported feed or N fertiliser. If N fertiliser is the choice the challenge is then ensuring it is applied early enough (in advance of the feed deficit) and to estimate if the likely size of the response will overcome the feed shortage.
Consider these questions to decide on application
N fertiliser should be applied only when there are no other factors limiting pasture growth.
Soil temperatures and moisture are the key drivers. A commonly accepted rule of thumb is to apply when soil temperature is more than 6oC and rising. Too little or too much soil moisture can have a large effect on yields and N uptake. In the absence of irrigation, suitable conditions (and therefore profitable use) generally occur between spring and early summer (before lack of soil moisture) and any use from late summer through autumn is higher risk, both environmentally and financially. N fertiliser is a growth multiplier. If conditions are unfavourable for growth, N fertiliser response will be small and slow. Pastures will also respond best to N when all other soil nutrient levels are satisfactory.
Consider the following:
The size of the response to N fertiliser will depend on several factors. However, growing conditions have the greatest influence, affecting overall response to fertiliser N and the time to fully express this response. Building on a bank of knowledge about your farm over time is a useful management tool to refine these guidelines. Mineral N in farm dairy effluent (FDE) applications will substitute for N fertiliser, as will N mineralised over time from the organic component of FDE. N fertiliser interaction with clover is an interesting conundrum.
As a rule of thumb, for every 3 kg fertiliser N applied, N fixation is reduced by 1 kg N/ha/yr. Therefore, in legume-rich swards, N fertiliser use needs to be carefully considered. However, it has been reported that rates in the order of 20-50 kg N/ha per application, plus good grazing management that minimises the competitive effects of ryegrass, are unlikely to have a mayor effect clover. Soils with high levels of organic matter have been found to have a higher soil N supply, which means the pasture they support do not require as much N fertiliser. Therefore, more DM per kg N applied may be achieved by targeting paddock with lower soil organic matter.
Higher rates of N at individual applications generally result in less DM return per kg N applied, with 50 kg N/ha upper limits per application recommended.
While most of the response is usually seen in the first harvest, a small residual response occurs in the next re-growth phase. There is rarely a residual effect beyond the second harvest. Fertiliser losses can be reduced at times by matching the appropriate fertiliser to the prevailing soil moisture and weather conditions. Ammonium nitrate and sulphate of ammonia are less susceptible to volatilisation when applied during hot, dry conditions than urea and DAP (di-ammonium phosphate). Urease inhibitors can also decrease losses from urea in periods when loss is expected.
Considering the following
Ensure fertiliser goes where it needs to go (not directly blown into ditches, surface waters and general non-productive areas) and stays there (does not run off slopes when soils are too dry or too wet), and apply only when the pasture can use it (right growing conditions). Using Spreadmark accredited spreading companies is recommended to ensure good placement of the fertiliser.
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.
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.
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.
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*
|Cost of N
|Cost of N
|Sulphate of Ammonia
|$905||$250 (220 kg S
|DAP(18-20-0-2)||$1,794||$1,192 (200 kg
of P @$4.00/kg)
|PhaSedN Quick Start
of S @ $1.12/kg)
|N-Protect S(30-0-0-32)||$1,272||$359 (315kg S
*As at July 2022: All prices are bulk for product only and exclude GST. Need to add instore cost, cartage and spreading cost.
**SustaiN (Ballance) and N Protect (Ravensdown) are products where urea is coated with a urease inhibitor, which reduces field losses of N due to volatilisation (gas loss). This is helpful when applying N in dry conditions, e.g. early summer.
***The extra cost of using a fertiliser coated with a urease inhibitor will be recouped if 2.8 kg N/ha of volatilisation is prevented.
PhaSedN (Ballance) and N-Protect S (Ravensdown) are a combination of urea coated with a urease inhibitor plus finely ground elemental sulphur.
PhaSedN QuickStart is a combination of urea coated with a urease inhibitor plus 32% sulphate sulphur and 68% finely ground elemental sulphur.
The cost of fertiliser, instore charges, transport and application should be added. These costs will vary for each farm. For example, if these costs total $140/tonne of urea applied this adds another 30 cents/kg N to the cost of applying N fertiliser. For urea this will bring the current cost up to $3.21/kg N.
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.
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.
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