Managing N Fertiliser Profitability Close Yield Gaps

By BCG Staff and Contributors
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Take Home Messages

  • In 2022, Yield Prophet® Lite 50 per cent treatment applied more N (137kg N/ha) and was more profitable (gross margin $1205/ha) than the N Bank 125kgN/ha treatment (69kg N/ha, gross margin $951/ha) but mean profit for the two treatments over five years was similar ($575 vs $524kg N/ha).
  • Over five years of the experiment, N decisions based on 50 per cent Yield Prophet® or 125kg/ha N bank strategy applied more N (63–74kg N/ha) and were $128–179/ha per year more profitable than the district average N rate (21–30kg N/ha).
  • The most profitable strategies all had neutral to positive partial N balances (more N applied in fertiliser than removed in grain) indicating soil organic N was not being mined.
  • High urea prices in 2022 were offset by higher grain prices, and the most profitable treatments and N application rates remained the same over the last five years

Aim

To evaluate different N management systems designed to profitably close the yield gap due to N deficiency and decrease soil organic matter decline.

Background

Australian wheat yields are only half what they could be for the rainfall received (Hochman et al., 2017). Nitrogen (N) deficiency is the single biggest factor contributing to this yield gap. This is also likely to be true for other non-legume crops (barley, canola and oats) and reduces farm profitability. Alleviating N deficiency would increase national wheat yields by 40 per cent (Hochman and Horan 2018).

On farms with no legume pastures, most of the crop N supply must come from N fertiliser. Grain legumes do not provide enough N to support yield of subsequent crops at the intensity at which they are currently grown. N fertiliser is a costly input and its use increases the cost of production and value-at-risk for growers. Growers fear over-fertilisation will result in ‘haying off’, which reduces both yield and quality. There is also concern that overapplied fertiliser not used by crops is lost to the environment through leaching, volatilisation and denitrification. Consequently, efforts are made to match N fertiliser inputs to seasonal yield potential. This is difficult in southern Australia due to the lack of accurate seasonal forecasts for rainfall. The difficulty in matching N supply to crop demand and a tendency for growers to be conservative in their N inputs is responsible for a large proportion of the yield gap explained by N deficiency. 

In 2018, BCG and La Trobe University began a multi-year experiment to evaluate the potential for different N management systems to profitably close the yield gap and slow organic matter decline; 2022 was the fifth season of the experiment.

Paddock Details

Location: Curyo
Soil type: Sandy loam top-soil with clay content and calcium carbonate increasing with depth
Paddock history: 2017: Lentil

 

Trial Details

Treatments: Refer to Table 2
Seeding equipment: Knife points, press wheels, 30cm row spacing
Replicates: Four

Trial Inputs

N Fertiliser

Starter fertiliser:

  • 2018: Urea @ 35kg/ha at sowing (host farmer management)
  • 2019: Granulock Z @ 60kg/ha at sowing
  • 2020: Granulock Z @ 60kg/ha at sowing
  • 2021: Granulock Z @ 60kg/ha + triple superphosphate @ 35kg/ha at sowing
  • 2022: Granulock Z @ 60kg/ha + triple superphosphate @ 35kg/ha at sowing

The experiment was kept free of weeds and disease as per current best practice management

Method

A multi-year experiment using a randomised complete block design was established in 2018 to evaluate the performance of different N management systems. Four different systems were tested:

  1. Matching N fertiliser to seasonal yield potential (Yield Prophet® and Yield Prophet® Lite, YP)
  2. Maintaining a base level of fertility using N fertiliser (N banks)
  3. Replacing the amount of N removed in grain each year with fertiliser in the next season (replacement)
  4. Applying national average N fertiliser rate (45kg/ha) each season (national average, NA)

All systems were compared to a nil control which received only starter fertiliser (7kg N/ha per year). Within the Yield Prophet® and N bank systems there were different treatments targeting different yield potentials (Table 3). In the Yield Prophet® treatment before 2021, water limited potential yield was determined at different levels of probability; the amount of N required to achieve these yields was applied, assuming a requirement of 40kg/ha N per t/ha wheat yield and 80kg/ha N per t/ha canola yield (Figure 1). From 2021 onward, Yield Prophet® Lite was used in a similar way. For the N bank treatments there were different target levels of N fertility (N bank targets). N fertiliser rates in these treatments were calculated as the N bank target value minus soil mineral N (kg/ha) measured prior to sowing.

All gross margins were calculated assuming a urea price of $1400/t and cash grain prices delivered Birchip early January 2023. All other variable costs are from the 2022 SAGIT Gross Margin Guide assuming medium rainfall (SAGIT 2022).

Results & Interpretation

Results & Interpretation

2020 time of sowing (TOS) effects contrasted with the 2019 trial at Kalkee which experienced a dry spring finish.In 2019 yields at Kalkee were favoured by earlier sowing, with early May sown treatments averaging 1.5t/ha higher than early June sown treatments (Figure 1). The late maturing Forester failed to reach GS71 for hay cut at either sowing date.Yield in 2019 was generally optimised with the application of 60kg N/ha split 2:1 between sowing and top dressing six weeks post sowing.

Results & Interpretation

2020 time of sowing (TOS) effects contrasted with the 2019 trial at Kalkee which experienced a dry spring finish.In 2019 yields at Kalkee were favoured by earlier sowing, with early May sown treatments averaging 1.5t/ha higher than early June sown treatments (Figure 1). The late maturing Forester failed to reach GS71 for hay cut at either sowing date.Yield in 2019 was generally optimised with the application of 60kg N/ha split 2:1 between sowing and top dressing six weeks post sowing.

Results & Interpretation

Comparison of the different systems over the five years of the experiment using 2022 costs and prices shows the relationship between mean N application and gross margin flattening out (Figure 2). Yield Prophet® 25, 50 and 75 per cent and N Bank 100 and 125kg N/ha are all equally profitable and have applied between ~50 and 94kgN/ha per year on average. The Yield Prophet® 50 per cent and N Bank 125kg N/ha treatments sit in the middle of this band and have applied 74 and 63kgN/ha on average. Assuming the district average N application is 30kg N/ha (Norton 2016), the fitted quadratic function suggests the Yield Prophet® 50 per cent and Nitrogen Bank 125kg/ha treatments have on average returned $179/ha and $128/ha more profit per year (Figure 2). 

The Yield Prophet® 50 per cent and N Bank 125kg/ha treatments also had a neutral to slightly positive five-year N balance (Figure 3), indicating soil organic N was not being mined and soil organic matter was likely being maintained. This contrasts to the Nil control which had a five-year N balance of minus 124kg/ha N. Based on the soil C:N ratio at the site of 9.7, this suggests ~1203kg/ha of soil organic carbon has been lost.

Commercial Practice and On Farm Profitability

Growers should soil test and use an environmentally appropriate fertiliser N management strategy such as Yield Prophet® Lite or N Banks to maximise profits. In this experiment, profit has been maximised at much higher rates of fertiliser N (~70kg N/ha N or 152kg/ha urea per year) than is usually applied in the district (21–30kg N/ha or 46–65kg/ha urea). Long-term profitability is likely to be increased by growers being less conservative with N fertiliser applications, particularly for those consistently achieving cereal grain proteins of less than 11.5% (ie. APW or ASW wheat).

In the very favourable season of 2022, Yield Prophet® treatments were more aggressive with N applications and were more profitable compared to N bank treatments. However, over the five years of the experiment, profitability of the best Yield Prophet® and N Bank treatments is very similar. The most profitable treatments in this experiment have neutral to slightly positive N balances, indicating a ‘win-win-win’ where profits are maximised, soil organic N is not mined and excessive mineral N is not accumulated that is then susceptible to losses. Growers should check the long-term N balances of their paddocks to ensure soil organic N is not being mined. A spreadsheet to do this is available here: https://www.bcg.org.au/understanding-crop-potential-and-calculating-nitrogen-toimprove-crop-biomass-workshop-recording/

High urea prices in 2022 and beyond are a legitimate concern, given the strong reliance of continuous cropping systems on synthetic N fertiliser for high yields and profits. However, increases in grain prices have offset rises in the price of urea, and results of this experiment show the most profitable N rates have not changed during the five years of the experiment. 

References

Norton, R (2016) Nutrient Performance Indicators. GRDC, Barton, ACT. Available at https://grdc.com.au/research/reports/report?id=6785.

Hochman, Z, Gobbett, DL, Horan, H (2017) Climate trends account for stalled wheat yields in Australia since 1990. Global Change Biology 23, 2071–2081.

Hochman, Z, Horan, H (2018) Causes of wheat yield gaps and opportunities to advance the water-limited yield frontier in Australia. Field Crops Research 228, 20–30.

Norton, R (2016) Nutrient Performance Indicators. GRDC, Barton, ACT. Available at https://grdc.com.au/research/reports/report?id=6785.

SAGIT (2022) ‘Farm Gross Margin and Enterprise Planning Guide 2022.’ Available at https://sagit.com.au/wp-content/uploads/2022/01/21112.01-Gross-Margins-Guide-2022_WEB.pdf [accessed 16 January 2022].

Acknowledgements

This research was funded by La Trobe University through the Securing Food, Water and the Environment Research Focus Area, the Mallee Catchment Management Authority, through funding from the Australian Government’s National Landcare Program and GRDC, through National Grower Network. We thank Juan Wang and Caixian Tang of La Trobe University for analysis of soil and plant samples and Paul Barclay for hosting the experiment.

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