Nitrogen for high rainfall pastures in Western Australia

General guidelines for nitrogen (N) management

• Apply N strategically, rather than by fixed recipe.
  Before each N application, estimate the likely N response (use lookup tables, experience, consultants) and compare the cost of the additional pasture produced to other purchased feed options. For example, a 10kg/ha dry matter response to 1kg/ha of urea N applied during the autumn/winter feed gap is usually a cheaper option than buying feed at this time of the year.
• Apply N when pasture is actively growing and can utilise the N.
  Check that soil moisture is adequate to sustain the regrowth, rainfall is likely in the regrowth period, temperatures are conducive to good pasture growth, there is a good species composition and other major soil nutrients are optimal.
• Apply N at rates of 25 to 50kg N/ha per application, no closer than 21 to 28 days apart.
  The recommended rate for intensively grazed pastures is from1.0 to 2kg N/ha per day. Calculate the application rate by multiplying the daily equivalent rate by the days between N applications (e.g. 1.0 or 1.5kg N/ha per day for 30 or 20 days). During the peak growth period, with good soil fertility and newer cultivars, it may be justified to increase the upper rate to 2kg N/ha per day.
• Maximise use the extra pasture growth to get the best finacial return.

Right rate

• The most efficient N fertiliser rates are 25–50kg N/ha at any one time.
  This is because the steepest pasture growth responses to N occur at these rates and drops off as rates increase. For example, 80kg N/ha on 1 hectare is likely to produce less additional growth than 40kg N on 2 hectares, due to decreasing N efficiency with increasing rate.
• Apply more than 25kg N/ha in any single application.
  Lower rates will often produce unpredictable N responses i.e. 20kg N on 2 hectares (40kg total) may produce less than 40kg N on 1 hectare.
• Do not apply above 50kg N/ha in any single application*.
  And do not apply N closer than 21 days (30kg N/ha in spring), to 28 (50kg N/ha) days apart, as this will increase N losses exponentially. *The exception may be on highly productive pastures, through their peak growth period, with a newer cultivar, and where soil moisture is not limiting, when pastures may respond to more than 60kg N/ha per application (above 2kg N/ha/day).

Right place

• Apply N to pastures with a high density of desirable species.
  Applying N to pastures where weed species have invaded will result in larger, healthier weeds and have no beneficial effect on feed supply for grazing cows.
• Apply N to pastures with a good ground cover.
  Gaps or bare areas in pastures will result in more N lost through leaching and/or volatilisation.
• Apply N to pastures that have no limitations to major soil nutrients.
  Soil tests will identify if other nutrients or pH are limiting growth and reducing the effectiveness of added N.
• Do not apply N to pastures that are stressed.
  Drought, waterlogging, or other stresses reduce the effectiveness of applied N.
• Consider applying less N to the front half of a paddock than the back.
  Cows transfer N towards the gate.
• Avoid applying N to animal hot spots.
  Gateways, water troughs, shelterbelts, and stock camps are prime N loss areas.

Right time

• Avoid grazing until growth has reached the right stage.
  At least the 2.5–leaf stage for temperate grasses (e.g. ryegrass), or the 3–leaf stage for tropical grasses (e.g. kikuyu), to maximise the nitrogen use efficiency, the energy: protein ratio in the diet and therefore the amount of N excreted or lost.
• Apply N as soon after grazing as possible.
  This is when plants need access to N for maximum regrowth potential. As a rule, for every day you delay applying N post-grazing, you can lose 1% of the potential N response.
• Apply N when the average soil temperature over the regrowth period is high enough.
• Temperate pasture grasses (e.g. ryegrass) generally respond to N fertiliser when soil temperatures are above 4°C, and subtropical pasture grasses (e.g. kikuyu) respond to N fertiliser when soil temperatures are above 10°C. Remember, this is the average soil temperature over the regrowth period, NOT just on the day of application. In South West Western Australia, soil temperatures rarely fall below 5°C.
• Only apply N when there is adequate soil moisture.
  Autumn and summer N responses on dryland pastures are highly dependent on adequate soil moisture.
  • Do not apply N unless there is adequate soil moisture in the root zone from either irrigation or rainfall, and there is a good prospect of irrigation or rainfall to follow through the regrowth period (e.g. summer and autumn in winter rainfall regions).
  • In temperate winter-dominant systems, following a wet summer with active pasture growth, there will be little N left in the soil at the autumn break, meaning there will be a reasonable response to N at the break. In contrast, if the summer is dry there will substantial mineralisation of organic N, but little pasture growth to utilise this, meaning a very poor response to N at the break. See soil mineralisation below.
  • If irrigating, apply water well before soil moisture content drops below the refill point and plant growth is retarded. Once the soil has started to dry out it is almost impossible to apply sufficient water to raise moisture back-up into the Readily Available Water (RAW) zone, the point at which plants will most vigorously grow and respond to N. Research undertaken by the Tasmanian Institute of Agriculture (TIA) determined that for each day irrigation start-up is delayed, pasture utilisation losses of 105kgDM/ha/day can be experienced.

Right source

• Urea is the cheapest source of N.

  • Assuming soil moisture is adequate for pasture growth, ammonia losses from urea fertiliser are usually not large enough to justify the unit price of other N sources.
  • If applying N to waterlogged soils, an ammoniated source (e.g. urea, ammonium sulphate) is better than using a nitrate source (e.g. UAN).
  • Research has shown that foliar N is ineffective at supplying the N requirements of pasture.
• Urease inhibitors are only required in situations where high ammonia loss is expected.
  As inhibitor coated fertilisers cost more per unit of N, and seldom produce additional pasture, they are only likely to be cost-effective if the N rate applied is cut back by the expected reduction in N loss. If you are expecting a loss of ammonia of around 20% from urea, and you use Green Urea to reduce this, then apply 15% less N as Green Urea. Urease inhibitors are only required in situations where high ammonia loss is expected, such as autumn on dryland systems (see managing ammonia losses).
• Other inhibitor-coated fertilisers demonstrate variable effectiveness.
  Nitrification inhibitors (e.g. ENTEC®) has been shown to produce similar yield responses with 20% reduced application rates on heavier northern soils after prolonged use. Currently available coated urea products (e.g. polymers) have not been shown to be cost effective on pastures.

Other nutrients:

The response to N could be limited by the availability of other nutrients. Use regular soil and tissue analysis to check the adequacy of macronutrients (P, K, S) and micronutrients (e.g. copper).

• Phosphorus: Di-Ammonium Phosphate (DAP) is potentially the cheapest source of N, if the P is needed at the same time.

  • When using DAP, calculate the N rate applied and consider ‘topping up’ with urea to ensure an adequate N fertiliser rate i.e. 100kg of DAP/ha will apply 20kg/ha of P but only apply 18kg N/ha, which may not produce a predictable N response.
  • When applying N and P fertiliser together, refer to phosphorus for high rainfall pastures and Gourley et al (2019) to optimise the application of P and minimising overland flow losses of P.
• Sulphur: Low soil available sulphur (S) can reduce the response to N. Ammonium sulphate or sulphur blends can be a useful source of both N and replacing soil S, particularly where single-superphosphate has not been applied for a few years.
  • Granulated ammonium sulphate is an expensive form of N and it will acidify the soil rapidly with regular use. An application of 30kg/ha of N applied from ammonium sulphate compared with urea will require 100kg/ha of lime to neutralise the additional acidity from using ammonium sulphate, adding a lime cost of $5/ha to the application.
  • S from ammonium sulphate can also leach out from free draining soils during high rainfall or irrigation, so only apply the sulphate when needed and at the recommended rate (refer to link to sulphur for high rainfall pastures).
  • If sulphur is applied in elemental from (S), this form needs to be oxidised to sulphate before plants can use it, so apply this form well before the sulphur is needed e.g. in autumn.
• Soil acidity and lime: Where annual N application rates exceed 250kg N/ha per year, a proactive strategy of soil testing and liming may be required to prevent soil acidification. Usually 2.5 tonnes of lime per hectare will be required every 2 to 3 years, where high rates of urea have been applied (this will be higher if ammonium sulphate is used regularly).

Soil N mineralisation

Around 1–2% of organic N will be mineralised from soil organic matter annually (approximately 150 to 250kg N/ha), but mainly supplied in spring and summer. Soil N mineralised and not utilised by the pasture over a dry summer means there will be surplus N in the soil at the autumn break; thus only apply N later in autumn once soil moisture is adequate and pasture growth has resumed. As mineralisation of organic N is low in winter, N fertiliser will be required to maximise pasture growth.

Applying N in late autumn on grass dominant pastures can be a useful tactic for beef graziers to reduce supplementary feed requirement when pasture growth is slow in mid-winter. This is especially so if earlier summer rainfall has depleted reserves of organic N from the previous year. Also, consider the requirements of soil moisture and actively growing pasture before embarking on N applications prior to or early in winter.

Managing nitrogen losses

Managing ammonia losses

• Ammonia loss is highest under hot, dry, windy conditions.
  Ammonia losses from urea are also highest during the first 48 hours after application, while the urea granule is breaking down to ammonia (called hydrolysis). The following practices will reduce the risk of ammonia loss:
  • Do not apply urea where soil moisture is limiting, on hot and windy days with low pasture cover. As a general principle, ammonia volatilisation losses from urea should be small, if best practice is followed.
  • Urea does not need to be watered into the soil in the cooler, wetter months (May to November in southern Australia). If there is enough soil moisture and rainfall to justify the urea, the urea will be able to absorb enough moisture to dissolve itself. See the Right time section above for more information.
  • Avoid applying urea fertiliser immediately after rainfall where soils are dry and temperature and evaporation are high, as volatilisation losses could increase to more than 30%. Under these conditions, irrigate after applying urea to greatly reduce ammonia loss.Ammonia volatilisation losses in summer, under dryland conditions, average around 14%, which still does not economically justify switching to other more expensive sources of N.
  • If urea fertiliser is applied in drier periods, November to March, without irrigation, you can apply fertiliser 2 to 3 days prior to grazing to minimise wind speed at ground level and reduce ammonia volatilisation during the critical loss period (first 48 hours). Care must be taken to avoid cows ingesting lumps of fertiliser as this could lead to ammonia toxicity.

Managing ammonia losses - spray irrigated pastures

• Apply N fertiliser within 24 hours prior to spray irrigation.
• In summer when evaporation is high, avoid applying urea fertiliser after irrigation, as this is likely to increase volatilisation losses.

Managing ammonia losses - flood irrigation

• Urea is best applied just before irrigation but minimise run off into drains, as this will carry dissolved urea. In some cases, not fertilising the last few metres of the irrigation bay will capture the urea dissolved in the irrigation headwater.
• If urea fertiliser is applied after flood irrigation, soil moisture should be adequate to dissolve the urea and minimise volatilisation, but avoid wheel damage to the wet soils.

Minimising nitrate leaching and denitrification

• Avoid applying N fertiliser to warm (>10°C) waterlogged soils, as this increases the risk of N loss through denitrification.
• If applying N to cold, wet soils use urea or ammonium based fertilisers and avoid nitrate-based fertilisers such as UAN.
• Avoid applying N fertiliser near streams/riparian zones and over drainage lines within a paddock.
• If irrigating, take care to avoid overwatering, as this may result in nitrate leaching as well as inefficient water use.
• Avoid applying high rates of N fertiliser to free draining soils during periods of high leaching potential (e.g. high rainfall). Frontal rainfall patterns in high rainfall areas generally have a large rain event followed by scattered showers. Applying N after the bulk of the rain has past, reduces leaching risk.
• Use of nitrification inhibitors may be warranted in highly leached or waterlogged sites to prevent losses to the environment (nitrate leaching, denitrification gaseous emissions).

Minimising surface runoff losses

• The volume of water lost as runoff determines the N lost in runoff – avoid overwatering and surface runoff.
  • Use a weather forecast to minimise runoff after N application. When soils are saturated, wait at least 2 days after rainfall for excess run-off water to drain, before applying N.
  • Where possible, re-use drainage water from irrigation.
• Do not apply N fertiliser near drains, channels, dams, lakes or riparian areas. In a dune and swale situation, avoiding applying N to the swale as this is likely to receive N through surface movement anyway.

Dung and urine management

• Minimise the time that cows spend in the laneways and ensure that runoff from laneways, feed pads, sacrifice paddocks or other standoff areas drain to pastures and not directly into waterways.
• Effluent from dairies and feed pads should be viewed as a valuable fertiliser resource, and nutrient testing should be used to ensure that no more than 50kg N is applied to a pasture at a time.
• High stocking rates and stocking intensity will result in high losses of N from hot spots in the farm.

Animal health and nitrogen

• Avoid high rates of N fertiliser on annual ryegrass and kikuyu, as these can accumulate potentially toxic levels of nitrate. Perennial ryegrass, fescue, cocksfoot and white clover are NOT known to accumulate toxic levels of nitrate.
• If nitrate toxicity is of concern, do not graze pastures 7 to 14 days after N fertilisation or if water limited as nitrate levels are highest. Likewise, do not graze pastures 14 to 18 days after N fertilisation if pastures are high in crude protein (e.g. spring or autumn with high N fertiliser rates) and animals are not receiving an energy supplement or lower quality hay or silage.
• Do not apply more than 60kg N/ha in a single application, particularly with higher pasture growth in autumn and spring.
• Avoid subjecting cows to rapid diet change e.g. from low to high quality pasture, or to pasture with capeweed or volunteer brassicas, especially dry cows or heifers. Likewise, never give starved, unadapted or dry cows, unrestricted access to highly N fertilised pastures
• Cows that are suffering from excess N in their diet tend to select for lower quality roughage. A bale of low quality ‘bedding’ hay in the corner of the paddock can be used as an indicator of protein or nitrate stress.

Acknowledgement

These guidelines were originally developed by Richard Eckard (University of Melbourne) as part of the More Profit from Nitrogen national program, and have been adapted for Western Australian high rainfall pasture conditions.