Take home messages

• In most areas of the southern region, 2022 is shaping up to be another good winter crop season. Soil moisture profiles are full, (saturated in some areas) and the rainfall outlook appears to be positive.
• However, after two seasons with big yields and nutrient removal, starting soil-nitrogen levels are typically low.
• Fertiliser and grain prices are high, so a tailored nitrogen strategy will be needed to optimise grain yield and returns.

Background

Growers and agronomists need to ensure cereals and canola have an adequate supply of nitrogen (N) to optimise costs and returns. This means managing nitrogen application rates with application timings and placement methods to maximise nitrogen use efficiency.

Nitrogen prices are high, but fortunately, potential grain prices are high as well. This combination means fertiliser is still a good investment. Compared to seasons with a combination of low fertiliser and grain prices, a similar quantity of extra grain is required to cover the investment in nitrogen and phosphorus fertiliser in 2022.

Nitrogen is essential for crop dry matter production, shoot density and potential yield in cereals. When crops begin to mature, nitrogen within the plant is redirected to developing grains. Carbohydrates are then deposited within the grain, and it is the level of carbohydrates that determine grain size and yield. The dilution of nitrogen with carbohydrates in grain also determines final grain protein levels.

In short dry springs, crop yields are reduced because they are unable to fill each grain, and the percentage of grain protein is much higher and often grain size is smaller compared to normal seasons. Longer soft finishes to the insufficient to maintain the high yield potential, the dilution of nitrogen levels in the grain may result in low grain protein levels.

The idea of using grain protein concentration to assess the likelihood of nitrogen responsiveness wheat in cropping systems in South Australia was described 50 years ago (Russell, 1963). The work suggested that yield responses were most likely when grain protein concentration was <11.4% (McDonald and Hooper, 2013). This is supported in trial results from the Incitec Pivot Fertilisers long term trial at Glenelg in Central NSW (Figure 2), where yield response to nitrogen plateaus in the 11-12% protein range and protein per cent continues to increase with additional nitrogen.

Figure 1. Grain nitrogen removal from the 2020 (canola) and 2021 (wheat) harvests at the Incitec Pivot Fertilisers long term nitrogen x phosphorus trial, Glenelg NSW, established 2007.

In winter cereals, it can be a juggling act to manage additional nitrogen inputs between pre-plant and booting and, in irrigated and high rainfall systems, through to flowering in order to achieve the targeted yield, grain size and protein results.

Throughout this process, it is critical that nitrogen is identified as the only limitation. As Table 1 shows, there are many other things that will affect crop yield and protein potential.

Table 1: Factors affecting crop yield and protein potential.

Adapted from “Soil Fertility and Fertilizer Management” Havlin, Beaton, Tisdale and Nelson 6th edition.

Nitrogen budgeting

The first step in planning nitrogen applications is a simple budgeting process to assess nitrogen demand (the nitrogen required to grow a target yield and protein) and the likely amount of nitrogen to be provided by the soil. Once you know the demand and the soil supply, you can determine the amount of fertiliser/nitrogen that will be required to make up the shortfall.

The guiding ‘4R’ principles of the right source, rate, time, and place of fertiliser requires knowledge of both crop demand and soil N supply. Seasonal conditions remain the primary driver of crop demand for N in dryland cropping systems and to a considerable extent, also fertiliser use efficiency.

Estimating existing nitrogen supply

There are several tools available to advisers to help determine the existing nitrogen supply, such as reviewing paddock fertiliser and crop histories, deep soil nitrogen test, in-crop NDVI, and shoot density. The best decisions are made using reputable information collected from a range of sources.

Paddock history

Information such as previous crop yield, previous grain protein levels, crop rotation, fallow weed control, soil moisture levels, and seasonal conditions can all inform soil nitrogen supply.

For example, where paddocks produced wheat crops in 2021 with less than 10.5–11%, it suggests nitrogen supply was limiting. There may be higher soil nitrogen levels in 2022 in paddocks coming out of a legume pasture or pulse crop phase in 2021. But relying solely on paddock history information can be misleading at times, and direct soil nitrogen measurements should be taken.

Deep soil nitrogen tests

Pre-plant deep soil nitrogen test results are an excellent resource for nitrogen management. If a paddock is low soil nitrogen and no significant nitrogen has been applied at seeding, there may be a requirement for early topdressed nitrogen at mid-tillering to establish the desired target shoot density by GS30.

Monitor the paddocks that you know have low soil nitrogen levels (either by deep N soil test results or paddock history). Don’t always wait until GS30 to assess crop performance, as early nitrogen may be required to achieve the desired target shoot density by GS30. The application of 15–30kg N/ha may be required to stimulate additional tillers. Urea ammonium nitrate (Easy N®) through streamers or dribble bars is ideally suited to this scenario with accurate placement, timely application and potentially less volatilisation losses than urea on alkaline soils.

Soil sampling after crop emergence can also play a role where deep N tests weren’t taken pre-plant. When sampling post-emergence, be careful to avoid any banded pre-plant nitrogen or starter nitrogen fertilisers as this will skew the results.

In-crop deep nitrogen test results should be used in conjunction with plant analysis (i.e. the percentage of total nitrogen in above ground dry matter), especially where dry matter is above 1000kg/ha, to estimate the nitrogen supply and demand balance.

Timing considerations

Nitrogen applications before GS30 can increase tiller numbers and dry matter, leading to yield increases if there is sufficient moisture available. The greatest demand for nitrogen is when the leaf area is expanding rapidly, and the crop is growing most rapidly.

During stem elongation (GS31–GS37), peak growth rates can be of the order of 200kg/ha/day and crops can accumulate 2–3kg N/ha/day (McDonald and Hooper, 2013). Some carry-over of nitrogen into grain protein may occur.

When nitrogen is applied after GS59, it is generally to boost grain protein and maintain yields in above average seasons. Nitrogen use efficiency is reduced at these later stages of application, especially if conditions are dry. Likely returns need to be assessed carefully.

Remember that it is only when the yield potential has been reached that additional nitrogen can contribute to higher grain protein levels.

Maximising nitrogen use efficiency

To maximise nitrogen use efficiency, apply nitrogen  when the crop is likely to respond, including:

• prior to periods of rapid growth
• when the soil moisture profile is full, and the season outlook is favourable
• to paddocks free from soil limitations such as compaction, sodicity, acidity and salinity
• where root diseases (such as crown rot, Rhizoctonia and take all) are non-limiting
• where leaf diseases (such as stripe rust, yellow leaf spot and Septoria) are non-limiting
• where weeds and insects are controlled

Where losses of nitrogen through volatilisation may occur, consider an inhibitor product, such as Green Urea NV®.

Figure 2. Yield and protein response to rates of phosphorus and nitrogen in 2013 at the Incitec Pivot Fertilisers long term nitrogen x phosphorus trial, Glenelg NSW.

Figure 3. Typical grain yield and protein responses to nitrogen in wheat. Source: Incitec Pivot Fertilisers.

Nitrogen losses when topdressing

When broadcast on the soil surface, both liquid or dry urea, or urea containing products or blends, can be susceptible to ammonia loss through volatilisation. The higher the initial soil pH and the pH in the reaction zone around the fertiliser granule, the greater the potential for volatilisation. Losses from alkaline soils in south-east Australia have been measured as high as 26% of total N lost through volatilisation over a period of 20 days on an alkaline clay applied in the first week of September (Turner et al. 2012).

Factors favouring NH3 volatilisation losses from topdressed urea are:

• light rain post-application, insufficient to wash the dissolved granule into the soil
• crop residue on the soil surface
• temperatures above 18°C
• alkaline soils, especially those with low cation exchange capacity
• wind
• application to a moist soil that dries after application.

The safest applications are those made to dry clay soils, in low humidity conditions with no wind and sufficient rainfall to move the urea into the soil within a few days of the application.

The cost of nitrogen losses on highly responsive situations will be more than just the value of nitrogen . If grain yield potential is compromised, which is highly possible on low nitrogen  soils, the outcome will be worse. Where starting soil nitrogen is low, the yield response from applied nitrogen fertiliser could be as much as 20kg grain per kg nitrogen. If 9kg/ha nitrogen is lost from an application of 100kg/ha nitrogen , the resulting yield reduction maybe 180kg/ha or $90/ha. Combine this with the value of the lost nitrogen  ($33/ha) and the reduction in income may be $123/ha. This makes an investment in products that can reduce volatilisation an excellent return on investment. (Note: calculations assume urea @ $1700 per tonne, wheat @ $500 per tonne.)

Green Urea NV® is a urea-based product that is treated with N-(n-Butyl)-thiophosphoric triamide (NBPT). When NBPT is added to urea, it inhibits the activity of the urease enzyme for a period of up to 14 days. During that period, the losses of ammonia from the surface applied urea are greatly reduced, and nitrogen is still present for the next rainfall event.

Table 2: Summary of results from 15 wheat topdress nitrogen trials conducted across SE Australia 2005-07.

Green Urea NV gave a statistically significant yield response compared with the control and urea. Green Urea NV also gave a significant protein increase compared with the control and urea.

Wheat productivity and nitrogen use efficiency was measured at another three sites in the Wimmera region of Victoria, Australia between 2012 and 2014. At those sites, NBPT (the urease inhibitor in Green Urea NV) increased yield (+ 7–11%) in two of three years compared to unamended urea (Wallace et al. 2020).

Incitec Pivot Fertilisers trial work in the Mallee in 2014 also demonstrated the effectiveness of Green Urea NV over urea with an early September topdress and 17 days before a welcome rainfall event of 35mm. The Green Urea NV treatment returned a significant grain yield response over urea (0.41t/ha), protein response (0.7%) and improved nitrogen use efficiency from 33.7% for urea to 60.5% for Green Urea NV.

In yet another field study, the management strategy significantly affected losses of fertiliser nitrogen depending on seasonal conditions, with urease inhibitors (NBPT) producing significant benefits in reducing losses of topdressed urea under dry seasonal conditions (Armstrong et al. 2021).

When topdressing large acreages, topdressing can commence four to six days before a substantial rainfall event with confidence that all the nitrogen is still available for incorporation into the soil profile when that rain front comes through.

All farming systems should aim to maximise nutrient use efficiency. When choosing the most appropriate product to apply, consideration should be given to the rate of application, the frequency of application, the timing of the application, the economic objectives, and any environmental consequences.

References

Armstrong R, Wallace A, Dunsford K (2021) GRDC Update 2021 Nitrogen fertiliser use efficiency ‘rules of thumb’ - how reliable are they?

McDonald G, Hooper P (2013) GRDC Update 2013 Nitrogen decision – Guidelines and rules of thumb.

Russell JS (1963) Nitrogen content of wheat grain as an indication of potential yield response to nitrogen fertilizer. Australian Journal of Experimental Agriculture and Animal Husbandry 3, 319-325.

Turner DA, Edis RE, Chen D, Freney JR, Denmead OT (2012) Ammonia volatilization from nitrogen fertilizers applied to cereals in two cropping areas of southern Australia. Nutrient Cycling in Agroecosystems 93, 113-126.

Wallace AJ, Armstrong RD, Grace PR, Scheer C, Partington DL (2020) Nitrogen use efficiency of 15N urea applied to wheat based on fertiliser timing and use of inhibitors. Nutrient Cycling in Agroecosystems 116, 41-56.

Contact details

Lee Menhenett
Incitec Pivot Fertilisers
0427 006 047
lee.menhenett@incitecpivot.com.au