Author: Reily Menhenett – Technical Agronomist, Incitec Pivot Fertilisers

Despite a late break to the season, winter rainfall has set the scene for a timely spring pasture flush. Making the most of this growth will be critical to restore farm fodder reserves. Your pasture paddocks will need enough nutrients to optimise production and cover the nutrient removal from the hay and silage cuts.

Using a Boosta (an NPKS fertiliser blend) is one way to meet the nutrient requirements in one pass.

Incitec Pivot offers several Boosta products. Selecting the best one for an individual situation is best done by answering a few simple questions.

  1. Set correct nitrogen rate?
  2. Do I require capital phosphorus/potassium?
  3. What is my nutrient removal from grazing/hay/silage?

Nitrogen (N) requirement

As always, following best practice nitrogen management in pasture systems will provide the best outcomes. Nitrogen rate is the primary consideration when selecting the most suitable Boosta. N rates should be a minimum of 25kgN/ha up to a maximum of 60Nkg/ha. Top end rates should be used to maximise dry matter growth and particularly in applications post silage cuts. In grazing situation where spring rotations are quick, ~20days, then N rates can be brought back to 20kgN/ha.  For hay or silage conservation, nitrogen should be applied at least 4 weeks prior to cutting.

The key aspect to achieving N responses is ensuring adequate soil moisture is present, this highlights a key management timing decision to get spring fertiliser applications underway early to take advantage of good growing conditions.  A 2019 IPF trial in Byaduk demonstrated increases in growth rates of a ryegrass pasture from 180% with 30kg N/ha to 256% with 90kg N/ha compared to no applied nitrogen. The average daily growth rates at different nitrogen rates between 14 August and 2 November are presented in figure 1. In this trial urea was applied every 30 days following a grazing.

Figure 3: Influence of nitrogen on spring growth rates.
Source: Incitec Pivot Fertiliser, 2019.

It is also important to keep in mind the potential for losses through volatilisation in pasture systems. Spring-time often brings more unreliable rain fronts and forecasts, increasing the potential for nitrogen losses through volatilisation. Incitec Pivot Fertilisers can convert straight urea to Green Urea and any Boosta product to Green Boosta, protecting the value of the nitrogen investment from direct volatilisation losses, and keeping in the nitrogen in the paddock to grow more fodder.

Understanding background fertility with soil tests

Knowing the paddock levels and fertiliser history of other macronutrients (phosphorus, potassium and sulphur) is also critical for selecting the optimal Boosta. Where capital applications are required, Boostas can offer an opportunity to address part of this requirement, helping to bring levels back into critical ranges.

Phosphorus (P)

Having a target level for P is important to get the best out of a nitrogen application. Figure 2 shows the variable response to 80kg/ha of nitrogen at different Olson P levels. For beef-sheep enterprises, maximum N response can be achieved around an Olson P of 16, while in Dairy systems, 22-25 is generally considered optimal (Gourley et al, 2020).

Figure 2: Background phosphorus requirements – N response by P level.
Source: Gourley et al 2007.

When building P levels it is important to know the phosphorus buffering index (PBI) for your soil type. This determines the P build up factor – the kg/ha P required to increase Colwell/Olson P by 1mg/kg. To determine an appropriate capital P application rate use the following formula: Capital P (kg/ha) = (Target P – Current P) x P build up factor (See Table 1).

Table 1: Phosphorus buildup factors by phosphorus buffering index (PBI).

Source: Adapted from Smart Fertilisers – Making Sensible Phosphorus Decisions

Potassium (K)

Soil supply of K in the south has historically been sufficient for pasture production. However, years of removal have run down soil reserves to the point where responses to potassium are now commonplace. While a soil test is the best way to check K levels in a paddock, an observed decline in sub clover, or a paddock with a history of hay/silage production can also be an indication that a pasture may be responsive to K. During spring, uptake of potassium can be in the order of 2-3 kg/ha/day, meaning an adequate supply is crucial to maximise growth.

The ability for a pasture to access K is directly correlated to root mass. In a year like this, where pastures have potentially been grazed earlier than is ideal, or harder than usual, root growth in the winter may have been compromised. As such, pastures may struggle to access potassium for deeper in the soil profile, increasing the chance of a response to K applied in spring.

When applying K to a pasture, bear in mind the potential risk of Grass Tetany. An imbalance of cations within the plant can induce magnesium deficiency in ruminants, so stock should be monitored carefully. Additionally, given the mobile nature of K in the soil, and the propensity for luxury uptake in plants, it is not recommended to apply more than 60kg/ha of K in one go, or a total of 120kg/ha in one year.

Sulphur (S)

To maintain ideal pasture production, an annual rate of 12kg/ha of sulphur is recommended in pasture system.

The sulphur nutrient cycle is similar to nitrogen, therefore, mineralisation is usually the chief source of sulphur for pasture systems. The ‘grower standard’ of 100kg/ha SuPerfect in the autumn is usually sufficient to meet maintenance sulphur requirements for a given season, and paddocks with a long history of SSP may not need a further application in spring. Soil test levels and fertiliser history can be good indicators of whether a pasture will be sulphur responsive.

Removal and maintenance

When cutting for hay and silage, export of nutrients from the paddock is especially high. While tissue testing is the most accurate way to determine exact nutrient removal from a paddock, the rule of thumb for nutrient removal in 1 tonne of hay or silage is 25N:2P:25K:2S. Removals from different types of hay and silage are presented in figure 3. Meeting these removals with application of a Boosta, prior to or after cutting to promote regrowth is important to ensure the pasture system stays resilient.

Figure 3: Nutrient removal per tonne of dry matter. Source Gourley et al. 2012.

Tying it all Together

Incitec Pivot Fertilisers offer several Boostas for pasture production (Table 2). The right Boosta will depend on both paddock fertility and pasture end use.

Hay and Fodda Boosta products are often used where hay or silage are the end use for the pasture. The higher rates of potassium help to balance the higher rates of removal with these fodder cuts. On the other hand, where a higher rate of nitrogen is more of a consideration, or where background fertility is higher, Grass and Pasture Boosta may be a superior choice for that paddock.

Table 2: IPF Boosta range

Source: IPF, 2025

Summary

Boosta products are an efficient and effective way to provide a blend of nitrogen, phosphorus, potassium and sulphur to promote spring growth in pasture systems.

Spring-time is a perfect time to undertake soil testing.  Paddock variability is visible, leading to a more representative sample.  Given the last two seasons have been below average, nutrient removal through decreased stocking rates and fodder production, may have decreased nutrient removal, and the addition of brought in feed loading nutrients on certain paddocks.  These factors provide a cause to undertake an intensive sampling regime to get spring fertiliser applications right and provide a great insight into autumn fertiliser requirements.

Determining nitrogen rate, based on grazing strategy or target hay/silage yield is the first step. Understanding background phosphorus, potassium and sulphur levels, as well as fertiliser history in the paddock will then help you determine whether capital or maintenance rates of these other macronutrients are necessary.

To plan you spring fertiliser application you need to answer these questions:

  1. What is my intended Nitrogen rate?
  2. Do I require capital phosphorus/potassium?
  3. What is my nutrient removal from grazing/hay/silage?

Selecting Boosta products on a paddock-by-paddock basis will help you to optimise your fertiliser investment this spring.

Further Information

For more information on the spring pasture management and Boosta strategies, reach out directly to IPF Agronomist, Reily Menhenett at reily.menhenett@incitecpivot.com.au or on 0474 093 167.

References

Gourley CJP (2020) Fertcare technical standards for nutrient management planning on Australian dairy farms, Fertilizer Australia, Canberra, Australia.

Gourley CJP, Dougherty WJ, Weaver DM, Aarons SR, Awty IM, Gibson DM, Hannah MC, Smith AP, Peverill KI (2012) Farm-scale nitrogen, phosphorus, potassium and sulfur balances and use efficiencies on Australian dairy farms. Animal Production Science 52, 929-944. https://doi.org/10.1071/AN11337

Laycock J, Menhenett L, Barlow K 2022. Making sensible phosphorus decisions, Smart Fertilisers smartfertilisers.com.au/making-sensible-phosphorus-decisions/