Author: Lee Menhenett – Technical Agronomist Cropping and Pasture

Dryland cropping is a fickle business when outcomes are largely driven be weather events beyond our control. Over the past four years in southern cropping zones, the weather gods have generally been favourable, leading to solid yields. However, in 2024 the curse of dry conditions and frosts saw yields significantly decline from the five-year average. Certain areas were affected more than others, with large parts of South Australia feeling the full extent of the damage.

As yields are highly variable, so too is the basis of achieving maximum water limited yields (WLY)– nitrogen. Armstrong et. al. (2019) undertook a study on 136 wheat paddocks in the Wimmera, Mallee and Western District of Victoria, assessing the various genetic, environmental and grower management attributes that affected Water Use Efficiency. These attributes were then ranked in order of relative importance. Graph 1 clearly demonstrates the importance of nitrogen (N) fertilisers, with soil mineral N also listed.

Graph 1: Relative importance of different environmental, genetic and management factors contribution to the WUE of wheat in growers’ paddocks (2013-2016) based on Conditional Forest Analysis. 

Source: Armstrong et. al. (2019)

The difficulty in N management is largely associated with the fact that N decisions and applications are made during earlier plant growth stages with no knowledge, or control over, later crop damaging environmental issues, such as heat shocks, frosts or minimal finishing rains.

While these later season issues can be devastating, there are key considerations and management options that can be adopted, and importantly adjusted, during the growing season.

Assessing plant available water

Without moisture, N discussions are a moot point. The best determination of available soil moisture is with soil moisture probes. On-farm probes provide instant data that reflects stored moisture from rainfall events, crop water use and implications of farm management activities, such as summer spraying of weeds. Soil type variation within paddocks and the cost of soil probes, prohibits large numbers to be established across individual farms. Access to on farm moisture probes or publicly available soil moisture monitoring programs, such as Agriculture Victoria’s Soil Moisture Monitoring Sites (link attached), allow some understanding of moisture availability as the season progresses. This soil moisture information is integral to matching N applications to water limited yield potential. When using public soil moisture data, be aware of soil type and paddock management history (previous crop, summer weed control, current crop) as these factors will be different to each individual paddock situation.

Using on farm rainfall records as a proxy for soil moisture status and water limited yield is also highly valuable. There are many methods available for calculating WLY, each has its strengths and weaknesses, but from my experience adopting a method (or a derivative of) and applying to your own paddocks over many years, will provide some great data, that is calibrated and relevant to each individual situation. Graph 2 shows annual and effective rainfall (effective rainfall is calculated as 30% of December to March rainfall plus 75% of GSR April – October). 

Yield (kg/ha)/effective rainfall (mm) provides a Water Use Efficiency (WUE) number shown over a number of years. The flood year of 2022 produced very low WUE, while 2024 produced very high WUE due to timely establishment and finishing rains. The summer fallow in 2024 also received highly effective rainfall of 140mm due to it falling over a three-week period, allowing deeper soil moisture movement and low surface evaporation. From Graph 3 the median WUE for wheat is 22.6 kg grain/mm, canola 12.1 and faba beans 12.7. Upper and lower extremes are greater in canola and faba beans compared to wheat. 

Based on this historical data, more confidence can be gained in estimating WLY therefore better matching N supply to the demand. 

Graph 2: Yearly and effective rainfall data overlayed by crop Water Limited Yield (annual and average).

Source: IPF 2025

Graph 3: Box and whisker graph of Water Use Efficiency for wheat, canola and faba beans.

Source: IPF 2025

Measuring soil mineral N

All budgets have a debit and a credit side of the ledger, N budgets are no different.  Setting yield targets based on Water Limited Yield provides the debit side of the equation. Credits are provided through in-crop N mineralisation, starter N fertiliser and soil mineral N. Whether in-crop mineralisation is included in budgets will be dependent on soil organic carbon levels. Ignoring mineralisation from soils higher (>2.5%) can lead to budgeting errors, given the higher amount of N potentially coming into the system, whereas lower organic carbon soils (<2.5%) contribute less N and accounting for this N supply essentially means soil carbon levels are being mined. 

Without deeper soil samples analysis of mineral N (nitrate and ammonium), budgeting becomes guess work. There is even greater value in deep soil samples if they are segmented.  A canola N rate and timing trial conducted in 2014 at Dookie, was sampled in segments to a depth of 90cm post-harvest. Optimal yield was attained at 80kgN/ha. The soil N results presented in Graph 4, show that at the optimal yield each sample depth segment and the total residual N values were the lowest of all treatments. The graph also shows increasing rates of N increased mineral soil values at each segment depth. Most mineral N resides in the top 30cm of soil, however, will move deeper in wetter years or when applied N rates exceed the optimum. Having soil N data on the amount of mineral N allows budgets to be closed, with the added benefit of segmenting the samples provides a picture of crop access.

Graph 4: Segmented post-harvest mineral nitrogen levels from a nitrogen rate and timing trial.

Source: IPF 2014

Nitrogen budgeting

As crop yield varies so does the requirement for N. This is the reason historical rainfall data should be used to identify individual localised crop water use efficiencies, and why current season rainfall should be tracked to adjust water limited yield. While yield will be a moving target throughout the growing season, protein in the budget should be fixed. Obviously, final grain protein percentage will vary based on several factors, including temperature and moisture of the season finish. Protein is a surrogate for yield. Main season spring wheat maximises yields with protein generally in the 11-11.5% range, while for winter wheat and barley, the protein target is 10-10.5%. 

Protein factor varies for crop type – for wheat it is 1.75 and all other crops 1.6

Nitrogen Use Efficiency varies based on application timings, rate of N and crop type.

Indicative efficiency percentages – spring wheat 40%, winter wheat 45-50%, barley 45-50% and canola 30%.  E.g. 100/40% = 2.5

Nitrogen budget:

  • Wheat (spring type)

Yield (adjust) X Protein 11.5% (fixed) X Protein Factor 1.75 (fixed) X Efficiency (40%) 2.5 = 50kgN/t

  • Wheat (winter type)

Yield (adjust) X Protein 10.5% (fixed) X Protein Factor 1.75 (fixed) X Efficiency (40%) 2 = 37kgN/t

  • Barley

Yield (adjust) X Protein 10.5% (fixed) X Protein Factor 1.6 (fixed) X Efficiency (40%) 2 = 34kgN/t

  • Canola

Yield (adjust) X Protein 18% (fixed) X Protein Factor 1.6 (fixed) X Efficiency (30%) 3 = 86kgN/t

Key considerations for 2025

Late breaking rains have caused issues with patchy and late crop emergence in southern cropping zones. Due to the late start, crops are outside the optimal sowing/emergence windows that can lead to yield penalties, particularly for canola and wheat. This is due to the decrease in time to develop yield components (primarily spikes and grain/spike) plus potentially flowering/filling grain in later/warmer spring conditions. Across most regions there is a soil moisture deficit compared to long-term average seasons. Therefore, yield targets need to be more conservative, coupled with a more tempered approach to N rates.

While N rates may need to be tempered, timing should start while crops are vegetive. N programs need to get underway to avoid plant stress and to take advantage of rainfall events that maybe fewer in a drier year. Early N applications can be made on canola to assist in dry matter accumulation without haying off risk. Unless soil N levels are high, cereals should have N applied prior to reproductive growth stages. Rates of N will be dependent on moisture, soil mineral N levels and farmers attitude to risk. Early tillering timing applications can be considered if N levels are low and if plant establishment numbers are low.

Products like Easy N allow smaller and more frequent applications of N to better match changes in Water Limited Yield as the season progresses. Easy N is also less volatile than urea when applied in situations of minimal rainfall events and open crop canopies. Green Urea is also a great option to significantly reduce N losses when urea maybe sitting on the soil surface for >2days.

It is not too late to undertake deep soil tests and close the N budget with confidence, and similarly it is not too late to monitor soil moisture levels (rainfall) and make continual adjustments to yield (and N budgets) as the season progresses, wetter or drier.

Further information

For more information feel free to contact Lee Menhenett, IPF Technical Agronomist Cropping and Pasture at lee.menhenett@incitecpivot.com.au

Microsoft Word – 2019ASA_Armstrong_Roger_#62.docx

Soil Moisture Monitoring | Soil Moisture Monitoring

References

Armstrong R., Hochman Z., Waldner F., Bell K.L., Perris R., Dunsford K., Hekmeijer P., and Munn M. (2019). Nitrogen supply, rotation and variety are critical predictors of the water use efficiency of wheat in grower’s paddocks in Victoria. Proceedings of the 2019 Agronomy Australia Conference, 25-29 August 2019, Wagga Wagga, Australia.